The Solfatara Magmatic-Hydrothermal System: Geochemistry, Geothermometry and Geobarometry of Fumarolic Fluids (Advances in Volcanology) 3030984702, 9783030984700

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Advances in Volcanology

Luigi Marini Claudia Principe Matteo Lelli

The Solfatara Magmatic-Hydrothermal System Geochemistry, Geothermometry and Geobarometry of Fumarolic Fluids

Advances in Volcanology An Official Book Series of the International Association of Volcanology and Chemistry of the Earth’s Interior Series Editor Karoly Nemeth, Institute of Natural Resources, Massey University, Palmerston North, New Zealand

More information about this series at https://link.springer.com/bookseries/ 11157

Luigi Marini Claudia Principe Matteo Lelli •

•

The Solfatara Magmatic-Hydrothermal System Geochemistry, Geothermometry and Geobarometry of Fumarolic Fluids

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Luigi Marini STEAM srl Pisa, Italy Matteo Lelli Institute of Geosciences and Earth Resources CNR - Italian National Research Council Pisa, Italy

Claudia Principe Institute of Geosciences and Earth Resources CNR - Italian National Research Council Pisa, Italy

ISSN 2364-3277 ISSN 2364-3285 (electronic) Advances in Volcanology ISBN 978-3-030-98470-0 ISBN 978-3-030-98471-7 (eBook) https://doi.org/10.1007/978-3-030-98471-7 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Preface

There are at least three reasons that pushed me to write this book on the Solfatara of Pozzuoli, with the competent help of my wife Claudia Principe for the geological-volcanological aspects and my young friend and colleague Matteo Lelli for the geochemical aspects. These three reasons are personal, but I really want to mention them. Reason # 1. My first scientific article, written together with Roberto Cioni and Egizio Corazza, was focused on the geochemistry of the Solfatara fumarolic gases (Cioni et al. 1984). Both Roberto and Egizio died last year, the cursed 2020, and this book is dedicated to both of them. The scientific paper of Roberto, Egizio, and myself appeared in 1984 as part of a special issue of the Bulletin Volcanologique (that has been called Bulletin of Volcanology starting from the following issue) collecting several scientific articles on the bradyseismic crisis that affected the Campi Flegrei caldera in 1982–1984. In that work, a conceptual geochemical model of the Solfatara magmatic-hydrothermal system was developed, based on the limited data available at the time. The model proved inadequate a few years later, due to the evolution of the system, as clearly and correctly pointed out by Caliro

Left: Roberto Cioni is sampling the geothermal gases at Larderello http://www.societageochimica.it/wp-content/ uploads/2020/05/Roberto-Cioni.pdf Right: Egizio Corazza is performing the gas chromatographic analysis of a gas mixture http://www.societageochimica.it/wp-content/uploads/2020/10/Egizio-Corazza.pdf v

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et al. (2007), who proposed a new conceptual geochemical model of the Solfatara magmatic-hydrothermal system based on a dataset much larger than that of Cioni et al. (1984). This new model will probably need to be revised and updated as well. It is the normal course of these things, in that geology is not an exact science and volcanic systems are very complex and extend to considerable depths, but can be directly studied from the surface only. Reason # 2. The fumarolic fluids of the Solfatara and the thermal waters of the Campi Flegrei have attracted me repeatedly. This interest first blossomed in the late 1970s and early 1980s, when I participated in the geochemical monitoring of selected thermal waters of the Campi Flegrei and the short-term production tests of the deep geothermal wells drilled by Agip-ENEL at Mofete, as a geochemist of Aquater, formerly Geotecneco, two companies of the ENI group. Afterwards, the fumarolic fluids of Solfatara were the focus of other scientific articles of mine (e.g., Cioni et al. 1989; Cioni and Marini 1990). In the meantime, Giovanni Chiodini had joined the Pisa group led by Roberto Cioni and a fruitful collaboration lasting for many years was born with him. Some scientific articles I wrote together with Giovanni, who was working at the Osservatorio Vesuviano, were focused on the Solfatara, as a whole or in part, such as that on the hydrothermal gas equilibria in the H2O-H2-CO2-CO-CH4 system (Chiodini and Marini 1998) and that on the CO2 degassing and energy release at the Solfatara (Chiodini et al. 2001). Then our paths separated and, in the following years, I must confess that I had a limited interest in the Solfatara and the Campi Flegrei, while Giovanni carried out researches of the highest quality, many of which were dedicated to the geochemistry of the Solfatara fluids and he and his talented collaborators produced an impressive amount of data, on which, as a matter of fact, this book is based almost entirely. It must be underscored that some of the activities performed by Giovanni and coworkers at the Solfatara as well as by other geo-scientists in other active volcanic areas of the world, are not free of risks. We cannot forget, in this regard, what happened on January 14, 1993, at Galeras volcano, Colombia, when six scientists were killed by a sudden explosion.1 Four were Colombians: Dr. Jose Arles Zapata, a geochemist at the Pasto observatory, Dr. Fernando Cuenca, a geophysicist from Bogota who had conducted a magnetic survey of the volcano, Dr. Nestor Garcia, an industrial chemist at the National University, Dr. Carlos Trujillo, a community college teacher in Pasto who liked to use the volcano as a classroom. The other two were Dr. Geoff C. Brown, a geologist at the Open University in Britain and Dr. Igor Menyailov, a Russian scientist from the Institute of Volcanology in Petropavlovsk on the Kamchatka peninsula. And again, it is worth to recall what happened on March 12 of the same year at Guagua Pichincha, Ecuador, when two volcanologists from the Geophysical Institute of the National Polytechnic School, Víctor Hugo Pérez and Álvaro Sánchez, were

1

https://archive.nytimes.com/www.nytimes.com/books/01/04/15/specials/volcano.html (last visit on 11/12/2021).

Preface

Preface

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monitoring the fumaroles of the dome inside the crater and were killed by a phreatic explosion.2 Geo-scientific data can have a very high price. Reason # 3. Starting from May 2019, I was assigned a free research task by INGV, the Italian National Institute of Geophysics and Volcanology, for carrying out research activities concerning the dynamics of the hydrothermal systems related to active volcanoes. Subsequently, in April 2020, I was included in the Scientific Evaluation Panel of the Volcanoes Department of the INGV for the project “From the unrest pre-eruptive dynamics to the forecasting of the expected eruption” concerning the Etna, Stromboli, and Campi Flegrei volcanoes. And so my interest in the magmatic-hydrothermal system of the Solfatara was reawakened. The understanding of its characteristics and past evolution representing the sine qua non condition to predict its future evolution, which is the essence of volcanic surveillance, became a stimulating challenge for me. This renewed interest of mine represented the seed that finally led to the writing of this book with the skilled contributions of Claudia and Matteo. Viareggio, Italy December 2021

Luigi Marini

References Caliro S, Chiodini G, Moretti R, Avino R, Granieri D, Russo M, Fiebig J (2007) The origin of the fumaroles of La Solfatara (Campi Flegrei, south Italy). Geochim Cosmochim Acta 71(12):3040–3055 Chiodini G, Marini L (1998) Hydrothermal gas equilibria: The H2O-H2-CO2-CO-CH4 system. Geochim Cosmochim Acta 62:2673–2687 Chiodini G, Frondini F, Cardellini C, Granieri D, Marini L, Ventura G (2001) CO2 degassing and energy release at Solfatara volcano, Campi Flegrei, Italy. J Geophys Res 106:16213–16222 Cioni R, Corazza E, Marini L (1984) The gas/steam ratio as indicator of heat transfer at the Solfatara fumaroles, Phlegraean Fields (Italy). Bull Volcanol 47(2):295–302 Cioni R, Corazza E, Fratta M, Guidi M, Magro G, Marini L (1989) Geochemical precursors at Solfatara Volcano, Pozzuoli (Italy). In: Latter JH (ed) Volcanic Hazards. IAVCEI Proceedings in Volcanology, vol 1. Springer, Berlin, Heidelberg, pp 384–398 Cioni R, Marini L (1990) The determination of deep temperatures by means of the CO-CO2-H2-H2O geothermometer: an example using fumaroles in the Campi Flegrei, Italy. A comment. Bull Volcanol 53(1):67–69

2

http://ceniza-ecuador.over-blog.com/2018/03/a45-tragico-accidente-en-el-volcan-guaguapichincha-12-de-marzo-de-1993.html (last visit on 11/12/2021).

Introduction

Solfatara volcano is situated close to the center of the Campi Flegrei caldera which, in the past, has alternatively experienced phases of resurgence and subsidence, traditionally indicated with the term bradyseism. Ludovico Sicardi (1895–1987) was an Italian scientist, who carried out pioneering studies on the Solfatara and other Italian volcanoes (Vesuvius, Stromboli, Vulcano, Panarea, Salina, and Etna), developing innovative instruments and methods to sample and analyze volcanic gases (Calabrese et al. 2020). He wrote several papers on the Solfatara of Pozzuoli, one of which, in Italian, was published on the Bulletin Volcanologique in 1956. In that paper, he noted that among the numerous craters of the Campi Flegrei, on the Gulf of Naples, that of the Solfatara of Pozzuoli is the only one that has probably preserved an uninterrupted fumarolic activity for at least two millennia. In 1538, a few kilometers away, a new volcanic vent opened and created its edifice in 48 hours. It is known as Monte Nuovo and reaches an elevation of 140 m, but the only remnant of that eruptive episode of such a short duration are some weak fumarolic emissions. The fumaroles of the Solfatara of Pozzuoli, on the other hand, reveal a truly unusual persistence, even if the thermal level of the exhalation is not excessive, since the alternation of different phases of activity is quite common in volcanic centers (Sicardi 1956). Today, owing to the on-going unrest of Solfatara volcano, there is an urgent need to estimate the increasingly higher temperatures and pressures present at depth by using suitable gas-geoindicators, whose implementation and testing is the major aim of this book. In fact, the currently used gas-geothermometers and geobarometers, especially those related to the H2O-H2-CO2-CO-CH4 system (e.g., Chiodini and Marini 1998) involve carbon monoxide, which is expected to re-equilibrate upon cooling at temperatures in the order of 210–250 °C which are presumably found at shallow depths. Furthermore, these currently used geochemical tools assume ideal gas behavior and, therefore, cannot be used above 250–300 °C approximately. Among the innovative approaches adopted in this work to overcome the limitations of the currently used gas-geoindicators and differentiating this book from numerous previous geochemical studies, not only those focused on the Solfatara, we recall the following ones:

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(1) Methane, being characterized by a sluggish behavior, was treated separately from fast-reacting carbon monoxide, in that mixing apples and oranges is not a good idea. (2) The deviations from the ideal gas behavior were considered, in order to implement geoindicators that can be used at the high pressures and temperatures which are presumably encountered along the decompression path of magmatic-hydrothermal fluids. (3) The kinetics of the relevant reactions, never considered before in the development of geothermometric and geobarometric techniques, was taken into account. To this purpose, both the numerous studies carried out by the chemical engineers for the optimization of the chemical processes in industrial plants and the geochemical investigations on the origin of methane were reviewed. These innovations were possible because of the availability, for the Solfatara, of a dataset including several geochemical parameters and extending over a time interval of 37 years, from 1983 to 2020, with a good sampling frequency. This makes the Solfatara a case history probably unique in the world. Clearly, if the time series had been longer and the sampling frequency had been higher it would have probably been possible to better understand short-duration processes. An example is the crisis of January 15, 1935, when the outlet temperature of fumarolic fluids reached 215 °C. Unfortunately, the closest available samples were obtained in 1923 and 1938, while no geochemical data was collected at that time. Although there are no ifs, ands, or buts in history, it is very likely that the availability of geochemical data would have made it possible to provide a more solid interpretation of what we tried to propose here in chapter “Historical Data”. We reiterate that long time series, with a good sampling frequency, and including all the necessary geochemical parameters are of fundamental importance and will be increasingly so in the future thanks to their continuous growth. Focusing on the content of this book, the background information is presented in chapters “Historical Data”, “Summary of the Campi Flegrei Geology”, “The Magmatic–Hydrothermal System Hosted in the Campi Flegrei Caldera with Emphasis on the Solfatara”, “Chemical and Isotopic Characteristics of Solfatara-Pisciarelli Fumarolic Fluids”, and “Soil CO2 Diffuse Degassing and Thermal Energy Release as Indicators of Volcanic Unrest in the Solfatara-Pisciarelli Area”, namely: (a) the temperature and chemical data acquired since the half of the nineteenth century are taken into account in chapter “Historical Data”; (b) a brief summary of the geo-volcanological framework of Campi Flegrei, including the Solfatara volcanic activity and the bradyseismic phenomenon affecting the homonymous caldera, is presented in chapter “Summary of the Campi Flegrei Geology”; (c) different aspects relevant for the elaboration of the conceptual models of the magmatic-hydrothermal system (or systems) hosted in the Campi Flegrei caldera are discussed in chapter “The Magmatic– Hydrothermal System Hosted in the Campi Flegrei Caldera with Emphasis on the Solfatara”, with a special focus on the magmatic-hydrothermal system

