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Hydrothermal Alteration and Associated Phenomena in Active Volcanoes: Mineralogy, Texture...
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Hydrothermal Alteration and Associated Phenomena in Active Volcanoes: Mineralogy, Texture and Isotope Geochemistry

A special issue of Minerals (ISSN 2075-163X). This special issue belongs to the section "Mineral Deposits".

Deadline for manuscript submissions: closed (31 May 2021) | Viewed by 18318

Special Issue Editor

Dr. Monica Piochi

E-Mail Website
Guest Editor
Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Napoli, Osservatorio Vesuviano, Italy
Interests: volcanic systems; magmatic processes; secondary mineralizations; hydrothermal systems; geophysical and petrological data merging; volcano monitoring
Methods and techniques: FT-IR; XRD; SEM; stable isotopes; isotope geochemistry; rock geochemistry

Special Issue Information

Dear Colleagues,

Hydrothermal alteration is the mutual transformation of rocks and infiltrating fluids under dynamic physico-chemical conditions sustained by endogenous heating reservoirs, in an attempt to reach the thermodynamic equilibrium. Over time and space, the circulation of hot aqueous fluids affects the rock voids and fractures and generates hydrothermal systems, the surface expression of which are hot springs, fumaroles, diffuse soil degassing and mud pools, which often are spectacular phenomena of active volcanoes. The host rocks respond to disequilibrium due to aggressive H+ and OH– (and other constituents) of fluids with complex and drastic mineralogical, chemical and textural changes (influencing the physico-mechanical properties of rock) that can be studied in order to track hydrothermal phenomena during space and time and to unravel the setting and dynamic of a volcano.

The study of alteration is often difficult and the interpretation of data hard, because the factors controlling the style of alteration processes (in terms of intensity, form, diffusion) are known, but their effective and respective roles far to be quantified: (1) the nature, composition and permeability of rocks; (2) the composition of the fluids (e.g. B, CO2, F, Cl, H2S, SO2); (3) the concentration, activity and chemical potential of their components, such as H+, CO2, O2, K+, H2S and SO2; (4) the temperature, pH and redox potential of the hydrothermal systems; (5) the abundance of fluids in comparison with the host rocks; (6) the time scale of process; (7) the contribute of microorganism and organic maths.

Hydrothermal alteration on materials sampled by field surveys and with geophysical supports is defined through the recognition of mineral assemblage(s) and chemical changes provided by various petrological and geochemical methods. Although methods are conventional, primary dilemmas arise from (1) selection of adequate numbers and types of samples and (2) separation of nearly pure alteration minerals, to avoid informative gaps and inaccurate analytical results. Moreover, single method useful to evaluate the intensity and style of alteration lacks. Furthermore, thermodynamic and isotopic equilibrium approach can fail or provide erroneous indications. In fact, the hydrothermal alteration assemblage can be out of equilibrium and minerals can have be not formed at the same time because hydrothermal systems are in a continuous dynamic evolution. Hydrothermal fluids can involve magmatic, meteoric, metamorphic and other not-juvenile components in proportions that can vary through time. Hydrothermal fluxes are varying in time and space, being difficult to assess.

The above topics are not purely academic since they can also play a large role in volcanic hazards assessment, in mineral resource explorations, in geotechnical engineering analyses, in natural pollution. Hydrothermal mineralization and dissolution can respectively seal or open rock voids, modifying the pathway of fluids and physico-mechanical properties of rocks. Sealing can generate overpressure of the hydrothermal systems, possibly leading to phreatic explosions. Rock weakness can favour instability of slopes. Hydrothermal systems can be also involved in the magma conduit processes enhancing the eruptive dynamics towards more intense explosions. Hydrothermal alteration is a circumstantial character of metal deposits driving the explorations of mineral resources.

In addition, hydrothermal systems host several organisms that show exceptional metabolic adaptation to a wealth of conditions—i.e., hot, anaerobic, acid, and saline conditions—and that are object of great attention in the understanding of life beginning and evolution, of genetic mutation induced by environmental changes, and the effect of CO2 abundance.

