Caracausi, Antonio

This study provides new mineral chemistry data together with micro-thermometric measurements on fluid inclusions hosted in ultramafic xenoliths (lherzolite, wehrlite, and dunite) brought to the surface by the last Mt. Vulture volcano activity (140 ka; southern Italy), and fed by melilitite-carbonatite magmas. Petrographic evidence and mineralogical compositions of Mt. Vulture xenoliths are consistent with an origin in the upper mantle. Fluid inclusions in rock-forming minerals of lherzolite and wehrlite xenoliths are CO2-dominated. The equilibrium temperature calculated by geothermometric estimates ranges from 1039 C (±36°C) to 1142°C (±15°C), and entrapment pressures of fluid inclusions with post-trapping re-equilibration correspond to the local crust–mantle boundary (32 km depth), and to a shallow reservoir located at 12–14 km depth. These results contribute to constrain the origin of these xenoliths and the depth of storage of magmas erupted from Mt. Vulture, where carbonatite-like metasomatism and mantle-derived CO2 degassing occur.

In this study, we present scaling relationships between fault lengths, fault slip-rates and historical seismicity for an active normal fault system, seismically accommodating crustal extension within the upper plate of the Ionian subduction zone (southern Italy). This crustal extension is confirmed by historical seismicity and instrumental geodesy, with GNSS-derived values of horizonal deformation within a range of 2-3 mm/yr throughout Calabria and the Messina Strait region. We collated data for fault slip-rates, fault lengths and historical earthquakes for a given fault to explore whether fault slip-rates are correlated with fault size and their geometric moment.We present new results showing a robust correlation between fault lengths and fault slip-rates, which supports the idea of a relationship for a given fault between fault slip-rates and the geometric moment.We discuss our results in terms of how these correlations should be used if regional deformation is accommodated by localised strain on faults mostly arranged along strike rather than distributed strain on multiple faults across-strike. For instance, we compare our empirical correlation between fault lengths and fault throw-rates over the Middle-Late Pleistocene in Calabria and the Messina Strait with those from Central and Southern Apennines over the Holocene, characterized by strain distributed on multiple faults across-strike and strain localised on faults mostly arranged along-strike, respectively.Tectonic and seismic hazard implications are discussed for future investigations based on fault slip-rates, fault size and historical seismicity.

This study is focused on fluids characterization and circulations through the crust of the Irpinia region, an active seismic zone in Southern Italy, that has experienced several high-magnitude earthquakes, including a catastrophic one in 1980 (M = 6.9 Ms). Using isotopic geochemistry and the carbon‑helium system in free and dissolved volatiles in water, this study aims to explore the processes at depth that can alter pristine chemistry of these natural fluids. Gas-rock-water interactions and their impact on CO2 emissions and isotopic composition are evaluated using a multidisciplinary model that integrates geochemistry and regional geological data. By analyzing the He isotopic signature in the natural fluids, the release of mantle-derived He on a regional scale in Southern Italy is verified, along with significant emissions of deep-sourced CO2. The proposed model, supported by geological and geophysical constraints, is based on the interactions between gas, rock, and water within the crust and the degassing of deep-sourced CO2. Furthermore, this study reveals that the Total Dissolved Inorganic Carbon (TDIC) in cold waters results from mixing between a shallow and a deeper carbon endmember that is equilibrated with carbonate lithology. In addition, the geochemical signature of TDIC in thermal carbon-rich water is explained by supplementary secondary processes, including equilibrium fractionation between solid, gas, and aqueous phases, as well as sinks such as mineral precipitation and CO2 degassing. These findings have important implications for developing effective monitoring strategies for crustal fluids in different geological contexts and highlight the critical need to understand gas-water-rock interaction processes that control fluid chemistry at depths that can affect the assessment of the CO2 flux in atmosphere. Finally, this study highlights that the emissions of natural CO2 from the seismically active Irpinia area are up to 4.08·10+9 mol·y-1, which amounts is in the range of worldwide volcanic systems.