Introduction

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of Solfatara volcano; (d) the chemical and isotopic characteristics of Solfatara-Pisciarelli fumarolic fluids are examined in chapter “Chemical and Isotopic Characteristics of Solfatara-Pisciarelli Fumarolic Fluids”; (e) the diffuse degassing of CO2 from soil and the related thermal energy release from the Solfatara-Pisciarelli area, two powerful indicators of the on-going volcanic unrest occurring at Solfatara volcano, are considered in chapter “Soil CO2 Diffuse Degassing and Thermal Energy Release as Indicators of Volcanic Unrest in the Solfatara-Pisciarelli Area”. A review of the different equations of state (EOS) that can be used to model the deviations from the ideality condition of the gas mixtures of interest was considered mandatory and was done in chapter “The Expansion (Decompression) of the Solfatara Fumarolic Fluids”. As a result of this review, the GERG-2008 EOS and the EOS of Gallagher et al. (1993) were used for computing the fugacity coefficients of CO2 and H2O, that is, the major components of Solfatara fluids, whereas the fugacity coefficients of all relevant gas species, including CO2 and H2O, were calculated by means of the well-known Peng-Robinson EOS. Calculations were performed for several possible expansion (decompression) paths of Solfatara fluids to provide a complete picture of the deviations from the ideal gas behavior at the temperatures and pressures possibly encountered by the Solfatara fluids along their ascent towards the surface. Chapter “The Geoindicators Involving H2O, CO2, CO, CH4 and H2” is devoted to a thorough re-examination of the five possible reactions occurring in the H2O-H2-CO2-CO-CH4 homogeneous gaseous system in order to choose the two most representative ones for the geothermometry and geobarometry of magmatic-hydrothermal fluids, obviously keeping CH4 and CO separate the one from the other. Chapter “The Chemical Kinetics of the Reactions Involving H2O, CO2, CO, CH4 and H2” is focused on the kinetics of relevant reactions between H2O, H2, CO2, CO, and CH4. A reaction path model was developed to simulate the heating of Solfatara fluids and applied to a congruous number of selected samples. New geothermometric-geobarometric techniques applicable at temperatures up to ca. 1000 °C and pressures up to ca. 3 kbar, both for the H2O-H2CO2-CO-CH4 homogeneous gaseous system and the heterogeneous gas-solid systems involving H2S, were implemented and applied to all Solfatara fluid samples, as described in chapters “Calibration and Application of the RWG and SS4 Indicators for the Possible Expansion Paths of Solfatara Fluids” and “The Redox Potential and Sulfur Gas Species”, respectively. To reach this goal, the deviations from the ideal gas behavior were taken into account in a simplified and generalizable form. Some final considerations are drawn in chapter “Conclusive Remarks” including the two possible scenarios of the future evolution of the Solfatara magmatic-hydrothermal system. All in all, we think that the geochemical tools proposed in this book might provide useful results for the volcanic surveillance of the Solfatara volcano and Campi Flegrei as a whole, which is a densely inhabited area with ca. 500,000 inhabitants at present. Nevertheless, the assessment of the current state of risk is beyond the aims of this book.

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Finally, we are sure that the case history of the magmatic-hydrothermal system of the Solfatara is a useful term of comparison for the geo-scientists engaged in the study and surveillance of other similar volcanic systems, elsewhere in the world, for the reasons presented in chapter “A Comparison Between the Solfatara and Other Magmatic-Hydrothermal Systems”.

References Calabrese S, Li Vigni L, Brugnone F, Capasso G, D’Alessandro W, Parello F, Ferla P (2020) The precious treasure of Mariano Valenza: the history of Ludovico Sicardi and the birth of geochemical volcano monitoring. Italian Journal of Geosciences 139 (3):413–435 Chiodini G, Marini L (1998) Hydrothermal gas equilibria: The H2O-H2-CO2-CO-CH4 system. Geochim Cosmochim Acta 62:2673–2687 Gallagher JS, Crovetto R, Sengers JL (1993) The thermodynamic behavior of the CO2H2O system from 400 to 1000 K, up to 100 MPa and 30% mole fraction of CO2. J Phys Chem Ref Data 22(2):431–513 Sicardi L (1956) La Solfatara di Pozzuoli. Bull Volcanol 18(1):151–158

Introduction

Contents

Historical Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 The Temperature of Solfatara Fumaroles from the Historical Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 The Chemical Composition of Solfatara Fumarolic Fluids Based on the Historical Sources . . . . . . . . . . . . . . . . . . . . . . . . . 3 The Temperature Anomaly of 15 January 1935 . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary of the Campi Flegrei Geology . . . . 1 The Solfatara Volcano . . . . . . . . . . . . . . . . 2 The Bradyseism in the Campi Flegrei . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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The Magmatic–Hydrothermal System Hosted in the Campi Flegrei Caldera with Emphasis on the Solfatara . . . . . . . . . . . . . . . 1 The Shallow-Hydrothermal Portion of the Magmatic–Hydrothermal System Hosted in the Campi Flegrei Caldera. . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Main Characteristics of the Deep Geothermal Wells Drilled in the Campi Flegrei Caldera . . . . . . . . . . . . . . . . 1.2 Hydrothermal Alteration Mineralogy. . . . . . . . . . . . . . . . . 1.3 The Geochemistry of the Geothermal Liquids Discharged by the AGIP-ENEL Boreholes . . . . . . . . . . . . 2 The Deep-Magmatic Portion of the Magmatic–Hydrothermal System Hosted in the Campi Flegrei Caldera . . . . . . . . . . . . . . . 2.1 Layer (C)—Thermometamorphic Rocks . . . . . . . . . . . . . . 2.2 Layer (D): Mesozoic Carbonate Rocks and the Crystalline Bedrock. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Layer (E): The Melt Zone . . . . . . . . . . . . . . . . . . . . . . . . . 3 The Surface Discharges of the Campi Flegrei Magmatic–Hydrothermal System . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 The Onshore Thermal Springs and Shallow Wells . . . . . . 3.2 The Onshore Fumaroles and Gas Vents . . . . . . . . . . . . . . 3.3 The Offshore Fumaroles and Gas Vents . . . . . . . . . . . . . . 3.4 The Sub-Lacustrine Hydrothermal Discharges in Lake Averno . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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26 26 31 33 40 41 42 43 45 45 46 48 50

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The Conceptual Model of the Magmatic–Hydrothermal System Hosted in the Campi Flegrei Caldera. . . . . . . . . . . . . . . . . . . . . . 5 The Conceptual Geochemical Models of the Solfatara Magmatic–Hydrothermal System . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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The Expansion (Decompression) of the Solfatara Fumarolic Fluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 1 The Outlet Temperature and Related Implications . . . . . . . . . . . . 64 2 The H2O-CO2 Binary System . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 2.1 Experimentally Measured P-T-X and P-T-X-V Data of CO2-H2O Mixtures . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 2.2 The Dimensionless Thermodynamic Properties of CO2-H2O Mixtures . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 2.3 The Excess Molar Enthalpy of CO2-H2O Mixtures . . . . . . 71 3 An Excursus on the Equations of State (EOS) . . . . . . . . . . . . . . 72 3.1 The First Steps of an Incomplete History that Began Over 350 Years Ago . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 3.2 The Van Der Waals’s Equation . . . . . . . . . . . . . . . . . . . . 73 3.3 The Cubic EOS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 3.4 The Virial EOS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 3.5 The Benedict-Webb-Rubin EOS and Its Modified Versions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 3.6 The Lee-Kesler EOS and Its Modified Forms . . . . . . . . . . 81 3.7 The EOS of Keenan, Keyes, Hill, and Moore . . . . . . . . . . 83 3.8 The EOS of Haar, Gallagher, and Kell . . . . . . . . . . . . . . . 84 3.9 The Schmidt-Wagner EOS . . . . . . . . . . . . . . . . . . . . . . . . 85 3.10 The Reference EOS of Wagner and Coworkers . . . . . . . . 86 3.11 The Technical EOS of Span and Lemmon . . . . . . . . . . . . 86 3.12 Equations of State for Mixtures . . . . . . . . . . . . . . . . . . . . 86 3.13 The Generalization and Extension of the Principle of Corresponding State . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 3.14 Mixture Properties from Helmholtz Energy EOS . . . . . . . 88 3.15 Conclusive Considerations and Way Forward . . . . . . . . . . 91 4 The Isenthalpic Decompression of Solfatara Fluids: Results of the GERG-2008 Thermodynamic Module of the CONVAL Software Package . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 4.1 The PTV Values Computed by the GERG-2008 Thermodynamic Module of the CONVAL Software Package for the Isenthalpic Expansion of Bocca Grande and Bocca Nuova Fluids and Related Implications . . . . . . 96 4.2 The Fugacity Coefficients of CO2 and H2O Computed by Using the GERG-2008 Thermodynamic Module of the CONVAL Software Package and the Virial EOS for the Isenthalpic Decompression of Bocca Grande and Bocca Nuova Fluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102

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5

The Fugacity Coefficients of H2O, CO2, H2, CH4, and CO Computed by Using the Peng-Robinson EOS for the Isenthalpic Decompression of Bocca Grande Fluids . . . . . . . . . . 6 Comparison of the Fugacity Coefficients of CO2 and H2O from the Peng-Robinson EOS, GERG-2008/CONVAL, and Gallagher et al. (1993) for the Isenthalpic Expansion of Bocca Grande Fluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Other Possible Expansion Paths of Solfatara Fluids . . . . . . . . . . 7.1 The Saturation Decompression Paths . . . . . . . . . . . . . . . . 7.2 The Linear P–T Decompression Path . . . . . . . . . . . . . . . . 7.3 Fugacity Coefficients at the Pressures and Temperatures of Coexistence of a Brine Containing 21 Wt% NaCl and the Related Vapor Phase . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4 Fugacity Coefficients at the Pressures and Temperatures of Coexistence of a Brine Containing 33.5 Wt% NaCl and the Related Vapor Phase . . . . . . . . . . . . . . . . . . . . . . 7.5 Fugacity Coefficients Along the Linear P–T Decompression Path . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Chemical and Isotopic Characteristics of Solfatara-Pisciarelli Fumarolic Fluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Chemistry of the Solfatara-Pisciarelli Fumarolic Fluids Based on Binary Diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Chronograms of Gas Concentrations in Bocca Grande and Bocca Nuova Fumarolic Fluids . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Chemistry of the Solfatara-Pisciarelli Fumarolic Fluids Based on Principal Component Analysis . . . . . . . . . . . . . . . . . . . . . . . . 4 The Triangular Diagram of N2-Ar-He . . . . . . . . . . . . . . . . . . . . . 5 Argon Isotopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Helium Isotopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Nitrogen Isotopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Sulfur Isotopes of H2S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Carbon Isotopes of CO2 and CH4 . . . . . . . . . . . . . . . . . . . . . . . . 10 Hydrogen Isotopes of H2O and Oxygen Isotopes of H2O and CO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.1 The Exchange of Oxygen Isotopes Between H2O Vapor and Gaseous CO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2 Modeling of Magmatic Degassing . . . . . . . . . . . . . . . . . . 10.3 Recycling of Condensed Steam . . . . . . . . . . . . . . . . . . . . . 10.4 Interpretation of the dD and d18O Values Available for the Solfatara Fumarolic Fluids . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

111

118 122 122 124

124

130 136 142 151 153 159 162 167 169 171 172 173 174 177 177 178 190 192 193

xvi

The Geoindicators Involving H2O, CO2, CO, CH4 and H2 . . . . . . 1 Theoretical Foundations of the Geoindicators Involving H2O, CO2, CO, CH4 and H2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Application of the Geoindicators Involving H2O, CO2, CO, CH4 and H2 to the Solfatara Fluids: Preliminary Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Application of the Geoindicators Involving H2O, CO2, CO, CH4 and H2 to the Solfatara Fluids Assuming Full Equilibrium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Application of the Geoindicators Involving H2O, CO2, CO, CH4 and H2 to the Solfatara Fluids Assuming Independent Equilibrium in a Pure Saturated Vapor Phase . . . . . . . . . . . . . . . 5 Comparison of CO Concentrations Consistent with the SS4 Equilibrium Temperature and Analytical Values . . . . . . . . . . . . . 6 Comparison of CH4 Concentrations Consistent with the RWG Equilibrium Temperature and Analytical Values . . . . . . . . . . . . . 7 Comparison of H2 Concentrations Consistent with the CCC Equilibrium Temperature and Analytical Values . . . . . . . . . . . . . 8 Effects of Steam Condensation . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Summary of the Results Provided by the Geoindicators Involving H2O, CO2, CO, CH4 and H2 for the Solfatara Fluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.1 The Full Equilibrium Approach . . . . . . . . . . . . . . . . . . . . 9.2 The Approach Based on the Independent Equilibrium in a Pure Saturated Vapor Phase . . . . . . . . . . . . . . . . . . . . 9.3 Considerations on Carbon Monoxide . . . . . . . . . . . . . . . . 9.4 Considerations on Methane . . . . . . . . . . . . . . . . . . . . . . . . 9.5 Considerations on Hydrogen . . . . . . . . . . . . . . . . . . . . . . . 9.6 Considerations on Steam Condensation. . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Chemical Kinetics of the Reactions Involving H2O, CO2, CO, CH4 and H2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 The Hydrogenation of CO2 and CO to CH4: A Synthesis of the Achievements of Paul Sabatier . . . . . . . . . . . . . . . . . . . . . 2 The Hydrogenation of CO2 and CO to CH4: The Geochemical Studies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 The Hydrogenation of CO2 and CO to CH4: Reaction Path, Reaction Mechanisms, and Rate Equations . . . . . . . . . . . . . . . . . 4 The Kinetics of the Steam Reforming of Methane Accompanied by the Water–Gas Shift Reaction . . . . . . . . . . . . . 5 Application of the Reaction Path Model Approach to the Solfatara Fluids: Results and Limitations . . . . . . . . . . . . . 5.1 Application of the Reaction Path Model Involving the Reactions WG, RSS3, and RSS4 to the Solfatara Fluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Application of the Reaction Path Model Involving Reactions WG and RSS3 to the Solfatara Fluids . . . . . . .