Finally, hydrothermal alteration minerals (particularly hydrated silicates, phyllosilicates, salts) and rocks can represent excellent analogues in experimental studies and the development of new instrumentation aimed at identifying planetary and extraterrestrial materials.

This special issue aims to refocusing on hydrothermal alteration and associated phenomena induced by low to moderately high temperature fluids (100–350 °C), from various viewpoints, combining topics such as:

  • Sampling site selection, sample treatment, analytic methods, experiments and theoretical and thermodynamic models: accuracy, limits, advances, guidelines, outcomes based on real case studies;
  • Mineral species, whole-rock geochemistry, isotope geochemistry, bioelements (C, N, P and S), metals, pollution elements (As, Hg, Pb) at exemplar hydrothermal settings: a synoptic framework for future comparative studies;
  • The hydrothermal alteration minerals and the various magmatic, meteoric and not-juvenile endogenous operating fluids types: cross-link between mineralogy, chemistry and isotope tracers;
  • The processes of hydrothermal alteration: hydration and hydrolysis under different and dynamic physico-chemical conditions;
  • Hydrothermal alteration patterns and associated mineral assemblages at active volcano;
  • The link between the hot fluids pathways, the hydrothermal alteration and the tectonic settings;
  • The hydrothermal alteration, the mechanical weakening and the dissolution of the wall rock, the sealing of voids and impermeable barriers: clues for the surface instability, seismogenesis and eruptive dynamics;
  • The contribute of geophysical methods in detecting and studying hydrothermal systems and in evaluating hydrothermal alteration at depth;
  • The role of bacteria and biological maths in hydrothermal alteration, mineralization and metal concentration;
  • The hydrothermal systems and the biosphere: diversity, evolution and structuring of hyperthermophile species.

Dr. Monica Piochi
Guest Editor

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Minerals is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2400 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • hydrothermal alteration
  • hydrothermal systems
  • active volcano

Published Papers (6 papers)