The redox state of the Earth’s upper mantle (i.e., oxygen fugacity, fO2 ) is a key variable that influences numerous processes occurring at depth like the mobility of volatile species, partial melting, and metasomatism. It is linked to the oxidation state of peridotite rocks, which is normally determined through the available oxythermobarometers after measuring the chemical composition of equilibrated rock-forming minerals and the Fe3C in redox-sensitive minerals like spinel or garnet. To date, accurate measurements of Fe3C = P Fe in peridotites have been limited to those peridotites (e.g., harzburgites and lherzolites) for which an oxythermobarometer exists and where spinel (or garnet) crystals can be easily separated and measured by conventional 57Fe Mössbauer spectroscopy. Wehrlitic rocks have been generally formed by the interaction of a lherzolite with carbonatitic melts and, therefore, have recorded the passage of (metasomatic) fluids at mantle conditions. However, no oxythermobarometer exists to determine their equilibrium fO2 . The aim of this study was to retrieve the fO2 of the mantle beneath Mt. Vulture volcano (Italy) through the study of a wehrlitic lapillus emitted during the last eruption ( 140 kyr ago) that contain olivines with multiple tiny spinel inclusions with sizes <40 μm. To our knowledge, the Fe oxidation state of these inclusions has been never determined with the Mössbauer technique due to their small sizes. Here, we present measurements of the Fe3C = P Fe using in situ synchrotron Mössbauer spectroscopy coupled with chemical and spectroscopic analysis of both host olivine and spinel inclusions. The results show Fe3C = P Fe ratios of 0.03–0.05 for olivine and 0.40–0.45 for the included spinels, the latter of which appear higher than those reported in literature for mantle spinel harzburgites and lherzolites. Given the evidence of the mantle origin of the trapped spinels, we propose that the high fO2 (between 0.81 and 1.00 log above the fayalite–magnetite–quartz buffer; FMQ) likely results from the interaction between the pristine spinel lherzolite and a CO2-rich metasomatic agent prior to the spinel entrapment in olivines at mantle depths.