Contents

197 198

204

206

209 215 217 219 221

224 224 224 225 225 226 226 226 229 230 232 235 239 242

242 248

Contents

xvii

5.3

Final Considerations on the Application of Reaction Path Modeling to Solfatara Fluids . . . . . . . . . . . . . . . . . . . 260 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281 Calibration and Application of the RWG and SS4 Geoindicators for the Possible Expansion Paths of Solfatara Fluids . . . . . . . . . . . 1 The Adopted Calibration Approach . . . . . . . . . . . . . . . . . . . . . . . 2 The RWG and SS4 Geoindicators for the Isenthalpic Decompression Path of Solfatara Fluids . . . . . . . . . . . . . . . . . . . 3 The RWG and SS4 Geondicators for the Saturation Decompression Path of Solfatara Fluids Involving a Brine Containing 21 wt% NaCl and the Related Vapor Phase . . . . . . . 4 The RWG and SS4 Geoindicators for the Saturation Decompression Path of Solfatara Fluids Involving a Brine Containing 33.5 wt% NaCl and the Related Vapor Phase . . . . . . 5 The RWG and SS4 Geoindicators for the Linear P–T Expansion Path of Solfatara Fluids . . . . . . . . . . . . . . . . . . . . . . . 6 Application of the RWG and SS4 Geoindicators to the Solfatara Fluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1 The RWG Equilibrium Temperature and Total Fluid Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 The SS4 Equilibrium Temperature and Total Fluid Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3 Final Considerations on the Temperatures and Total Fluid Pressures Indicated by the RWG and SS4 Reactions at Equilibrium . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The 1 2 3

Redox Potential and Sulfur Gas Species . . . . . . . . Preliminary Considerations . . . . . . . . . . . . . . . . . . . . The Redox Potential . . . . . . . . . . . . . . . . . . . . . . . . . The Sulfur-Bearing Gas Species . . . . . . . . . . . . . . . . 3.1 Hydrogen Sulfide . . . . . . . . . . . . . . . . . . . . . . 3.2 Sulfur Fugacity . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Sulfur Dioxide Fugacity . . . . . . . . . . . . . . . . . 3.4 Carbonyl Sulfide Concentration . . . . . . . . . . . 4 Gas-Solids Equilibria Involving H2S . . . . . . . . . . . . . 4.1 Gas-Solids Equilibria Involving Pyrite . . . . . . 4.2 Gas-Solids Equilibria Involving Anhydrite . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . .

. . . . . . . . . . . .

. . . . . . . . . . . .

. . . . . . . . . . . .

. . . . . . . . . . . .

. . . . . . . . . . . .

. . . . . . . . . . . .

. . . . . . . . . . . .

. . . . . . . . . . . .

285 286 287

290

293 294 296 296 299

305 305 307 308 311 317 317 318 320 323 325 325 332 347

Soil CO2 Diffuse Degassing and Thermal Energy Release as Indicators of Volcanic Unrest in the Solfatara-Pisciarelli Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 351 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 361

xviii

A Comparison Between the Solfatara and Other Magmatic-Hydrothermal Systems . . . . . . . . . . . . . . . . . . . . . . . . . . 363 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369 Conclusive Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375

Contents

Historical Data

Abstract

The outlet temperature and the chemical composition of Bocca Grande fumarole were measured since the half of the nineteenth century, albeit from time to time rather than in a systematic way. The temperature peak of 215°C recorded on 15 January 1935 is significantly greater than the possible uncertainty of ca. 20°C on the outlet temperature determination. Based on the thermodynamic properties of pure water, the temperature of 215°C at 1 atm pressure constrains the enthalpy of fumarolic gases at ca. 2905 kJ/kg. This value is ca. 100 kJ/kg higher than the maximum enthalpy of saturated steam, which is 2803 kJ/kg at 236°C, 31.2 bar, suggesting that, at that time, a superheated steam phase was present in the plumbing system of the Bocca Grande fumarole, possibly at all depths, due to dry up. It was already clear to the scientists of the nineteenth century that the main component of Bocca Grande fumarolic fluids is by far H2O, accompanied by CO2, H2S, N2, H2, CH4, NH3, He and Ar, in order of decreasing importance. Sulfur dioxide was reported in some investigations, but De Luca (1871) already recognized that SO2 was produced by the O2-driven oxidation of H2S, upon mixing of fumarolic gases with atmospheric air.∎∎∎ As noted by Sicardi (1956) “the bibliographic roots of the Solfatara are above all literary and

only Strabo, as a geographer, fits in providing descriptions of scientific interest. The Solfatara phenomenology, however, always lacked that grandeur for which the other volcanoes of the Italian peninsula are so tied to ancient mythology and myths. The place situated on the very pleasant gulf of Baia and Pozzuoli, so loved by the Romans, was well known to them and the contrast between the luxuriant richness of the nearby countryside and its acrid squalor must have appeared very vivid. The desolation of the Solfatara is described by Petronius Arbiter with such lively lyricism that today nothing can be removed from the imagination of that time, because the same words adapt to our impressions, to the current reality very similar to that of twenty centuries ago.1 Today, we find the same sulfur field and the stinking vapors remembered by Strabo, in the same squalid and mortal realm where Petronius saw only Chaos without any sign of spring. This concomitance is certainly not sufficient to affirm that the Solfatara has always been like this. However, there are not enough Petronio in his Satyricon wrote: “Est locus exciso penitus demersus hiatus Parthenopen inter magnaeque Dicarchidos arva, Cocyti perfusus aqua; nam spiritus, extra qui furit effusus, funesto spargitur aestu.” According to https://www. loebclassics.com/view/petronius-satyricon/1913/pb_LCL015. 303.xml, the English translation is: “Between Parthenope [Naples] and the fields of the great town of Dicarchis [Pozzuoli] there lies a spot plunged deep in a cloven chasm, wet with the water of Cocytus [one of the six rivers of the underworld in Greek mythology]: for the air that rushes furiously outward is laden with that baleful spray”. 1

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 L. Marini et al., The Solfatara Magmatic-Hydrothermal System, Advances in Volcanology, https://doi.org/10.1007/978-3-030-98471-7_1

1

2

Historical Data

reasons to think differently since in its structure it does not reveal a tormented life. An exhaustive examination of the historical sources from the first century BC to the nineteenth century AD was carried out by Sicardi (1944). These historical sources suggest the probable occurrence of fumarolic activity inside the Solfatara crater since Roman times, as noted by Breislak (1811) among the others. The first measurements of the temperature and chemical composition of fumarolic fluids dates back to the nineteenth century, with the works of Charles Joseph Sainte-Claire Deville in 1856– 1862, who called “Bocca della Solfatara” the fumarole that today is known as “Bocca Grande”, Claude-Henri Gorceix in 1867, and Sebastiano De Luca in 1869 and 1871. Further details are given in the following two sections.

1

The Temperature of Solfatara Fumaroles from the Historical Sources

A good starting point of this section is the scientific paper of Mercalli (1907) who reported on the temperatures measured in the past as follows: “At the ‘Bocca della Solfatara’, Ch. S. C. Deville found 88° C in 1856; Gorceix determined 155° C in 1867; Seb. De Luca measured just over 114° C in 1871; H. Johnston-Lavis obtained 156° C in 1889; finally Nasini observed 130 and 132° C in 1897”. These disparate results could lead us to believe that there are greater variations in activity than actually occurring. In fact, it is necessary to recall that, between 1856 and 1867, the vapors of the ‘Bocca della Solfatara’ came out of a narrow cave, partly artificial, into which one could only enter on all fours and with difficulty for a few meters. It is therefore evident that the disparate temperatures obtained depend mostly on the different placement of the thermometer more or less close to the true point of origin of the fumarole. In 1899, between my trips of July 16 and November 2, on a day that I could not specify, a portion of the vault of that small cave collapsed.

For some months, the ‘Bocca della Solfatara’ remained almost blocked; then at least a part of the fallen material was excavated, and so now the fumarole can be approached closer and the true temperature can be determined with less uncertainty. After that time, I measured the temperature of this fumarole 12 times with a maximum thermometer of the Müller house in Bonn, checked by Prof. O. Scarpa in the Technical Physics Cabinet of the Royal Polytechnic High School in Naples. However, I would like to point out that even now it is not easy to always put oneself in identical conditions for determining the temperature; because the vapors do not come to light from a stable opening, but come out, hissing, from a heap of sand and loose lapillus, which are in constant motion, especially when digging a small cavity a few decimeters deep, to apply the thermometer. The two photographs [in Fig. 1] show the state of the ‘Bocca della Solfatara’ in 1901–1903, following the landslide and indicate the precise point [A inside the black star in Fig. 1b], where I took the temperature of the fumarole. The temperatures I observed at the ‘Bocca grande’2 are the following: Date of measurement

Temperature (°C)

October 28, 1900

153

February 27, 1901

126

August 11, 1901

152.5

February 23, 1902

153

March 27, 1902

152

May 21, 1903

154

December 24, 1903

151

January 28, 1905

157.5

April 11, 1906

154.5

March 1, 1907

154.5

March 10, 1907

154

March 25, 1907

153.5

I must warn that the minimum of 126 °C probably depended on having applied the 2

To be noted that here the fumarole is indicated with a different name from the one used above.

1

The Temperature of Solfatara Fumaroles …

3

Fig. 1. a The ‘Bocca della Solfatara’, on February 27, 1901 (picture by Mercalli) and b on June 2, 1903 (picture by Aguilar). The A inside the black star indicates the

position of the main vent after the landslide, where temperatures were measured by Mercalli (1907). Both pictures are from Mercalli (1907)

thermometric bulb too superficially; because on that day the fumarole made a loud noise and threw the grains at a height of about one meter, that is, it did not appear to have less activity than usual. Discarding this observation as not good, we see that the other 11 temperatures are quite concordant, and, in general, much higher than those given by previous observers; among which perhaps only Dr. Johnston-Lavis placed the thermometer at the true point of exit of the vapors”. Mercalli (1907) reports also that the temperatures of the other Solfatara fumaroles were up to 102.5 °C, whereas the temperature of the Pisciarelli fumaroles was 97.5 °C. To be kept in mind that the measured temperatures depend not only on the point but also on the depth reached by the probe, as reported for Bocca Grande by Dall’Aglio et al. (1972) who determined (i) a temperature of 141 °C at 20 cm depth on 23 July 1970, when M. Sato measured 162 °C at 2 m depth and (ii) a temperature of 143 °C at 20 cm depth on 7 October 1970, when 151 °C were determined at 2 m depth. Sicardi (1941) compiled 57 outlet temperatures of Bocca Grande which were measured from 1856 to 1939 by several authors (SainteClaire Deville 1856; De Luca 1869; Gorceix 1872; Johnston-Lavis 1890; Nasini et al. 1904; Mercalli 1907, 1910; Signore 1924, 1929, 1935;

Majo 1931; Parascandola 1936; Ginori Conti 1938) and himself, whereas Sicardi (1970) reported 12 data for the period 1949–1969 again by other authors (e.g., Parascandola 1955; Santi 1956) and himself. All the data obtained since October 1900 for the outlet temperature of Bocca Grande (apart from the value of 126 °C which was considered unreliable by the author, see above) as well as those reported by Dall’Aglio et al. (1972) are shown in the chronogram of Fig. 2. The chronogram shows: (i) the presence of two peaks, one on 31 July 1927 with a temperature of 174.5 °C, the other on 15 January 1935 with a temperature of 215 °C and (ii) a possible downward shift of 10–20 °C in the baseline after the two peaks, especially after the second one, compared to its position before the peaks. Most of these observations must be taken with caution, considering a possible uncertainty of ca. 20 °C in the measured temperatures, depending on the point and depth at which the probe is placed, but the temperature peak of 15 January 1935 is significantly greater than the possible uncertainty on the measured temperatures and, therefore, calls for a detailed examination. The sources quoted by Sicardi (1941) for the anomalously high temperatures of the period are the communications given by prof. G. B. Rizzo, Director of the Institute of Earth Physics of the

4

Historical Data

Fig. 2. Chronogram of the outlet temperature of Bocca Grande in the period 1900–1969 based on the data compiled by Sicardi (1941, 1970) and those reported by Dall’Aglio et al. (1972)

Royal University of Naples, to the local newspapers. The same sources are also reported by Signore (1935), who wrote what follows: “Finally in January 1935 the activity of the Solfatara receives a new increase, which raises the temperature of the Bocca grande, according to the press release of the Institute of Earth Physics, first to 195 °C and then to over 215 °C and produces the opening of the new vent” [in the southern part of the crater, close to the mud volcano known as Fangaia, see Fig. 3.11]. Further considerations are postponed to Sect. 3.