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Research

37 pages, 6490 KiB  
Article
Hydrothermal Alteration at the San Vito Area of the Campi Flegrei Geothermal System in Italy: Mineral Review and Geochemical Modeling
by Monica Piochi, Barbara Cantucci, Giordano Montegrossi and Gilda Currenti
Minerals 2021, 11(8), 810; https://doi.org/10.3390/min11080810 - 27 Jul 2021
Cited by 7 | Viewed by 3072
Abstract
The Campi Flegrei geothermal system sets in one of the most famous and hazardous volcanic caldera in the world. The geothermal dynamics is suspected to have a crucial role in the monitored unrest phases and in the eruption triggering as well. Numerical models [...] Read more.
The Campi Flegrei geothermal system sets in one of the most famous and hazardous volcanic caldera in the world. The geothermal dynamics is suspected to have a crucial role in the monitored unrest phases and in the eruption triggering as well. Numerical models in the literature do not properly consider the geochemical effects of fluid-rock interaction into the hydrothermal circulation and this gap limits the wholly understanding of the dynamics. This paper focuses on fluid-rock interaction effects at the Campi Flegrei and presents relevant information requested for reactive transport simulations. In particular, we provide: (1) an extensive review of available data and new petrographic analyses of the San Vito cores rearranged in a conceptual model useful to define representative geochemical and petrophysical parameters of rock formations suitable for numerical simulations and (2) the implemented thermodynamic and kinetic data set calibrated for the San Vito 1 well area, central in the geothermal reservoir. A preliminary 0D-geochemical model, performed with a different contribution of CO2 at high (165 °C) and low (85 °C) temperatures, firstly allows reproducing the hydrothermal reactions over time of the Campanian Ignimbrite formation, the most important deposits in the case study area. Full article
(This article belongs to the Special Issue Hydrothermal Alteration and Associated Phenomena in Active Volcanoes: Mineralogy, Texture and Isotope Geochemistry)
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21 pages, 7211 KiB  
Article
Sulphide Globules in a Porcellanite–Buchite Composite Xenolith from Stromboli Volcano (Aeolian Islands, Southern Italy): Products of Open-System Igneous Pyrometamorphism
by Alberto Renzulli, Marco Taussi, Frank J. Brink, Stefano Del Moro and Richard W. Henley
Minerals 2021, 11(6), 639; https://doi.org/10.3390/min11060639 - 16 Jun 2021
Cited by 1 | Viewed by 2863
Abstract
Pyrometamorphism is the highest temperature end-member of the sanidinite facies (high-temperature, low-pressure contact metamorphism) and comprises both subsolidus and partial melting reactions which may locally lead to cryptocrystalline-glassy rocks (i.e., porcellanites and buchites). A wide range of pyrometamorphic ejecta, with different protoliths from [...] Read more.
Pyrometamorphism is the highest temperature end-member of the sanidinite facies (high-temperature, low-pressure contact metamorphism) and comprises both subsolidus and partial melting reactions which may locally lead to cryptocrystalline-glassy rocks (i.e., porcellanites and buchites). A wide range of pyrometamorphic ejecta, with different protoliths from Stromboli volcano, have been investigated over the last two decades. Among these, a heterogeneous (composite) glassy sample (B1) containing intimately mingled porcellanite and buchite lithotypes was selected to be studied through new FESEM–EDX and QEMSCAN™ mineral mapping investigations, coupled with the already available bulk rock composition data. This xenolith was chosen because of the unique and intriguing presence of abundant Cu–Fe sulphide globules within the buchite glass in contrast with the well-known general absence of sulphides in Stromboli basalts or their subvolcanic counterparts (dolerites) due to the oxygen fugacity of NNO + 0.5–NNO + 1 (or slightly lower) during magma crystallization. The investigated sample was ejected during the Stromboli paroxysm of 5 April 2003 when low porphyritic (LP) and high porphyritic (HP) basalts were erupted together. Both types of magmas are present as coatings of the porcellanite–buchite sample and were responsible for the last syn-eruptive xenoliths’s rim made of a thin crystalline-glassy selvage. The complex petrogenetic history of the B1 pyrometamorphic xenolith is tentatively explained in the framework of the shallow subvolcanic processes and vent system dynamics occurred shortly before (January–March 2003) the 5 April 2003 paroxysm. A multistep petrogenesis is proposed to account for the unique occurrence of sulphide globules in this composite pyrometamorphic xenolith. The initial stage is the pyrometamorphism of an already hydrothermally leached extrusive/subvolcanic vent system wall rock within the shallow volcano edifice. Successively, fragments of this wall rock were subject to further heating by continuous gas flux and interaction with Stromboli HP basalt at temperatures above 1000 °C to partially melt the xenolith. This is an open system process involving continuous exchange of volatile components between the gas flux and the evolving silicate melt. It is suggested that the reaction of plagioclase and ferromagnesian phenocrysts with SO2 and HCl from the volcanic gas during diffusion into the melt led to the formation of molecular CaCl in the melt, which then was released to the general gas flux. Sulphide formation is the consequence of the reaction of HCl dissolved into the melt from the gas phase, resulting in the release of H2 into the melt and lowering of fO2 driving reduction of the dissolved SO2. Full article