Deeply-sourced fluids are released in volcanically and tectonically active regions through conduits such as fumaroles, natural springs, and permeable soils. The origin and transport of the fluids in volcanic and tectonic systems are a key research theme in Earth Sciences, which is of particular importance for geo-hazard mitigation and resource exploration. This Research Topic aims to present recent advances in fluid geochemistry and its application in volcanically and tectonically active regions. Under this context, 10 papers covering a series of research themes in fluid geochemistry were published in this Research Topic, as briefly summarized below. People living close to active fault zones are threatened by earthquake hazard and therefore monitoring the status of active faults is important to mitigate the damage caused by future earthquakes. Caracausi et al. reported data from a novel infrastructure designed for multidisciplinary and continuous monitoring of the Alto Tiberina fault, Italy. Monitoring results (including seismic, geodetic, and geochemical data) from The Alto Tiberina Near Fault Observatory (TABOO-NFO) would shed new light on earthquake prediction studies in other countries. Fidani et al. conducted a comprehensive statistical analysis of CO2 time series registered at the Gallicano test site, Italy, and identified the correlations between low- magnitude earthquakes and CO2 anomalies in spring waters. Li et al. studied the spatial variations in soil Rn and CO2 emissions in the Wuzhong-Lingwu region, NW China, as well as the possible controlling factors of earthquakes, stress state, and deep-to-shallow crustal structures. Their findings offer new insight into combining geochemical characteristics of soil gas and seismological methods to estimate regional seismic hazards. Under the context of continuous collision between Indian and Asian continents, the Tibetan Plateau and its surrounding regions have drawn increasing concern from the Earth science community because of intensive and frequent earthquake events. Liu et al. reported the first estimates of diffuse soil CO2 flux (~1.2 Mt yr–1) for the Anninghe-Zemuhe fault in the Southeast Tibetan Plateau and found close relationships between spatial variations in soil CO2 fluxes and that of regional seismic activity. Based on the geochemistry of hot spring waters, Liu et al. explored the controls of the Jinshajiang fault zone (SW China) on hydrothermal fluid circulation, water-rock interaction, and earthquakes, which highlighted the role of hot spring water discharging from fracture zones in receiving the hydrological information on seismic activity. Also published in this Research Topic, Liu et al. presented an example of post-earthquake hydrological changes based on carbon isotope data of spring waters collected after the 2021 Mw 7.4 Maduo earthquake in eastern Tibetan Plateau. They quantitatively identified enhanced mobilization of the shallow soil organic carbon following the 2021 Maduo earthquake and suggested that earthquakes could disturb the circulation of subsurface fluids and their interaction with the country rocks and sediments on short timescales. Wang et al. investigated origin and circulation of geothermal waters in the Karakoram strike-slip fault zone in western Tibet. Their results show that geothermal water is correlated with the epicenter and focal depth of earthquakes, especially for high-temperature spring water with deeper circulation and extremely high Li, B, Fe, and As concentrations. Three papers in this Research Topic focus on the degassing of historically active volcanoes. Gherardi et al. investigated helium isotopes on gas extracted by crushing from melt and fluid inclusions in minerals from Plinian and inter-Plinian tephra and lavas of Vesuvius, Italy. Their results show that i) 3He/4He values are buffered within an extended, deep-seated reservoir at about 10 km filled with magma rising from the mantle, and ii) magma ponding at crustal depth could be considered a key mechanism that might have the potential to homogenize the helium isotope signal. Located in the hinterland of Northeast Asia, the active Arxan volcanic field remains less studied for the characteristics of its present-day volcanic degassing. Pan et al. focused on diffuse soil CO2 fluxes and found that annual CO2 emission flux from the volcanic field to the atmosphere is ~0.63 × 105 t and is comparable to that of the Iwojima volcano in Japan. This is the first flux estimate for soil CO2 emissions of the Arxan volcanic field. Cui et al. presented a geochemical study on the hot spring water and gases from the Arxan volcanic field. They identified ~3%–23% mantle helium inputs and thus heat supply in the hydrothermal fluids, suggesting that the residual mantle-derived melts beneath the Arxan volcanic field are still releasing fluids/volatiles and heating the overlying hydrothermal systems.

Mantle-derived noble gases in volcanic gases are powerful tracers of terrestrial volatile evolution, as they contain mixtures of both primordial (from Earth's accretion) and secondary (e.g., radiogenic) isotope signals that characterize the composition of deep Earth. However, volcanic gases emitted through subaerial hydrothermal systems also contain contributions from shallow reservoirs (groundwater, crust, atmosphere). Deconvolving deep and shallow source signals is critical for robust interpretations of mantle-derived signals. Here, we use a novel dynamic mass spectrometry technique to measure argon, krypton, and xenon isotopes in volcanic gas with ultrahigh precision. Data from Iceland, Germany, United States (Yellowstone, Salton Sea), Costa Rica, and Chile show that subsurface isotope fractionation within hydrothermal systems is a globally pervasive and previously unrecognized process causing substantial nonradiogenic Ar-Kr-Xe isotope variations. Quantitatively accounting for this process is vital for accurately interpreting mantle-derived volatile (e.g., noble gas and nitrogen) signals, with profound implications for our understanding of terrestrial volatile evolution.