2

The Chemical Composition of Solfatara Fumarolic Fluids Based on the Historical Sources

Sicardi (1941) compiled also the chemical analyses of Solfatara fumarolic fluids, including those of Bocca Grande, which are listed in Table 1. The results of these historical analyses are given on a dry-gas basis although the prevalence of water vapor over the other gas constituents was already clear to the scientists of

Table 1 Chemical analyses (on a dry-gas basis) of the fumarolic fluids of Bocca Grande from the historical sources Author

Year

T

CO2

H2S

SO2

O2

N2

CH4

H2

°C

vol%

vol%

vol%

vol%

vol%

vol%

vol%

0.0086

0.0682

0.0960

0.04515

Gorceix

1867

>120

84.10

6.00

1.70

8.40

De Luca

1871

>115

99.10

0.90

0.00

0.00

Nasini

1897

132

98.43

0.60

0.15

0.82

Salvatore

1923

147

99.19

0.47

0.262

Sicardi

1928

154

98.90

0.50

0.60

Sicardi

1937

143

99.12

0.48

0.52

Ginori Conti

1938

148

98.66

0.94

0.265

2

The Chemical Composition of Solfatara Fumarolic Fluids …

the nineteenth century. Not shown in Table 1 is the chemical analysis of Sainte-Claire Deville (1856) who found that SO2 was the only Sbearing gas species, with a concentration of 24.5° vol%. To be noted that the fluid he analyzed was heavily mixed with atmospheric air, with 14.5° vol% O2 and 61.0°vol% N2. Six years later, Sainte-Claire Deville (1862) described the gases emitted from Bocca Grande as an SO2-free mixture of CO2 and H2S (Sicardi 1941) in accordance with all the other historical data reported in Table 1. A few years later, De Luca (1871), in addition to the fluid sampled from the Bocca Grande vent whose composition is given in Table 1, collected also six gas mixtures at a distance of 50 cm from the same vent and found that they were mainly made up of N2 + O2, with 83.1–87.0 vol%, accompanied by 12.5–15.9 vol% CO2, and 0.4– 0.6 vol% SO2, whereas H2S resulted to be undetectable. Furthermore, De Luca (1871) recognized that SO2 was produced by the O2-driven oxidation of H2S according to the reaction: H2 S þ 1:5O2 ¼ SO2 þ H2 O

ð1Þ

In spite of these early findings, Cheminée et al. (1968) found an SO2 concentration of 1.9 vol% on a dry gas basis for a gas sample obtained in September 1965 from an unspecified Solfatara vent with outlet temperature of 140 ± 10 °C. The gas sample was mainly made up of CO2, 95.3 vol%, accompanied by 0.9 vol% H2S, 1.7 vol% N2 (+rare gases), and 0.2 vol% O2. It was collected into an evacuated gas bottle containing 50 g of anhydrous molten CaCl2 used to trap water, but addition of air evidently took place, leading to partial conversion of H2S to SO2. The water concentration of Bocca Grande fluids was measured by Salvatore (1923b) and Ginori Conti (1938). Salvatore (1923b) performed five measurements within 10 days. Assuming that CO2 represents on average 99.19° vol% of dry gases, based on the analytical results of Salvatore (1923a), the average H2O and CO2 concentrations turn out to be 909,160 lmol/mol

5

and 90,100 lmol/mol, respectively, with an uncertainty of 2050 lmol/mol on both values. Ginori Conti (1938) measured a gas/steam ratio of 80.4°L/kg. Hypothesizing that the gas volume is referred at 0 °C and 1 atm, this gas/steam ratio corresponds to a H2O concentration of 939,300 lmol/mol and an incondensable gas concentration of 60,700 lmol/mol. Hence, both gas mixtures are much richer in water and much poorer in incondensable gases than those collected at Bocca Grande in the 1983–2020 period, having H2O concentration varying from 689,200 to 867,400 lmol/mol, around a mean of 793,800 lmol/mol and a median of 804,600 lmol/mol, with a standard deviation of 42,800 lmol/mol (Chiodini et al. 2021 and references therein). Salvatore (1923a) and Ginori Conti (1938) measured also CH4 and H2 (see Table 1). Salvatore (1923a) reports also a He + Ar concentration of 0.0021°vol% on a dry gas basis, whereas Ginori Conti (1938) gives also the concentrations of NH3, He and Ar on a total fluid basis, which are as follows: NH3 = 170 mol/mol, He = 0.5070 lmol/mol, Ar = 0.7733 mol/mol. To be noted that the He concentration measured by Ginori Conti (1938) is significantly lower than the values of Bocca Grande, from 1989 to 2020, ranging from 0.92 to 2.93 lmol/mol (Chiodini et al. 2021 and references therein). These low He and CO2 concentrations in 1938 are reasonable based on the correlation between these two gas constituents (see Sect. 1 in Chap. “Chemical and Isotopic Characteristics of Solfatara-Pisciarelli Fumarolic Fluids”).

3

The Temperature Anomaly of 15 January 1935

Let us accept that the outlet temperature of Bocca Grande achieved the value of 215 °C on 15 January 1935 (see Sect. 1) and that fumarolic fluids were water-rich, considering that a H2O concentration of 909,160 lmol/mol was determined in 1923, that is, ca. 7 years before (see Sect. 2). If so,

6

the enthalpy of this gas mixture at discharge conditions was close to 2905 kJ/kg, based on the thermodynamic properties of pure water (Kretzschmar and Wagner 2019). This value is ca. 100 kJ/kg higher than the maximum enthalpy of saturated steam, which is 2803 kJ/kg at 236 °C, 31.2 bar, suggesting that a superheated steam phase was present at all depths in the plumbing system of the Bocca Grande fumarole, possibly due to dry up. Since there might be different causes leading to dry up, it is preferable to refrain from speculating further on this point. The depressurization of the system might have triggered the subsequent inflow of external cold waters (meteoric and/or marine) into the fumarolic conduits causing (a) a decrease in the outlet temperature, in agreement with the values measured on 19 January, 22 January, and 1 February 1935, namely, 154, 150, and 151 °C, respectively (Signore 1935; Sicardi 1941), and (b) an increase in H2O concentration, in accordance with the value of 939,000 lmol/mol that was determined in 1938 by Prince Ginori Conti (see Sect. 2). It is unclear if the anomalously high temperature of 15 January 1935 is a longterm effect of the Irpinia earthquake of 23 July 1930, with an equivalent magnitude of 6.7 (Guidoboni et al. 2018, 2019). For sure, the short-term effects of that earthquake comprised explosions throwing pieces of mud at heights of 10–15 m above the rim of the crater (Signore 1935).3

References Breislak S (1811) Introduzione alla geologia. Stamperia Reale, Milano, p 490 Cheminée J, Letolle R, Olive P (1968) Premiéres données isotopiques sur des fumerolles de volcans italiens. Bulletin Volcanologique 32(3):467–475 Chiodini G, Caliro S, Avino R, Bini G, Giudicepietro F, De Cesare W, Ricciolino P, Aiuppa A, Cardellini C, Petrillo Z, Selva J, Siniscalchi A, Tripaldi S (2021) Hydrothermal pressure-temperature control on CO2 emissions and seismicity at Campi Flegrei (Italy). J Volcanol Geoth Res 414:107245 3 Quite surprisingly, the year of the Signore paper, 1929, is prior to that of the reported event, 1930.

Historical Data Dall’Aglio M, Martini M, Tonani F (1972) Rilevamento geochimico delle emanazioni vulcaniche dei Campi Flegrei. Quaderni de ‘La Ricerca Scientifica’. CNR— Consiglio Nazionale delle Ricerche, Roma, vol 83, pp 152–181 De Luca S (1869) Osservazioni sulla temperatura interna della grande fumarola della Solfatara di Pozzuoli. Rendiconto dell’Accademia delle Scienze Fisiche e Matematiche. Società Reale di Napoli, Anno VIII, pp 34–38 De Luca S (1871) La composizione dei gas che si svolgono dalle fumarole della Solfatara di Pozzuoli. Rendiconto dell’Accademia delle Scienze Fisiche e Matematiche. Società Reale di Napoli, Anno X, pp 181–188; 211–221 Ginori Conti P (1938) L’attività endogena quale fonte di energia. Reale Accademia Nazionale dei Lincei. Classe di scienze fisiche, matematiche e naturali. Seduta del 3 aprile 1938, Roma, p 39 Gorceix C-H (1872) Sur les gas des solfatares des Champs Phlégréens. Annales Chimie et Physique (ser 4) 25:559–566 Guidoboni E, Ferrari G, Mariotti D, Comastri A, Tarabusi G, Sgattoni G, Valensise G (2018) CFTI5Med, Catalogo dei Forti Terremoti in Italia (461 a.C.-1997) e nell’area Mediterranea (760 a.C.1500). Istituto Nazionale di Geofisica e Vulcanologia (INGV). https://doi.org/10.6092/ingv.it-cfti5 Guidoboni E, Ferrari G, Tarabusi G, Sgattoni G, Comastri A, Mariotti D, Ciuccarelli C, Bianchi MG, Valensise G (2019) CFTI5Med, the new release of the catalogue of strong earthquakes in Italy and in the Mediterranean area. Sci Data 6:80. https://doi.org/10. 1038/s41597-019-0091-9 Johnston-Lavis HJ (1890) Osservazioni geologiche lungo il tracciato del grande emissario-fognone di Napoli dalla Pietra sino a Pozzuoli. Bollettino del Regio Comitato Geologico d’Italia Kretzschmar HJ, Wagner W (2019). Tables of the Properties of Water and Steam. In: International Steam Tables. Springer Vieweg, Berlin, Heidelberg. https://doi.org/10.1007/978-3-662-53219-5_5 Majo E (1931) La Solfatara di Pozzuoli. Annali dell’Osservatorio Vesuviano 3:61–72 Mercalli G (1907) Sullo stato attuale della Solfatara di Pozzuoli. Atti dell’Accademia Pontaniana 37:1–16 (memoria n. 6) Mercalli G (1910) Osservazioni sulla temperatura del vapore emanante all Solfatara di Pozzuoli. Atti Società Italiana per il Progresso delle Scienze. 4a Riunione Nasini R, Anderlini F, Salvadori R (1904) Ricerche sulle emanazioni terrestri italiane. II—Gas del Vesuvio e dei Campi Flegrei, delle Acque Albule di Tivoli, del Bullicame di Viterbo, di Pergine, di Salsomaggiore. Memorie della Reale Accademia Nazionale dei Lincei. Classe di scienze fisiche (serie 5) 5:25–82 Parascandola A (1936) Il rione delle Mofete nei Campi Flegrei. Bollettino della Società dei Naturalisti in Napoli 48:81–94

References Parascandola A (1955) Notizie sul soffione di recente apparso alla Solfatara di Pozzuoli. Ric Sci 25:3113 Sainte-Claire Deville Ch (1856) Sur les phénomènes éruptifs de du Vesuve et de l’Italie meridionale. Comptes Rendus de l’Académie des sciences 43:745– 751 Sainte-Claire Deville Ch (1862) Sur les emanations volcaniques des Champs Phlégréens. Comptes Rendus de l’Académie des sciences 54:528–536 Salvatore E (1923a) Sui gas della Solfatara di Pozzuoli. Zeitschrift für Vulkanologie 7:149–154 Salvatore E (1923b) Sulla determinazione del vapor d’acqua nelle esalazioni fumaroliche della Solfatara di Pozzuoli. Zeitschrift für Vulkanologie 7:215–217 Santi B (1956) Sulla nuova manifestazione (Soffione) nel cratere della Solfatara. Boll Soc Geol It 75:282–283 Sicardi L (1944) L’attività della Solfatara di Pozzuoli attraverso la documentazione storica avanti l’ultimo ottantennio. Atti della Società Italiana di Scienze Naturali e del Museo Civico di Storia Naturale in Milano 83(2):97–114

7 Sicardi L (1970) Recenti misure termiche alla Solfatara di Pozzuoli e qualche ancora necessaria osservazione. Bollettino Della Società Dei Naturalisti in Napoli 79:137–144 Sicardi L. (1941) Sulle manifestazioni dell’attività fumarolica della Solfatara di Pozzuoli nell’ultimo ottantennio (1856–1939). Rivista di Scienze Naturali (Natura) 32:53–74 Sicardi L (1956) La Solfatara di Pozzuoli. Bulletin Volcanologique 18(1):151–158 Signore F (1924) Relazione su di una escursione fatta il 10 maggio 1923 nella plaga puteolana. Bollettino della Società dei Naturalisti in Napoli 36:2–8 Signore F (1929) Sur la variation d’activite du volcan de Boue (“Fangaia”) de la Solfatare de Pouzzoles (Naples) par suite du grand tremblement de terre de l’Irpinia, 23 Juillet 1930. Bulletin Volcanologique 3 (2):49–52 Signore F (1935) Attività vulcanica e bradisismo nei Campi Flegrei. Annali dell’Osservatorio Vesuviano 3:173–192

Summary of the Campi Flegrei Geology

Abstract

This summary of the Campi Flegrei geology comprises: (i) the geo-volcanological and structural characteristics of Campi Flegrei and the Solfatara monogenetic edifice, (ii) the 1198 Solfatara (phantom?) eruption, which was reported for the first time about four centuries after its possible occurrence and has been a matter of debate in the volcanological literature, (iii) the bradyseism affecting the whole Campi Flegrei caldera, including alternating periods of resurgence and subsidence. Although the Campi Flegrei bradyseism is known to have occurred at least since Roman times, only two unrest episodes are well documented. One preceded the 1538 Monte Nuovo eruption, whereas the other begun in 1950 and is still ongoing, with ground deformations accompanied by seismicity and variations in the chemistry and flowrate of the fumarolic fluids discharged at Solfatara and Pisciarelli. Nevertheless, there is a lack of consensus on the processes governing the on-going unrest episode, in that the involvement of a magmatic input or the pressurization of the magma chamber was proposed by some authors, the pressurization of the overlying hydrothermal system was suggested by other authors, whereas a combination of both processes was invoked in other investigations.∎∎∎

The Solfatara is a monogenetic volcanic edifice situated inside the Campi Flegrei volcanic field, where persistent volcanic activity has taken place since at least *47,000 years, based on the oldest volcanic products cropping out in the area (Di Girolamo et al. 1984; Rosi and Sbrana 1987). Most erupted volcanic products are trachytic to trachyphonolitic tuffs and tuffites interstratified with sediments (silts, sands and marls), whereas lava flows and domes are subordinate (Rosi and Sbrana 1987; Fig. 1). The only historic eruption, which occurred in 1538 CE, involved a highly differentiated peralkaline phonolitic–trachytic magma and produced the ash and scoria cone of Monte Nuovo (Rosi et al. 1983). The prominent structural element of Campi Flegrei is a subcircular depression with a diameter of 8–12 km whose inland (northern) portion is marked by a series of topographic highs between Monte di Procida to the west and the hills of Posillipo and Camaldoli to the east (Rosi et al. 1983) and whose offshore (southern) part encompasses the Gulf of Pozzuoli (Sbrana et al. 2021). According to several authors (e.g., Rittman 1950; Rosi et al. 1983; Rosi and Sbrana 1987; Lirer et al. 1987; Barberi et al. 1991; Scandone et al. 1991; Orsi et al. 1996), this depression is a caldera generated by one or more volcano-tectonic collapses. In particular, Barberi et al. (1991) identified at least three nested

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 L. Marini et al., The Solfatara Magmatic-Hydrothermal System, Advances in Volcanology, https://doi.org/10.1007/978-3-030-98471-7_2

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Summary of the Campi Flegrei Geology

Fig. 1 Geological and structural sketch map of Campi Flegrei. Legend 1 = Quaternary undifferentiated deposits, areas of strong anthropization, soil cover; 2 = Products of the post-caldera subaerial recent activity; 3 = Products of the post-caldera mainly subaerial ancient activity; 4 = Products of the post-caldera mainly submarine

activity (Neapolitan Yellow Tuff deposits); 5 = Products of the subaerial pre-caldera activity; 6 = Volcanic vents; 7 = Faults and fractures; 8 = Campi Flegrei caldera rim; 9 = Post-caldera volcano-tectonic collapses; 10 = Craters; 11 = Starza marine terrace (from Rosi and Sbrana 1987)

collapses, namely an outer one, *13 km wide, an intermediate one, *11–12 km across, and an inner one, *4–5 km wide, by merging and elaborating surface geological–volcanological evidence (Rosi et al. 1983), subsurface information acquired by the deep geothermal wells drilled by Agip-ENEL in the Mofete, San Vito, and Licola areas (see Sect. 3 in Chap. “Summary of the Campi Flegrei Geology”), and inland and offshore gravimetric and aeromagnetic data (Fig. 2).