(This article belongs to the Special Issue Hydrothermal Alteration and Associated Phenomena in Active Volcanoes: Mineralogy, Texture and Isotope Geochemistry)
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23 pages, 12845 KiB  
Article
Mineralogy and Geochemistry (HFSE and REE) of the Present-Day Acid-Sulfate Types Alteration from the Active Hydrothermal System of Furnas Volcano, São Miguel Island, The Azores Archipelago
by Iuliu Bobos and Celso Gomes
Minerals 2021, 11(4), 335; https://doi.org/10.3390/min11040335 - 24 Mar 2021
Cited by 5 | Viewed by 3299
Abstract
Acid-sulfate alteration is comprised by clays, sulfate, sinter and native sulphur minerals crystallized as neoformation products from dissolution of primary minerals during water-rock interaction. Smectite, kaolinite, halloysite-7 Å and opal-A occur in assemblages with alunite. Smectite represents a mechanical mixture between two (propylitic [...] Read more.
Acid-sulfate alteration is comprised by clays, sulfate, sinter and native sulphur minerals crystallized as neoformation products from dissolution of primary minerals during water-rock interaction. Smectite, kaolinite, halloysite-7 Å and opal-A occur in assemblages with alunite. Smectite represents a mechanical mixture between two (propylitic and acid-sulfate) alteration types. High amounts of high-field strength elements (HFSE) and rare earth elements (REE) were measured in acid-sulfate rocks. The Nb vs. Ta and Zr vs. Hf show a positive trend and a widely scattered relationships, suggesting a large fractionation during acid-sulfate alteration. Higher ∑REE amounts (up to 934.5 ppm) were found in clay-sulfate assemblages and lower ∑REE amounts in sinter (opal-A ± sulfate, 169.05 ppm) than to fresh rocks (up to 751.2 ppm). The acid-sulfate rocks reveal a distinctive gull-wing chondrite-normalized pattern with a negative Eu anomaly and light- and heavy-REE “wings” similar to the gull-wing pattern of fresh rocks. The Eu/Eu* shows a large fractionation of acid sulfate rocks from 0.16 to 0.78 with respect to fresh trachyte products (0.10 to 0.38). Variation of (La/Sm)N and (La/Yb)N ratio show a large fractionation of light-REE and heavy-REE. The Y vs. Dy and Y vs. Ho show a very good positive correlation coefficient and a large Y fractionation in acid-sulfate rocks with respect to fresh rocks. Full article
(This article belongs to the Special Issue Hydrothermal Alteration and Associated Phenomena in Active Volcanoes: Mineralogy, Texture and Isotope Geochemistry)
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19 pages, 5748 KiB  
Article
Volcanic Holocrystalline Bedrock and Hydrothermal Alteration: A Terrestrial Analogue for Mars
by Anna Chiara Tangari, Lucia Marinangeli, Fabio Scarciglia, Loredana Pompilio and Eugenio Piluso
Minerals 2020, 10(12), 1082; https://doi.org/10.3390/min10121082 - 2 Dec 2020
Cited by 4 | Viewed by 2975
Abstract
Clay minerals have been detected on Mars to outcrop mainly as alteration of ancient bedrock, and secondarily, as deposition from aqueous environments or interlayered with evaporitic deposits on Mars. In order to better constrain the alteration environments, we focused on the process to [...] Read more.
Clay minerals have been detected on Mars to outcrop mainly as alteration of ancient bedrock, and secondarily, as deposition from aqueous environments or interlayered with evaporitic deposits on Mars. In order to better constrain the alteration environments, we focused on the process to form clays from volcanic rocks and experimentally reproduced it at different temperature and pH. A fresh, holocrystalline alkali-basalt sample collected in the Mount Etna volcanic sequence has been used as analogue of the Martian unaltered bedrock. Previous works considered only volcanic glass or single mineral, but this may not reflect the full environmental conditions. Instead, we altered the bulk rock and analyzed the changes of primary minerals to constrain the minimum environmental parameters to form clays. We observed that under acidic aqueous solution (pH ~ 3.5–5.0) and moderate temperature (~150–175 °C), clinopyroxene and plagioclase are altered in smectite in just a few days, while higher temperature appear to favor oxides formation regardless of pH. Plagioclases can also be transformed in zeolite, commonly found in association with clays on Mars. This transformation may occur even at very shallow depth if a magmatic source is close or hydrothermalism is triggered by meteoritic impact. Full article