Petrology and fluid inclusions (FI) geochemistry are increasingly used in tandem to constrain the compositional features and evolution of the lithospheric mantle. In this study, we combine petrography and mineral chemistry with analyses of noble gases (He, Ne and Ar) and CO2 in olivine, orthopyroxene- and clinopyroxene-hosted FI, as well as radiogenic isotope (Sr-Nd-Pb) systematics of ultramafic xenoliths collected at La Grille volcano in Grande Comore Island, aiming at better characterizing one of the most enigmatic and controversial portions of the western Indian Ocean lithospheric mantle. Xenoliths have been divided in three groups on the basis of their textural features: Group 1 (Opx-bearing), Group 2 (Opx-free) and Group 3 (Cumulate). Overall, petrographic observations and mineral phase compositions indicate that the sampled lithospheric portion experienced variable degrees of melting (from 5% to 35%), recorded by Group 1 most refractory harzburgites and lherzolites, as well as modal metasomatic processes as evidenced by the crystallization of cpx at the expense of opx in Group 1 fertile lherzolites and wehrlite and by Group 2 xenoliths. Crystallization of slightly oversaturated basic silicate melts seems also to have occurred, as shown by Group 3 xenoliths. A positive trend between temperature and ƒO2 is evident, with Group 2 and 3 xenoliths testifying for hotter and more oxidised conditions than Group 1. The variability of the 4He/40Ar* ratio (0.02–0.39) in Group 1, significantly below typical values of a fertile mantle (4He/40Ar* = 1–5), can be explained by the variable degrees of partial melting coupled to metasomatic enrichment that may account for modifying 4He/40Ar*, as also indicated by the mineral composition. He-Ar-CO2 relationships support the presence of a metasomatic CO2-rich process post-dating the melt extraction and the cumulate formation. The air-corrected 3He/4He isotopic ratios (6.30 to 7.36 Ra) are intermediate between the MORB mantle signature (8 ± 1Ra) and the SCLM (6.1 ± 0.9 Ra). The Ne and Ar isotopic signatures (20Ne/22Ne, 21/Ne/22Ne and 40Ar/36Ar) are consistent with mixing between an air-derived component and a MORB-like mantle, supporting the hypothesis for a lithospheric origin of the Comoros magmas, and arguing against any deep mantle plume-related contribution. This is also corroborated by combining Ne with He isotopes, showing that La Grille ultramafic xenoliths are far from the typical plume-type compositions. Sr-Nd-Pb isotope systematics in opx and cpx from La Grille additionally support a MORB-type signature for the lithospheric mantle beneath the area.

South Matese, Apennines, is a hydrothermally and seismically active extensional area characterized by CO2outgassing and Mw≤7.1 earthquakes. There, meters-sized pockets of incohesive pulverized dolostone are hosted within Mesozoic carbonates at the hanging wall of seismically active normal faults. The aim of this paper is to understand the pulverization process. The pulverized dolostone is finely comminuted (down to a few microns), but primary structures, mainly bedding, are preserved. The grain size distribution is similar to that of previously studied pulverized rocks associated with active faults and dissimilar to that of carbonate cataclasites and fault gouges. The pulverized pockets are surrounded by zones (halos), in which the loose grains are cemented, in their original position, by microcrystalline calcite, resulting in a cemented micro-mosaic breccia. Stable isotopes from the cement are compatible with calcite precipitation from rapidly CO2-degassing shallow waters. Comparing our observations with results of laboratory experiments on carbonate pulverization through rapid decompression of pore-hosted CO2, the best explanation for the pulverized dolostone may lie on local accumulations of pressurized CO2-rich gas, suddenly decompressed during earthquakes. The limited permeability of the gas-saturated dolostone must have prevented a prompt escape of the gas from the rock, which was therefore anhydrously pulverized by the rapid expansion of the trapped gas. The sudden decompression must have suctioned bicarbonate-rich groundwaters, from which microcrystalline calcite rapidly precipitated, fossilizing the freshly pulverized dolostone. Calcite precipitation formed an impermeable shield around the pulverized pockets, which, therefore, remained internally uncemented. This process may have occurred over multiple cycles at depths shallower than the CO2subcritical–supercritical boundary (ca. -800m). Although hypothetical, the proposed mechanism is for the first time suggested for an active tectonic environment. The gas rapid decompression could have been triggered by coseismic processes (e.g., dynamic unloading or transient tensile pulses) previously proposed for the formation of other pulverized rocks. The presented case may improve our knowledge of possible chemical-physical processes connected with the subsurface storage of CO2in seismically active areas.