According to Barberi et al. (1991): (i) The outer collapse is related to the *35,000 year-old Campanian Ignimbrite eruption, a catastrophic eruption that emitted *80 km3 of dense-rock equivalent (DRE) trachytic magma, producing pyroclastic flow deposits that covered an area of 7000 km2. In this work, reference to the Campi Flegrei caldera is to the Campanian Ignimbrite collapse. (ii) The intermediate collapse is associated to the Neapolitan Yellow Tuff eruption that occurred *12,000 a BP and emitted

Summary of the Campi Flegrei Geology

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Fig. 2 Structural map of the Campi Flegrei superimposed onto the Bouguer anomaly high-pass filter map. Legend of volcanic structures 1 = caldera rim; 2 = minor, post Neapolitan Yellow Tuff, volcano-tectonic collapsed areas;

3 = crater rim; 4 = lava dome; 5 = eruptive center. Reprinted from Barberi et al. (1991), Copyright © 1991, with permission from Elsevier

*51 km3 of DRE trachytic magma. This caldera is called Neapolitan Yellow Tuff caldera in this work. (iii) The inner collapse was formed in the last phase of activity, *4,000 a BP, in the Agnano–Astroni–Solfatara sector. Based on this evidence, Barberi et al. (1991) recognized that the volcanic and structural evolution of Campi Flegrei is characterized by the recurrence of the same eruption mechanism, including the formation of a ring fracture, magma ascent, the eruption from several centers positioned along the ring fracture, and the collapse of

the central sector into the emptied magma chamber. The decrease with time of both the diameter of the collapsed area and the total sinking suggests the concurrent decrease in the volume of the magma stored in the chamber still present in the center of the caldera (Barberi et al. 1991). Accepting this explanation, it is permissible to hypothesize the current presence of a small magma batch below the Solfatara crater, that is, positioned inland, near the center of the caldera. The numerous scientific studies carried out more recently have improved the geological

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Summary of the Campi Flegrei Geology

Fig. 3 Location map of the vents of the post-15 ka eruptions. Reprinted from Smith et al. (2011), Copyright © 2011, with permission from Elsevier

concerning this subject, in that, according to knowledge of Campi Flegrei providing further some authors, the Neapolitan Yellow Tuff details although some controversial aspects still was produced during a single eruption remain. For example: (Deino et al. 2004) occurred 14.9 ± 0.4 (i) The age of the Campanian Ignimbrite (2r) ka BP, whereas according to other eruption, either 34.29 ± 0.09 (1r) ka BP, authors (e.g., Rosi and Sbrana 1987) these based on radiocarbon dating of charred volcanic products are grouped in a single wood or 39.85 ± 0.14 ka BP (95% configeological unit but were emitted from sevdence level) based on 40Ar/39Ar dating of eral eruptive centers at different times. sanidine phenocrysts (Giaccio et al. 2017), and the DRE volume of the erupted magma, (iii) A database of the compositions of the volcanic ashes (tephra) emitted by the postranging from 54 km3 (Scarpati et al. 2014) 3 15 ka eruptions in the Campi Flegrei, that to 180–280 km (Costa et al. 2012). is, during the post-Neapolitan Yellow Tuff (ii) The DRE volume of the Neapolitan Yellow activity was implemented by Smith et al. Tuff magma, varying from 10 km3 (Scan(2011) through >1900 new chemical analdone et al. 1991) to at least 50 km3 (Scarpati yses. These authors presented also an et al. 1993). There are controversial aspects

Summary of the Campi Flegrei Geology

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Fig. 4 Scheme of the main evolution stages of the Campi Flegrei. Legend 1 = Thermometamorphic rocks; 2 = Tuffites; 3 = Subaerial tuffs; 4 = Products of the Campanian Ignimbrite eruption; 5 = Neapolitan Yellow tuffs; 6 = Tephra and tuffs of the recent, mainly subaerial activity; 7 = Lava domes; 8 = Lavas, subvolcanic rocks and volcanic conduits. Reprinted from Rosi et al. (1983), Copyright © 1983, with permission from Elsevier

updated chronology based on published radiocarbon dates and 40Ar/39Ar ages and grouped the already known vents in four epochs (location in Fig. 3). In spite of recent improvements and open questions, the works of synthesis elaborated by the volcanologists of the Pisa School (Rosi et al. 1983; Rosi and Sbrana 1987; Barberi et al. 1991) still remain the groundbreaking researches on the Campi Flegrei geology. The scheme of the main evolution stages of the Campi Flegrei (from Rosi et al. 1983) is shown in Fig. 4.

1

The Solfatara Volcano

The Solfatara volcanic edifice has a sub-circular crater, with a diameter varying between 610 and 710 m, an area of *0.35 km2, and a perimeter of *2.15 km2 (Isaia et al. 2015). The crater is bounded by a ring of pyroclastic products with a maximum thickness of *30 m. The diameter of the Solfatara cone varies between 1.1 and 1.5 km (Isaia et al. 2015). The well-stratified pyroclastic deposits of Solfatara volcano have primary slope from subhorizontal to gently dipping, up to 25°.

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Summary of the Campi Flegrei Geology

Fig. 5 Geological map of the Solfatara area. From Marotta et al. (2019), which is licensed under a Creative Commons Attribution 4.0 CC BY International License

They are interposed between the lavas of the Accademia lava dome and the pyroclastic deposits of the Astroni volcano (Fig. 5) and have a limited distribution area (Rosi and Sbrana 1987; Cipriani et al. 2008; Isaia et al. 2015). According to Rosi and Sbrana (1987), the Solfatara tephra comprise a basal phreatomagmatic coarse breccia overlain by few meters of stratified pyroclastic surges. The breccia is very poorly sorted and constituted by a fine-grained matrix containing large blocks of green tuff, lavas, and dispersed scoriaceous bombs. The whole breccia deposit is affected by hydrothermal alteration and is weakly hardened. The

pyroclastic surges are made up of layers of pisolitic ashes and coarse ashes with base surge structure enclosing some decametric beds of well sorted air-fall pumice lapilli. These surges mantle the eastern-northeastern crater wall and have overflowed the crater rim to the East and West. The juvenile fraction of the Solfatara pyroclastic products has trachyte composition. Based on the volatile content of melt inclusions and using a suitable solubility model, Cipriani et al. (2008) proposed that magma crystallized at pressures between 200 and 685 bar, corresponding to depths between 1 and 3 km, before the eruption.

1

The Solfatara Volcano

The main tectonic elements affecting the Solfatara area and controlling its crater morphology are relatable to the so-called Apenninic and anti-Apenninic trends, striking WNW and ENE, respectively (Rosi and Sbrana 1987). According to Isaia et al. (2015), morphological, stratigraphic, structural, and geoelectrical data indicate that the Solfatara tephra ring is connected with a maar-diatreme structure of phreatomagmatic origin extending to depths of 2–3 km; the upper part is made up of disaggregated rocks and collapse breccias forming a stairstep-like structure which is cut by steep concentric ring faults. As already mentioned above, the Solfatara monogenetic edifice formed during the recent phase of post-caldera subaerial activity which begun *4,000 years ago and is covered by the *3,700 years old deposits of Astroni volcano (Rosi and Sbrana 1987). This post-caldera subaerial phase of volcanic activity was heralded by a tectonic event that occurred 5,400 years ago and caused the uplift of the northern sector of the Gulf of Pozzuoli, as testified by the Starza marine terrace (Rosi et al. 1983). This uplift might have been controlled by the injection of new magma at shallow depth, although this is only a conjecture. At the end of 1500—beginning of 1600, the two chroniclers Giulio Cesare Capaccio and Scipione Mazzella reported that strong earthquakes were felt in the Neapolitan area and an eruption took place at Solfatara in 1198, that is, about four centuries earlier than the publication of these news (Scandone et al. 2010). The absence of bibliographic sources contemporary or closer to the event and, perhaps even more so, the lack of both corresponding morphological alterations in the Solfatara crater and products emitted during the eruption suggest that the two chroniclers mentioned above are scarcely reliable or completely unreliable as underscored by several authors. Some authors have questioned the authenticity of these historical sources, but have not excluded the occurrence of the 1198 eruption (e.g., Scacchi 1849; Mercalli 1883; Rittmann 1929), other authors have admitted that it was, at most, a minor phreatic (hydrothermal) explosion

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(Scandone et al. 2010), while still other authors have totally ruled out it (e.g., Sicardi 1944, 1956; Guidoboni 2010). Based on a diplomatic letter written in October 1498 and reporting that several earthquakes were felt in Naples and in the Campi Flegrei at that time, Guidoboni (2010) suggested a possible transcription error from MCDXCVIII (which is 1498 in Roman numbers) to MCXCVIII (which is 1198 in Roman numbers), that is a missing D, corresponding to 500 years. On the other hand, it is possible that the fumarolic activity of the Solfatara intensified following the earthquake of magnitude 7.1 that occurred in 1456 in the Neapolitan area and caused about 30,000 casualties. This event may have prompted the chroniclers of the time to create the phantom eruption of 1198, when another important seismic activity took place in the area. As anticipated in Chap. “Historical Data”, historical sources suggest that fumarolic activity inside the Solfatara crater is probably occurring since Roman times. This long-lasting and intense degassing process has determined the continuous production of relatively large amounts of steam condensates at shallow depths practically consisting of sulfuric acid solutions. These acidic solutions attacks the rocks causing the formations of authigenic minerals, mainly elemental sulfur and alunite, as well as gypsum, alunogen, pickeringite, potassium alum, hematite, and pyrite. These minerals typical of the so-called advanced argillic alteration (Pirajno 2009) were encountered in the fumarolic encrustations and in the muds of the hot pools at Solfatara and Pisciarelli (Piochi et al. 2015). Different clay minerals and amorphous silica are also formed by this pervasive water–rock interaction process, converting most of the rocks into mud. As a result, the internal crater wall was repeatedly interested by landslides which produced several debris flow deposits (Isaia et al. 2021) and exposed the pre-Solfatara volcanic substratum in the crater wall (Fig. 6). The occurrence of all these processes is very common in areas affected by fumarolic activity, but the debris flows related to landslides should not be confused with the debris flow deposits emplaced by hydrothermal

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Summary of the Campi Flegrei Geology

Fig. 6 Aerial view of the Solfatara Crater. Reprinted from Scandone et al. (2010), Copyright © 2010, with permission from Elsevier

(phreatic) events (Barberi et al. 1992; Marini et al. 1993) or magmatic eruptions such as that of Ruiz volcano, Colombia (Takahashi 2007). To the best of our knowledge, no debris flow deposits related to hydrothermal eruptions were recognized at Solfatara volcano so far. As a matter of fact, one of the most energetic historical activity at Solfatara was triggered by the M 6.4 Irpinia earthquake of 23 July 1930 and was an event of mud fountains that reached maximum heights of 10–15 m above the crater rim (Signore 1929). Nevertheless, we do not intend to exclude the possible occurrence, in the future, of a hydrothermal eruption triggered by an earthquake, which appears a realistic scenario considering the current over-pressurization state at depth of the Solfatara magmatic-hydrothermal system (see Chap. “Conclusive Remarks”).