(This article belongs to the Special Issue Hydrothermal Alteration and Associated Phenomena in Active Volcanoes: Mineralogy, Texture and Isotope Geochemistry)
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10 pages, 4268 KiB  
Article
CO2 and H2S Degassing at Fangaia Mud Pool, Solfatara, Campi Flegrei (Italy): Origin and Dynamics of the Pool Basin
by Dmitri Rouwet, Giancarlo Tamburello, Tullio Ricci, Alessandra Sciarra, Francesco Capecchiacci and Stefano Caliro
Minerals 2020, 10(12), 1051; https://doi.org/10.3390/min10121051 - 25 Nov 2020
Cited by 3 | Viewed by 1848
Abstract
The Fangaia mud pool provides a “window” into the hydrothermal system underlying the degassing Solfatara crater, which is the most active volcanic centre inside the restless Campi Flegrei caldera, Southern Italy. The present study aimed at unravelling the degassing dynamics of CO2 [...] Read more.
The Fangaia mud pool provides a “window” into the hydrothermal system underlying the degassing Solfatara crater, which is the most active volcanic centre inside the restless Campi Flegrei caldera, Southern Italy. The present study aimed at unravelling the degassing dynamics of CO2 and H2S flushing through the pH 1.2 steam-heated Fangaia mud pool, an ideal field laboratory as a proxy of an active crater lake. Our results from MultiGAS measurements above Fangaia’s surface show that H2S scrubbing, demonstrated by high CO2/H2S ratios, was most efficient in the portions of the basin affected by diffusive degassing. Convective bubbling degassing instead was the most effective mechanism to release gas in quantitative terms, with lower CO2/H2S ratios, similar to the Solfatara crater fumaroles, the high-T end member of the hydrothermal system. Unsurprisingly, total estimated CO2 and H2S fluxes from the small Fangaia pool (~184 m2 in June 2017) were at least two orders of magnitude lower (CO2 flux < 64 t/d, H2S flux < 0.5 t/d) than the total CO2 flux of the Campi Flegrei caldera (up to 3000 t/d for CO2), too low to affect the gas budget for the caldera, and hence volcano monitoring routines. Given the role of the rising gas as “sediment stirrer”, the physical and chemical processes behind gas migration through a mud pool are arguably the creating processes giving origin to Fangaia. Follow-up studies of this so far unique campaign will help to better understand the fast dynamics of this peculiar degassing feature. Full article
(This article belongs to the Special Issue Hydrothermal Alteration and Associated Phenomena in Active Volcanoes: Mineralogy, Texture and Isotope Geochemistry)
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25 pages, 5447 KiB  
Article
Variability of Carbonate Isotope Signatures in a Hydrothermally Influenced System: Insights from the Pastos Grandes Caldera (Bolivia)
by Cédric Bougeault, Christophe Durlet, Emmanuelle Vennin, Elodie Muller, Magali Ader, Bassam Ghaleb, Emmanuelle Gérard, Aurélien Virgone and Eric C. Gaucher
Minerals 2020, 10(11), 989; https://doi.org/10.3390/min10110989 - 7 Nov 2020
Cited by 8 | Viewed by 2579
Abstract
Laguna Pastos Grandes (Bolivia), nesting in a volcanic caldera, is a large, palustrine-to-lacustrine system fed by meteoric and hydrothermal calco–carbonic fluids. These different fluid inputs favor a complex mosaic of depositional environments, including hydrothermal springs, pools, and an ephemeral lake, producing abundant present-day [...] Read more.
Laguna Pastos Grandes (Bolivia), nesting in a volcanic caldera, is a large, palustrine-to-lacustrine system fed by meteoric and hydrothermal calco–carbonic fluids. These different fluid inputs favor a complex mosaic of depositional environments, including hydrothermal springs, pools, and an ephemeral lake, producing abundant present-day carbonates developing over a Holocene carbonate crust dated by U–Th. Present-day carbonates (muds, concretions, and microbialites) recorded a large range of isotope variations, reaching 13.9‰ in δ13C and 11.1‰ in δ18O. Sedimentological and geochemical data indicated that the main processes influencing the isotope record were: (i) rapid CO2 degassing and temperature decreases along hydrothermal discharges; (ii) strong evaporation favored by the arid high-altitude Andean climate, locally enhanced by capillary water rise within microbial mats or by wind-induced spray falling on vadose concretions. Unlike past or present perennial lake systems in Central Andes, the short residence time of brine waters in the ephemeral central lake prevents enrichment of lacustrine carbonates in 13C and 18O. The very low fraction modern F14C in these present-day carbonates demonstrates that incorporation of fossil magmatic carbon related to the volcanic context also prevents any radiocarbon dating. The use of isotopes for the interpretation of ancient continental series should always be accompanied by a thorough characterization of the environmental setting. Full article
(This article belongs to the Special Issue Hydrothermal Alteration and Associated Phenomena in Active Volcanoes: Mineralogy, Texture and Isotope Geochemistry)
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