This paper presents new chemical and isotopic data on gases from deep oil and gas fields, bubbling gases, dissolved gases in groundwaters and dry seeps of the Southern Po River Basin (Emilia-Romagna, Italy), aiming to (i) characterize and differentiate the various types of deep natural gases; (ii) identify the source(s) of methane and light hydrocarbons in shallow aquifers and surface gas-rich emissions; (iii) propose a conceptual model of natural fluid migration pathways in the sedimentary prism of the Southern Po River Basin. Based on the isotopic composition of CH4 and C2–C4 n-alkanes, CH4/(C2H6+C3H8) ratio, relative proportion of the C7 hydrocarbons and relative concentration of cyclic compounds with respect to the total cyclic abundance, three main deep reservoirs of hydrocarbons are identified in the subsurface of the Southern Po River Basin: (1) microbial gas hosted in Pliocene-Pleistocene marine sediments, (2) thermogenic gas hosted in Miocene deposits and (3) thermogenic gas produced in Triassic carbonates. Helium isotopes of these deep fluids indicate an almost pure crustal origin (Rc/Ra values = 0.014–0.04), with negligible contributions from mantle-derived helium. A variable contribution of atmosphere-derived fluids is highlighted by low 4He/20Ne (down to 5.42) and 40Ar/36Ar (≤319.5) values. Comparison of chemical and isotopic signatures of deep and surficial hydrocarbon occurrences suggests that methane in shallow groundwaters or gas seeps is sourced by microbial gas migrating upward from deep Plio-Pleistocene reservoirs, with no detectable contributions of Triassic or Miocene thermogenic hydrocarbons. At shallow depths (roughly around 20–50 m.b.g.l.), Plio-Pleistocene microbial methane appears to be mainly stored in anoxic aquifers. However, where CH4 further migrates upwards and reaches aerobic environments (e.g., aquifers or soils), it readily undergoes a process of exothermic microbial oxidation mediated by methanotrophic bacteria. Where the structural architecture of the sedimentary sequence favors the migration of fluids, the methanotrophic biofilter is bypassed and CH4 is discharged through soil diffuse degassing or gas bubbling at water wells. We argue that microbial consumption might be able to bio-sequester significant amounts of Plio-Pleistocene deep-sourced methane in the form of CO2 and biomass. Such process might be widespread in the subsurface of the Southern Po River Basin and, possibly, in other foreland basins worldwide.

With the aim of deepening our understanding of deep-seated fluids upwelling and mixing in large regional aquifers, we performed a hydrogeochemical study of twenty-two springs in the Contursi area (upper Sele river valley, southern Apennines) by means of the measurements of chemical-physical parameters, major ions, trace elements, and stable and radioactive isotopes. Besides, we realized two updated geo-structural cross-sections inorder to reconstruct the groundwater flowpath in the study area. The hydrogeochemical composition, as well a the water temperature allow to identify-three main groups of groundwater: Cold and Low salinity Groundwater (CLGW), Intermediate Salinity Groundwater (ISGW), and Thermal Salinity Groundwater (TSGW). The CLGW group, mostly emerging at the boundary of carbonate aquifers, is characterized by alkaline earth-bicarbonate hydrofacies. Instead, ISGW and TSGW, situated in the inner zone of the valley, show gradually a hydrogeochemical evolution towards sodium-chloride type hydrofacies domain with the highest salinity value. Stable isotope (δ18O-δD) of CLGW reveal the local meteoric origin of groundwater, while isotopic signatures of ISGW and TSGW is associated with the deep fluids inflow. CLGW hydrogeochemistry is clearly related to dissolution of carbonate rocks. On the other hand, for ISGW and TSGW an additional contribution from evaporitic rocks is supported by saturation indices values (gypsum and anhydrite) and validated by isotopic signature of dissolved sulphate (δ34S-δ18O). The application of two models based on tritium data (i.e., the piston-flow and well-mixed reservoir) attributes longer and deeper groundwater flowpaths to TSGW. Through geothermometric calculations (e,g., K-Mg and SiO2-quartz), the equilibrium temperature of deep fluids reservoir is also extrapolated (i.e., 75–96 ◦C). The results of the adopted hydrogeochemical multi-component approach allowed us to propose an interpretative model of groundwater flowpath for the Contursi area, where deep-seated tectonic discontinuities play a significant role for the upwelling of saline deep thermal fluids in shallow aquifers.