2

The Bradyseism in the Campi Flegrei

Vertical ground movements known as bradyseism, comprising cycles of uplift (inflation) and subsidence (deflation), have affected the Campi Flegrei caldera since at least Roman times (Fig. 7; Parascandola 1947). Actually, the bradyseismic uplift (inflation) should be termed caldera resurgence, whereas the bradyseismic subsidence (deflation) should be called caldera subsidence, to be consistent with the modern volcanological terminology and the processes observed at other volcanoes worldwide. Antonio Niccolini was the first to recognize the phenomenon and to describe it (Niccolini 1829, 1839, 1846), thanks to the excavation,

2

The Bradyseism in the Campi Flegrei

17

Fig. 7 Elevation changes of the Serapeum floor according to Parascandola (1947, redrawn). The green and red lines indicate caldera subsidence and resurgence, respectively

begun in 1750, of the ruins of the ancient roman market known as Serapeum of Pozzuoli (Fig. 8). Niccolini was commissioned in June 1824 to drain these ruins from the stagnant waters, but he realized that this operation was impossible because sea level was gradually rising, flooding the edifice. Moreover, Niccolini recognized the

occurrence of an earlier marine submersion as testified by the holes made by the marine bivalves Lithodomus lithophagus (Linnaeus 1758) on the marble columns of the Roman edifice at heights of 6.30 ± 0.03 m above the second floor of the market and 8.40 ± 0.15 m above the first floor (Del Gaudio et al. 2010). The

Fig. 8 Engraving showing the Serapeum of Pozzuoli in 1845 (from Niccolini 1846)

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bradyseism phenomenon was later made famous and attributed to ground movements instead of sea level variations by Charles Lyell in his “Principles of Geology” (Lyell 1850). Two other unrest episodes are well documented. The first one preceded by several years the 1538 Monte Nuovo eruption, affected the whole Campi Flegrei caldera that raised by *17 m in its center at Pozzuoli, where earthquakes sometimes were strong enough to collapse buildings (Di Vito et al. 1987; Dvorak and Gasparini 1991; Guidoboni and Ciuccarelli 2011; Giacomelli and Scandone 2012). In 1470–1472, the increased gas emission from Solfatara and Pisciarelli killed the vegetation in the nearby areas (Guidoboni and Ciuccarelli 2011). The frequency of the earthquakes felt by Pozzuoli inhabitants increased during the two years preceding the eruption. In the 36 h before the eruption, the vent site was affected by a sudden uplift of *6 m (Parascandola 1947; Dvorak and Gasparini 1991; Guidoboni and Ciuccarelli 2011). Then, the Monte Nuovo eruption began on the 29th of September, lasted a week, and consisted in a small phreatomagmatic event that built the homonymous cone (Parascandola 1947; Guidoboni and Ciuccarelli 2011). The second unrest episode begun in 1950 and is still ongoing. The deformation has been accompanied by seismicity and variations in the chemical composition and flowrate of the fumarolic fluids emitted at Solfatara and Pisciarelli. The magnitude of the recent vertical ground movements is largely based on geodetic observations, that began in 1905 (Fig. 9; Del Gaudio et al. 2010), and on GPS monitoring since 2000 (De Martino et al. 2014). Based on the geodetic data, it seems likely that a continuous subsidence took place after the Monte Nuovo eruption, possibly until 1950, when the phenomenon inverted with a maximum uplift of 0.73 m in 1952. After a period of *17 years in which a small deflation took place, a new inflation episode with moderate seismicity (M < 2.5) occurred between 1969 and 1972 resulting in a maximum uplift of 1.77 m. It was followed by a period of minor deflation lasting

Summary of the Campi Flegrei Geology

*10 years and by another period of inflation from 1982 to 1984 with maximum uplift of 1.79 m at the Pozzuoli pier. This time an intense seismicity, with more than 15,000 earthquakes of magnitude up to 4.2 and depth of most hypocenters shallower than 3 km was recorded in the area encompassing the Solfatara crater, the town of Pozzuoli, and the Pozzuoli Bay (Corrado et al. 1977; Barberi et al. 1984; Aster and Meyer 1988), where the uplift gradient was maximum (Berrino et al. 1984). According to Aster and Meyer (1988), 10% of the 228 hypocenters are below the Gulf of Pozzuoli at depths between 3.5 and 5.0 km, along a steeply dipping plane, whereas 90% of the hypocenters are onshore, are found at depths of 1.8–4.0 km (apart from few exceptions), and are centered close to the Solfatara crater, about 1 km NE of the Pozzuoli pier. After the important uplift phase of 1982– 1984, a process of slow subsidence began in 1985, with minor uplift episodes of short duration in 1989 and 2000, while a temporary arrest of the subsidence took place in 1994. Since 2004, the general trend inverted and a continuous uplift has been recorded, in spite of the variable uplift rate. The vertical ground movement has been accompanied by modest seismicity, represented by swarms of small-magnitude earthquakes. As shown by Fig. 10, the uplift phase is still in progress, with an increase in elevation of *0.8 m in June 2021 with respect to 2004 at the GPS RITE station, based on the monthly Surveillance Bulletin of June 2021 (INGV 2021). Moreover, the overall horizontal displacements recorded by the GPS network of 25 stations during the period January 2016–June 2021 indicate that the source of deformation is located in the Gulf of Pozzuoli a few hundred meters south of the RITE station and that the whole Campi Flegrei caldera seems to be affected by this phenomenon, including the northernmost QUAR station, whereas nil to negligible displacements are recorded at the stations positioned outside the Campi Flegrei caldera, that is, LICO to the west and FRUL, MAFE and NAMM to the east (Fig. 11). Similar inferences on the stability of the ground deformation source and on the

2

The Bradyseism in the Campi Flegrei

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Fig. 9 Elevation changes of the Serapeum floor and benchmark BM 25A (Datum Point) from 1905 to 2009 referred to the sea level in 1905. Reprinted from Del Gaudio et al. (2010), Copyright © 2010, with permission from Elsevier

Fig. 10 Time series of the weekly changes in elevation at the GPS RITE station (Pozzuoli—Rione Terra) from 2000 to June 2021. From INGV (2021), which is licensed under

a Creative Commons Attribution 4.0 CC BY International License

effects of the bradyseism over the whole Campi Flegrei caldera were previously drawn by De Martino et al. (2014) for the period 2000–2013. In recent years, seismicity was recorded more or less in the same area and in the same depth range of the 1982–1984 uplift phase (e.g., Castaldo et al. 2019 and references therein; see Sect. 2.3 in Chap. “The Magmatic–hydrothermal

System Hosted in the Campi Flegrei Caldera with Emphasis on the Solfatara”), in spite of the significantly lower magnitude of the earthquakes. For instance, in June 2021, the most energetic event, recorded on June 19, 2021 at 18:27 UTC, had Mdmax = 1.7 ± 0.3 (INGV 2021). Summing up, the only documented uplift episode caused by the emplacement of new

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Summary of the Campi Flegrei Geology

Fig. 11 Map of the horizontal GPS displacements recorded in the Campi Flegrei area from January 2016 to June 2021. From INGV (2021), which is licensed under a Creative Commons Attribution 4.0 CC BY International License

magma at shallow depth and preceding a volcanic eruption is that of Monte Nuovo in 1538 CE. In contrast, the magmatic control on the Starza uplift is hypothetical as already underscored above. Therefore, it is not surprisingly the lack of consensus on the processes controlling the 1982–1984 caldera resurgence and the ongoing uplift since 2004, in that some authors proposed the involvement of a magmatic input or the pressurization of the magma chamber, other authors suggested the pressurization of the overlying hydrothermal system, whereas a combination of both factors was invoked in other studies. In spite of this still open debate in the scientific community, the last hypothesis is considered to be the most convincing one by Todesco and coworkers, based on coupled thermohydro-mechanical simulations of the whole

hydrothermal-magmatic system (Chiodini et al. 2003; Todesco et al. 2004; Todesco 2008, 2009). According to these models: (i) the variable behavior of the hydrothermal system appears to be controlled by the varying intensity of magmatic degassing, although peaks in CO2/H2O ratio in fumarolic fluids discharged at the surface are observed several months after the magmatic degassing pulses and (ii) periods of high injection rate of magmatic fluids into the overlying hydrothermal system can drive significant amounts of rock deformation, consisting of a quick caldera resurgence phase followed by a much slower caldera subsidence phase. An updated discussion of the different models which have been proposed to explain the Campi Flegrei bradyseism is given by Cannatelli et al. (2020), Smale (2020), and Lima et al. (2021).

2

The Bradyseism in the Campi Flegrei

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21 reconstruction of vertical ground movements during 1905–2009. J Volcanol Geotherm Res 195(1):48–56 Di Girolamo P, Ghiara MR, Lirer L, Munno R, Rolandi G, Stanzione D (1984) Vulcanologia e petrologia dei Campi Flegrei. Bollettino della Società Geologica Italiana 103(2):349–413 Di Vito M, Lirer L, Mastrolorenzo G, Rolandi G (1987) The 1538 Monte Nuovo eruption (Campi Flegrei, Italy). Bull Volcanol 49(4):608–615 Dvorak JJ, Gasparini P (1991) History of earthquakes and vertical ground movement in Campi Flegrei caldera, southern Italy: comparison of precursory events to the AD 1538 eruption of Monte Nuovo and of activity since 1968. J Volcanol Geotherm Res 48:77–92 Giaccio B, Hajdas I, Isaia R, Deino A, Nomade S (2017) High-precision 14C and 40Ar/39Ar dating of the Campanian Ignimbrite (Y-5) reconciles the timescales of climatic-cultural processes at 40 ka. Sci Rep 7(1):1–10 Giacomelli L, Scandone R (2012) History of the exploitation of thermo-mineral resources in Campi Flegrei and Ischia, Italy. J Volcanol Geotherm Res 209:19–32 Guidoboni E (2010) History and volcanology: dialogue overdue? The case of a false eruption in medieval Italy. EOS Trans Am Geophys Union 91(26):231–231 Guidoboni E, Ciuccarelli C (2011) The Campi Flegrei caldera: historical revision and new data on seismic crises, bradyseisms, the Monte Nuovo eruption and ensuing earthquakes (twelfth century 1582 AD). Bull Volcanol 73(6):655–677 INGV (2021) Bollettino di Sorveglianza. Campi Flegrei. Giugno 2021, p 31. https://www.ov.ingv.it/ov/ bollettini-mensili-campania/Bollettino_Mensile_ Campi_Flegrei_2021_06.pdf. Accessed 4 Aug 2021 Isaia R, Vitale S, Di Giuseppe MG, Iannuzzi E, D’Assisi TF, Troiano A (2015) Stratigraphy, structure, and volcano-tectonic evolution of Solfatara maar-diatreme (Campi Flegrei, Italy). Geol Soc Am Bull 127(9– 10):1485–1504 Isaia R, Di Giuseppe MG, Natale J, Tramparulo FDA, Troiano A, Vitale S (2021) Volcano-tectonic setting of the Pisciarelli fumarole field, Campi Flegrei caldera, Southern Italy: insights into fluid circulation patterns and hazard scenarios. Tectonics 40(5): e2020TC006227 Lima A, Bodnar RJ, De Vivo B, Spera FJ, Belkin HE (2021) Interpretation of recent unrest events (bradyseism) at Campi Flegrei, Napoli (Italy): comparison of models based on cyclical hydrothermal events versus shallow magmatic intrusive events. Geofluids 16. Article ID 2000255. https://doi.org/10.1155/2021/ 2000255 Linnaeus C (1758) Systema Naturae per Regna Tria Naturae, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Holmiae, Imprensis Laurentii Salvii Lirer L, Luongo G, Scandone R (1987) On the volcanological evolution of Campi Flegrei. EOS Trans Am Geophys Union 68(16):226–234

22 Lyell C (1850) Principles of geology. J. Murray, London, pp 489–498 Marini L, Principe C, Chiodini G, Cioni R, Fytikas M, Marinelli G (1993) Hydrothermal eruptions of Nisyros (Dodecanese, Greece). Past events and present hazard. J Volcanol Geotherm Res 56:71–94 Marotta E, Peluso R, Avino R, Belviso P, Caliro S, Carandente A, Chiodini G, Macedonio G, Avvisati G, Marfè B (2019) Thermal energy release measurement with thermal camera: the case of la Solfatara Volcano (Italy). Remote Sens 11(2):167 Mercalli G (1883) Vulcani e fenomeni vulcanici in Italia. Arnaldo Forni Editore, Bologna, p 374 Niccolini A (1829) Rapporto sulle acque che invadono il pavimento dell’antico edifizio detto il Tempio di Giove Serapide. Stamperia Reale, Napoli, p 46 Niccolini A (1839) Tavola metrica-cronologica delle varie altezze tracciate dalla superficie del mare fra la costa di Amalfi ed il promontorio di Gaeta nel corso di diciannove secoli. Tipografia Flautina, Napoli, p 52 Niccolini A (1846) Descrizione della Gran Terma Puteolana volgarmente detta Tempio di Serapide preceduta da taluni cenni storici per servire alla dilucidazione de’ fenomeni geologici, e de’ problemi architettonici di quel celebre monumento e considerazioni su i laghi Maremmani. Stamperia Reale, Napoli, p 257 Orsi G, De Vita S, Di Vito M (1996) The restless, resurgent Campi Flegrei nested caldera (Italy): constraints on its evolution and configuration. J Volcanol Geotherm Res 74(3–4):179–214 Parascandola A (1947) I fenomeni bradisismici del Serapeo di Pozzuoli. Stabilmento tipografico G. Genovese, Napoli, p 156 Piochi M, Mormone A, Balassone G, Strauss H, Troise C, De Natale G (2015) Native sulfur, sulfates and sulfides from the active Campi Flegrei volcano (southern Italy): genetic environments and degassing dynamics revealed by mineralogy and isotope geochemistry. J Volcanol Geotherm Res 304:180–193 Pirajno F (2009) Hydrothermal processes and mineral systems. Springer, Berlin, Heidelberg, p 1250 Rittman A (1950) Sintesi geologica dei Campi Flegrei. Bollettino della Società Geologica Italiana 69(2):117– 128 Rittmann A (1929) Die Solfatara. Naturwissenschaften 17 (34):659–663 Rosi M, Sbrana A, Principe C (1983) The Phlegraean fields: structural evolution, volcanic history and eruptive mechanisms. J Volcanol Geotherm Res 17(1– 4):273–288 Rosi M, Sbrana A (1987) Stratigraphy. In: Rosi M, Sbrana A (eds) Phlegrean fields. Quaderni de La Ricerca Scientifica 9(114):10–39 Sbrana A, Marianelli P, Pasquini G (2021) The Phlegrean fields volcanological evolution. J Maps 17(2):545– 558. https://doi.org/10.1080/17445647.2021.1982033 Scacchi A (1849) Memorie Geologiche sulla Campania e relazione dell’incendio accaduto nel Vesuvio nel mese

Summary of the Campi Flegrei Geology di Febbrajo del 1850. Estratte dal Rendiconto della R. Accademia delle Scienze di Napoli, p 48 Scandone R, Bellucci F, Lirer L, Rolandi G (1991) The structure of the Campanian Plain and the activity of the Neapolitan volcanoes (Italy). J Volcanol Geotherm Res 48(1–2):1–31 Scandone R, D’Amato J, Giacomelli L (2010) The relevance of the 1198 eruption of Solfatara in the Phlegraean Fields (Campi Flegrei) as revealed by medieval manuscripts and historical sources. J Volcanol Geotherm Res 189(1–2):202–206 Scarpati C, Sparice D, Perrotta A (2014) A crystal concentration method for calculating ignimbrite volume from distal ash-fall deposits and a reappraisal of the magnitude of the Campanian Ignimbrite. J Volcanol Geotherm Res 280:67–75 Scarpati C, Cole P, Perrotta A (1993) The Neapolitan Yellow Tuff. A large volume multiphase eruption from Campi Flegrei, southern Italy. Bull Volcanol 55 (5):343–356 Sicardi L (1944) L’attività della Solfatara di Pozzuoli attraverso la documentazione storica avanti l’ultimo ottantennio. Atti della Società Italiana di Scienze Naturali e del Museo Civico di Storia Naturale in Milano 83(2):97–114 Sicardi L (1956) La Solfatara di Pozzuoli. Bulletin Volcanologique 18(1):151–158 Signore F (1929) Sur la variation d’activite du volcan de Boue (“Fangaia”) de la Solfatare de Pouzzoles (Naples) par suite du grand tremblement de terre de l’Irpinia, 23 Juillet 1930. Bulletin Volcanologique 3 (2):49–52 Smale L (2020) A re-interpretation of long-term deformation at Campi Flegrei caldera, Italy and perceptions of the causes of caldera unrest. PhD thesis, Department of Earth Sciences, University College London, p 340 Smith VC, Isaia R, Pearce NJG (2011) Tephrostratigraphy and glass compositions of post-15 kyr Campi Flegrei eruptions: implications for eruption history and chronostratigraphic markers. Quat Sci Rev 30(25– 26):3638–3660 Takahashi T (2007) Debris flow: mechanics, prediction and countermeasures. Taylor and Francis. London 448 pp Todesco M (2009) Signals from the Campi Flegrei hydrothermal system: role of a “magmatic” source of fluids. J Geophys Res Solid Earth 114(B5):B05201. https://doi.org/10.1029/2008JB006134 Todesco M, Rutqvist J, Chiodini G, Pruess K, Oldenburg CM (2004) Modeling of recent volcanic episodes at Phlegrean fields (Italy): geochemical variations and ground deformation. Geothermics 33(4):531–547 Todesco M (2008) Hydrothermal fluid circulation and its effect on caldera unrest. In: Gottsmann J, Marti J (eds) Caldera volcanism: analysis, modelling and response. Dev Volcanol 10:393–416

The Magmatic–Hydrothermal System Hosted in the Campi Flegrei Caldera with Emphasis on the Solfatara

Abstract

The available information relevant to reconstruct the conceptual models of both the magmatic–hydrothermal system hosted in the Campi Flegrei caldera and the Solfatara magmatic-hydrothermal system was summarized and reviewed. This information includes (but is not limited to): the main characteristics of the deep geothermal wells drilled in the area, the hydrothermal alteration mineralogy and the geochemistry of the fluids encountered by the AGIP-ENEL geothermal wells, the tomographic modeling of active and passive seismic data, the chemical and isotopic characteristics of onshore thermal springs and shallow wells, onshore fumaroles and gas vents, offshore fumaroles and gas vents, and sub-lacustrine hydrothermal discharges. Taking into account the general conceptual model of volcano-hosted magmatic-hydrothermal systems (Fournier, 1999) and adopting the methodology commonly used by geothermal scientists (Cumming 2009, 2016), the conceptual model of the magmatic-hydrothermal system hosted in the Campi Flegrei caldera was reconstructed. The local stratigraphy comprises: (a) water-saturated volcanic products above 0.6 km depth; (b) volcanic and marine deposits extensively affected by propylitic, phyllitic, and argillic hydrothermal alteration, between 0.6 and 2.7 km depths; (c) a layer of thermometamorphic rocks at depths

between 2.7 and 4 km approximately; (d) Mesozoic carbonate rocks (such as those cropping out around the Campanian Plain) and the crystalline bedrock from ca. 4 to ca. 7.5 km depths (e) a magma reservoir whose top is situated at ca. 7.5 km depth. Levels (c) and (d) host over-pressurized gases and hypersaline brines that are expelled from the crystallizing magma and CO2-rich gases produced in-situ by different reactions. Actually, CO2rich fluids were discharged by well San Vito 1. Thus, levels (c) and (d) are the “engine” governing the bradyseism as proposed by several authors. Levels (b) and (c) are separated by a relatively impermeable zone produced by self-sealing, mainly quartz deposition, at temperature close to 400°C (Fournier 1999). Actually, this quartz-bearing zone was encountered in well San Vito 1, at depths between ca. 2.5 and 2.8 km and temperatures of ca. 360 to 385°C. This quartz-rich zone represents the boundary between the underlying deep-magmatic portion of the magmatic-hydrothermal system, where fluid pressure is controlled by the lithostatic regime, and the overlying shallow-hydrothermal portion of the magmatic-hydrothermal system, where fluid pressure is governed by the hydrostatic regime (Fournier 1999). In the shallow-hydrothermal portion, the circulation of hydrothermal fluids of marine origin (at least for the most) is restricted to the brittle rocks of the propylitic zone, where

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 L. Marini et al., The Solfatara Magmatic-Hydrothermal System, Advances in Volcanology, https://doi.org/10.1007/978-3-030-98471-7_3

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The Magmatic–Hydrothermal System Hosted in the Campi Flegrei …

permeability is fracture-controlled, whereas the overlying impermeable, plastic rocks of the phyllitic and argillic zones act as the clay cap of the geothermal system. Similar to the basinal-type geothermal systems, the extension of the Campi Flegrei shallow geothermal reservoir is marked, to a first approximation, by the neutral chloride thermal waters emerging at the surface, or intercepted by shallow wells, close to the coastline. However, the onshore thermal manifestations (both thermal waters and gas emissions) cluster in the two distinct areas of Mofete and Solfatara-Agnano. Hydrothermal circulation might be restricted, almost completely, to these two areas, which would host two different, comparatively small hydrothermal systems. Alternatively (and this is our preferred interpretation), the two areas of Mofete and Solfatara-Agnano mark two separate upflows of a unique, relatively large hydrothermal system, extending between them, southwards (as indicated the offshore gas vents) and northwards (as proven by the deep boreholes of San Vito). Finally, the two conceptual models of the Solfatara magmatic-hydrothermal system proposed by Cioni et al. (1984) and Caliro et al. (2007) are recalled. Both conceptual models are realistic, in that they describe the system of interest during the 1982-1984 bradyseismic crisis and after its cessation, respectively.∎∎∎ The purpose of this chapter is twofold. First, the present state of knowledge of the magmatic–hydrothermal system hosted in the Campi Flegrei caldera, with emphasis on the Solfatara, is summarized and briefly discussed, thus building on what was presented in the previous chapter. Second, the conceptual models of the Solfatara magmatic–hydrothermal system proposed in relevant studies are examined and discussed. Unfortunately, it was not possible to examine all the pertinent scientific works due to their considerable number and their rapid growth with time. Therefore, the most relevant works were necessarily selected, taking into account both the general conceptual model of volcano-hosted magmatic–hydrothermal systems (Fournier

1999) and the methodology commonly adopted by geothermal scientists to reconstruct the conceptual models of these systems (Cumming 2009, 2016). According to the general conceptual model of Fournier (1999), volcano-hosted magmatic–hydrothermal systems comprise (Fig. 1a): (i) a lower part, where hypersaline brines and gases exsolved from the underlying crystallizing magma accumulate at lithostatic pressure within a volume of plastic rocks, and (ii) an upper part, where meteoric-derived hydrothermal fluids circulate through brittle rocks at hydrostatic pressure. As discussed in Sect. 1.3, marine waters may circulate into the upper part of the system at hydrostatic pressure, instead of meteoric waters or in addition to them, in islands and coastal areas, as is the case of Campi Flegrei. The two parts are separated by a thin, relatively impermeable zone produced through self-sealing (mainly quartz deposition1), where temperature is usually close to 400 °C. From time to time, this self-sealed zone is breached by an upward surge in magma or other mechanisms causing the quick expulsion of hypersaline brines and gases from the usually plastic region into the brittle domain, at lower pressure and temperature (Fig. 1b). The ensuing increase in fluid pressure and temperature within the brittle domain leads to faulting and fracturing, which increase permeability and the rate of discharge of magmatic–hydrothermal fluids. The general conceptual model of volcanohosted magmatic–hydrothermal systems proposed by Fournier (1999) was modified and applied to the Campi Flegrei by Lima et al. (2021) and references therein. Nevertheless, there is no reason and no evidence supporting the presence of the low-permeability layer B postulated by Lima et al. (2021). The methodology of Cumming (2009, 2016) is focused on the upper portion of volcano-hosted 1

The reduction in permeability close to an intrusive heat source caused by quartz precipitation was discussed by several authors (e.g., Fournier 1985, 1999; Wells and Ghiorso 1991; Lowell et al. 1993; Moore et al. 1994; White and Mroczek 1998; Saishu et al. 2014; Scott and Driesner 2018).

The Magmatic–Hydrothermal System Hosted in the Campi Flegrei …

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Fig. 1 Schematic conceptual model of a volcano-hosted magmatic–hydrothermal system, where the top of the crystallizing magma is at depths ranging from 1 to 3 km, and its change in state. a The brittle to plastic transition (BPT) occurs at ca. 370–400 °C and dilute, prevailingly meteoric waters circulate at hydrostatic pressure in brittle rocks above the BPT, while dominantly magmatic gases and highly saline brines accumulate in plastic rocks at lithostatic pressure, below the BPT. b Episodic and temporary breaching of the usually selfsealed zone allows magmatic fluids to escape into the overlying hydrothermal system. Reprinted from Fournier (1999), Copyright © 1999, with Fair Use Permission from the Society of Economic Geologists

magmatic–hydrothermal systems, which is that of interest for their industrial exploitation and, therefore, is investigated by means of surface exploration techniques, first, and deep drilling, afterwards. Thus, the works of Cumming (2009,

2016) provide the guidelines for the reconstruction of the conceptual models through the processing of the available data, usually including: (i) maps of surface geology also showing tectonic and volcano-tectonic structures, (ii) images

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The Magmatic–Hydrothermal System Hosted in the Campi Flegrei …

of the low resistivity clay cap derived from magnetotelluric (MT) surveys, and (iii) the geochemistry of hot springs and fumaroles. Following the general conceptual model of volcano-hosted magmatic–hydrothermal systems of Fournier (1999), the upper “shallowhydrothermal” portion of the Campi Flegrei magmatic–hydrothermal system, where fluid pore pressure probably follow the hydrostatic regime, and the lower “deep-magmatic” portion of the system, in which fluid pore pressures is probably close to the lithostatic regime, are considered in two distinct sections. To be noted that thermometamorphic reactions occur in the “deepmagmatic” sector of the Campi Flegrei magmatic– hydrothermal system, generating important amounts of different gas species, as discussed below.

1

The Shallow-Hydrothermal Portion of the Magmatic– Hydrothermal System Hosted in the Campi Flegrei Caldera

Mesozoic–Cenozoic carbonate basement cropping out in the Apennine thrust belt all around the Campanian plain. The knowledge of the hydrothermal alteration mineralogy allows one to distinguish the clay cap from the hydrothermal reservoir and to gain indications on the P–T–X conditions that were present when the hydrothermal minerals were deposited (see Sect. 1.2). To follow a logical order, the main features of the deep geothermal wells drilled in the Campi Flegrei caldera are summarized in the next section.

1.1 Main Characteristics of the Deep Geothermal Wells Drilled in the Campi Flegrei Caldera Several boreholes were drilled from 1939 to 1943 and from 1953 to 1955 by SAFEN at Mofete, Monte Nuovo, Agnano–Solfatara, and Licola (Penta 1949) and from 1978 to 1985 by AGIP-ENEL at Mofete, San Vito, and Licola (Carella and Guglielminetti 1983; Guglielminetti 1986; Chelini and Sbrana 1987). The location of the geothermal wells drilled at depth >500 m by SAFEN and of the deep geothermal wells drilled by AGIP-ENEL is shown in the Google Earth map of Fig. 3. Among the wells drilled by SAFEN, the following four reached depths >500 m below ground level:

Referring the dataset usually available for reconstructing the conceptual models of volcanohosted geothermal systems, the bad news is the lack of the MT data over most of the Campi Flegrei caldera, although this gap was partly filled for the Solfatara crater and nearby areas (see Sect. 1.2), while the good news is the detailed knowledge of both the local stratigraphic (a) Well CF23 (also called Agnano 1), which was drilled on the eastern slopes of the Solsequence and the hydrothermal alteration minfatara relief and reached a total depth of eralogy thanks to the data acquired through the 1841 m, where a maximum temperature of deep geothermal wells drilled by AGIP-ENEL at 325 °C was measured (Minucci 1964). Two depths of 1600–3000 m below ground level in aquifer zones were encountered by the well the areas of Mofete and San Vito. at 110 and 1445 m depth. Under optimum As shown by the N–S geological cross-section conditions, the well section from 1560 to across the Campi Flegrei and Pozzuoli Bay 1712 m depth (i.e., the deep aquifer zone) (Fig. 2; from Lima et al. 2009), the local stratidischarged a total flow rate decreasing from graphic sequence comprises coarse-grained to 4481 to 4249 kg/h, with a concurrent fine-grained clastic sediments, and different voldecrease in the mass steam fraction from canic and volcano-clastic rocks overlying the

1

The Shallow-Hydrothermal Portion …

Fig. 2 N–S geological cross-section across the Campi Flegrei and Pozzuoli Bay. Legend: 1. Holocene volcanics; 2. Neapolitan Yellow Tuff; 3. Main sediments post 39 ka; 4. Campanian Ignimbrite and previous tuffs; 5. Middle Pleistocene sandstones, siltstones and volcanics; 6. Middle Pleistocene marine sediments (sandstones and siltstones); 7. Fine-grained Middle Pleistocene marine sediments (claystones and siltstones); 8. Middle Pleistocene deep water debris flows; 9. Lower Pleistocene

27

marine sediments; 10. Continental deposits and conglomerates; 11. Meso-Cenozoic carbonate substrate; 12. Crystallized magma; 13. Volcanic bodies; 14. Magma body; 15. Thermometamorphic boundary; 16. Impermeable zone surrounding the crystallizing magma body; 17. Pozzuoli Anticline; 18. Pozzuoli Bay Syncline; 19. 1983– 84 earthquake hypocenters; 20. Deep geothermal wells. Reprinted from Lima et al. (2009), modified, Copyright © 2009, with permission from Elsevier

Fig. 3 Location map of the geothermal wells drilled at depth >500 m by SAFEN (yellow pins) and the deep geothermal wells drilled by AGIP-ENEL (magenta pins)

0.539 to 0.396. Assuming that these values refer to atmospheric conditions, the total discharge enthalpy decreased from 1636 to 1312 kJ/kg, indicating enthalpytemperatures of 345 to 294 °C if a single

liquid is considered to be present in reservoir. These enthalpy temperatures 20 °C higher and 31 °C lower than maximum measured temperature of 325 respectively.

the are the °C,

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The Magmatic–Hydrothermal System Hosted in the Campi Flegrei …

(b) Well CF 21 at Mofete, which was drilled to a total depth of 1218 m and found a reservoir temperature of 233 °C (Chelini et al. 1982). (c) Well CLV 7 at Mofete, which reached a total depth of 584.9 m, where a maximum temperature of 225 °C was measured. The borehole discharged a considerable flow of water and steam for 15 months (Penta 1949). (d) Well CLV 17 at Mofete, which attained a total depth of 521.7 m, where a maximum temperature of 225 °C was determined. The borehole discharged a flow of water and steam (Penta 1949). (e) Well CMV 1 at Monte Nuovo, which reached a total depth of 676.9 m, but measured temperatures oscillated from 62 to 77 °C (possibly 80 °C) from 15 m downwards (Penta 1949). (f) Well CF 22 at Licola, which was drilled to a total depth of 1600 m and encountered a reservoir temperature of 170 °C (Chelini et al. 1982). The main characteristics of the deep geothermal wells drilled by AGIP-ENEL at Mofete are as follows (Carella and Guglielminetti 1983): (i) Well Mofete 1 (MF-1) was drilled vertically to a total depth of 1606 m and encountered two productive zones, the shallower one in the depth interval 550– 896 m at temperature of 247 °C and the deeper one in the depth interval 1223–1469 m at temperature of 295 °C. (ii) Well Mofete 2 (MF-2) was drilled vertically to a total depth of 1989 m and met a productive zone from 1272 to 1989 m depth at temperature of 337–342 °C. (iii) Well Mofete 3d (MF-3d) was drilled vertically until 1749 m and then it was deviated south-westwards until the total depth of 1909 m. It met two productive zones, the shallower one in the depth interval 430–665 m at temperature of 230 °C and the deeper one in the depth interval 1302– 1900 m at temperature of 275 °C.

(iv) Well Mofete 5 (MF-5) was drilled vertically until the total depth of 2700 m. Also this well found two productive zones, the shallower one in the depth interval 1627– 1960 m at temperature of 286 °C and the deeper one in the depth interval 2310–2699 m at temperature of 362 °C. (v) Well Mofete 7d (MF-7d) was drilled vertically until 1476 m and then it was deviated towards ESE until the total depth of 1648 m. It met a productive zone from 1042 to 1646 m depth at temperature of 301 °C. (vi) Well Mofete 8d (MF-8d) was drilled vertically until 800 m and then it was deviated southwards until the total depth of 907 m. It encountered a productive zone from 660 to 907 m depth at temperature of 234 °C. (vii) Well Mofete 9d (MF-9d) was drilled vertically until 1582 m and then it was deviated towards ESE until the total depth of 1745 m. This well has two productive zones, the shallower one in the depth interval 694–1004 m at temperature of 210 °C and the deeper one in the depth interval 1342–1745 m at temperature of 308 °C. All in all, there is undoubtedly an up-doming of the isotherms in the Mofete area, with culmination between wells MF-1 and MF-2, as shown by the cross-section of Fig. 4, whereas the isotherms deepen considerably in well MF-5, which is positioned outside the Campi Flegrei caldera (Chelini and Sbrana 1987). This means that there is an upflow zone of the geothermal system in the Mofete area, consistent with the presence of weak steaming grounds and hydrothermally altered rocks at the surface. Besides, at least three different geothermal aquifers were encountered by the wells drilled in the Mofete area, a shallow one (500–900 m) at temperatures of 210–250 °C, an intermediate one (1800– 2000 m) at temperatures of 295–310 °C, and a

1

The Shallow-Hydrothermal Portion …

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Fig. 4 Distribution of hydrothermal minerals, boundaries between hydrothermal zones (dotted gray lines) and isotherms (solid red lines) in the Mofete area. Depth in meters. The vertical scale is the same as the horizontal scale (from Chelini and Sbrana 1987, redrawn by Smale 2020)

deep one (2500–2700 m) at temperatures of 340–360 °C (Carella and Guglielminetti 1983). The lateral extension of these aquifers is poorly known, but is probably limited, also considering that well MF-2 is connected with an aquifer which was not encountered by all the other wells (Guglielminetti 1986). According to Guglielminetti (1986), the high-temperature Mofete geothermal reservoir is of limited volume, is characterized by a general lack of permeability, and has a power capacity of 10–15 MW. The three boreholes drilled by AGIP-ENEL in the San Vito area, that is, San Vito 1 (SV-1), San Vito 8d (SV-8d), and San Vito 3 (SV-3), are

known less well than those of the Mofete area. The well San Vito 1 was drilled to a total depth of 3046 m, where the temperature was estimated to be higher than 419 °C based on the partial melting of a piece of zinc left at well bottom for a sufficient time (Bruni et al. 1985). Based on the cross-section of Fig. 5 (Chelini and Sbrana 1987), showing the outcomes of wells SV-1 and SV-3, the isotherms are relatively flat in the San Vito area, at least above *1.8 km depth, whereas at greater depths the isotherms deepen moving from well SV-3 (that reached a total depth of 2360 m) to well SV-1. Furthermore, isotherms are considerably lowered in the San

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The Magmatic–Hydrothermal System Hosted in the Campi Flegrei …

Fig. 5 Distribution of hydrothermal minerals, boundaries between hydrothermal zones (gray dashed lines) and isotherms (red solid lines) in the San Vito area. Depth in meters. The vertical scale is the same as the horizontal scale (from Chelini and Sbrana 1987, redrawn)

Vito area, compared to the Mofete area, confirming that there is an upflow zone at Mofete (see above). In spite of this lowering of the isotherms at San Vito, the deepest part of well SV-1 shows a very high geothermal gradient, from 335 °C (Chelini and Sbrana 1987, page 100) or

355 °C (Fig. 5) at 2500 m to over 420 °C at 3040 m (Chelini and Sbrana 1987). The extrapolation of this geothermal gradient of 120 or 157 °C/km at greater depths indicates the occurrence of 957 or 1122 °C at the top of the melt zone positioned at 7.5 km depth (see

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The Shallow-Hydrothermal Portion …

Sect. 2). The circulation of hydrothermal fluids in the San Vito area is poorly known because of the low quality of the data acquired through short-term production tests. Apart from the phreatic aquifer, no other aquifer with an appreciable lateral extension is probably present above 1.8 km depth, corresponding to the bottom of the clay cap (Chelini and Sbrana 1987). The well Licola 1, located outside the Campi Flegrei caldera, encountered temperatures substantially lower than those found inside the caldera, up to a maximum of 236 °C at the total depth of 2.5 km (Chelini 1984). Furthermore, the static temperature profile of this well shows a constant geothermal gradient close to 85 °C/km which is evidently controlled by conductive heat transfer (Wohletz et al. 1999). Carlino et al. (2012) reviewed the geothermal exploration activities carried out in the Campanian volcanic areas of Campi Flegrei, Vesuvio, and Ischia. The outcomes of a 501 m deep borehole drilled in December 2012 in the Bagnoli plain, in the eastern side of the Campi Flegrei caldera, are reported by Carlino et al. (2018) and references therein.

1.2 Hydrothermal Alteration Mineralogy The rocks encountered by deep geothermal boreholes drilled in the Campi Flegrei area are dominated by alkali-trachytic tuffs and tuffites interbedded with subordinate marine sediments (siltites, sandstones, and marls), lava flows, and domes (Chelini and Sbrana 1987). These original lithotypes were extensively affected by the interaction with water and gases at progressively higher temperatures with increasing depth, leading to the destruction of primary, rock-forming minerals and the production of secondary, hydrothermal minerals. Based on the study of the hydrothermal alteration mineralogy found in the deep geothermal wells Mofete 1, Mofete 2, Mofete 5, San Vito 1 and San Vito 3 (Figs. 4 and 5), the following hydrothermal alteration zones (from top to bottom) were recognized by Chelini and Sbrana (1987):

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(i) The argillic zone, which is characterized by abundant montmorillonite (smectite) accompanied by minor illite, chlorite, and low-temperature zeolites. The base of the argillic zone generally coincides with the 150–160 °C isotherm. (ii) The illite–chlorite zone (or phyllitic zone), which is marked by a considerable increase in chlorite and illite as well as by the first appearance of mixed-layer clay minerals. The base of the illite–chlorite zone coincides with the 250 °C isotherm at Mofete and with temperatures of 220– 270 °C in the San Vito area. (iii) The Ca–Al–silicate zone (or propylitic zone). Epidote is the index mineral of this zone, although it begins to appear in the deepest parts of the illite–chlorite zone. Epidote is accompanied by abundant adularia, albite, silica minerals (chiefly quartz), sulfide minerals (pyrite, sphalerite, and pyrrhotite), calcite, and hightemperature zeolites. Chlorite and illite are also present, but in lower amounts compared to the illite–chlorite zone, whereas mixed-layer clay minerals disappear close to the base of the Ca–Al–silicate zone, which coincides with the 325 °C isotherm in the Mofete area and with temperatures of 270–360 °C in the San Vito area. Since the neoformed mineral phases represent more than 60 volume% of the rock, the propylitic alteration causes a strong lithification of the original solid materials and, consequently, a marked increase in both density and P-wave velocity compared to those of the illite–chlorite zone. (iv) The thermometamorphic zone, which is marked by (a) a nearly complete textural re-arrangement, especially in the sedimentary lithotypes, (b) the appearance of amphibole, biotite, diopside, scapolite, and garnet and (c) the persistence of sulfide minerals (decreasing with increasing depth), quartz, and epidote at Mofete but not at San Vito. To be noted that the disappearance of epidote at San Vito is probably due to the high CO2 partial

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The Magmatic–Hydrothermal System Hosted in the Campi Flegrei …

pressure of the fluids hosted in this zone of the hydrothermal reservoir (see below). To be noted also that scapolite is absent in wells Mofete 1 and 2, is scarce in the deepest parts of San Vito wells and is abundant in well Mofete 5, which produced a brine with a TDS close to 200 g/kg (under reservoir conditions) from the depth interval 2310–2700 m, where scapolite occurs. The simplified general formula of scapolite is
 Nax Ca4
x Aly Si12
y O24 ðCl, CO3 Þ assuming that its composition varies between that of the endmembers marialite ½Na4 Al3 Si9 O24 Cl and meionite ½Ca4 Al6 Si6 O24 CO3  (Hassan and Buseck 1988) and neglecting the contribution of other chemical components possibly present (Shaw 1960; Evans et al. 1969). Based on these assumptions, it is possible that CO3-rich scapolite is present at San Vito (as already supposed by Chelini and Sbrana 1987), whereas the occurrence of Cl-rich scapolite in two cores sampled in well Mofete 5 at 1714 and 2695 m depth was confirmed by SEM analyses (Piochi et al. 2015). Taken together, the argillic zone and the underlying phyllitic zone have an extremely low permeability due to the plastic behavior of the phyllosilicate-rich rocks affected by these types of hydrothermal alteration. Therefore, these phyllosilicate-rich lithotypes constitute the impermeable clay cap of the hydrothermal system hosted in the Campi Flegrei caldera. The clay cap extends to depths of 890–1370 m below ground level at Mofete and 1350–1780 m below ground level at San Vito. In contrast, the rocks affected by propylitic and thermometamorphic alteration have brittle behavior, permitting the development of fractures, acting as high-permeability pathways for fluid circulation. However, it is likely that the two zones are not hydraulically connected or not

always hydraulically connected, being separated by a thin, relatively impermeable zone generated through self-sealing (e.g., quartz deposition), at temperature generally close to 400 °C, consistent with the conceptual model of Fournier (1999). Indeed, abundant hydrothermal quartz was found in well San Vito 1, at depths between ca. 2500 and 2800 m and temperatures of ca. 360– 385 °C, at the transition between the Ca–Al– silicate (propylitic) zone (above) and the thermometamorphic zone (below), as shown by Fig. 5. Therefore, it seems likely that two distinct reservoirs occur in the deepest portion of the San Vito 1 well, namely: (a) above, a hydrothermal reservoir characterized by propylitic alteration, where fluid pressure is governed by the hydrostatic regime, and (b) below, a magmatic reservoir marked by thermometamorphic alteration, where fluid pressure is controlled by the lithostatic regime. Although the thermometamorphic zone was found also in the deepest portion of well Mofete 5, measured temperatures turned out to be significantly lower than at San Vito 1, suggesting that this zone has been affected by cooling and the authigenic minerals are reminiscent of a past situation. The depth of the clay cap is poorly known away from the deep wells because the geoelectrical methods adopted in the geothermal exploration carried out by both SAFEN in 1939–1955 and AGIP-ENEL in the ‘70s had a limited penetration, partly due to the obvious logistic limitations in a densely inhabited area (Cassano and La Torre 1987). More recently, some resistivity models were implemented for the Solfatara crater area (Bruno et al. 2007; Byrdina et al. 2014; Isaia et al. 2015; Gresse et al. 2017), along a 1 kmlong profile joining Solfatara and Pisciarelli (Troiano et al. 2014), and along a *5.6 km-long profile connecting the Solfatara—Pisciarelli area and the Agnano crater (Siniscalchi et al. 2019). The latter one was generated through 2-D inversion of audiomagnetotelluric data and showed (Fig. 6):

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Fig. 6 Resistivity model of the Solfatara–Pisciarelli– Agnano area, also showing the local seismicity (at distance