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Aiming at understanding the source of the fluids that mineralizing within seismically active fault zones, we investigate the noble gas isotopes (i.e., helium (He), neon (Ne), and argon (Ar)) in the fluid inclusions (FIs) trapped in the calcite veins sampled along high‐angle fault zones of the Contursi hydrothermal basin, southern Italy. The latter basin lies in close vicinity of the MW = 6.9, 1980 Irpinia earthquake and exposes numerous fault scarps dissecting Mesozoic shallow‐water carbonates. The analyses of noble gases (He, Ne, Ar) are conducted to identify the origin of the volatiles circulating along the faults at the time of calcite precipitation. Then, outcomes of this discussions are compared with currently outgassing of deep‐sourced CO2 coupled to mantle‐derived He in that area, whose output is larger than those from some volcanic areas worldwide. The results indicate that He in FIs is dominated by a crustal radiogenic component (4He), and by an up to 20% of a mantle‐derived component (3He), with a highest isotopic signature of 1.38 Ra. This value is consistent with the highest percentage of mantle‐derived He associated to high‐flux CO2 gas emission in the investigated area (1.41 Ra). We propose that the variability of the He isotopic signature measured in primary FIs can result from early trapping of fluid inclusions or post trapping processes and seismic activity that modify the pristine He isotopic signature (i.e., derived from the crust and/or mantle) in groundwater along the faults during periods of background seismicity. Such investigations are fundamental to understand fluid migration in fault systems and the role of fluids in processes of earthquake nucleation. Key Points: Paleofluids in the studied seismogenic fault derive from the mixing between crustal and mantle (∼20%) derived fluidsThe variability of the He isotopic signature registered in fluid inclusions results from either early trapping processes (due to past possible earthquakes events) or post trapping processes by addition of radiogenic 4He produced within fractured calcite veins over time (vein aging)The pristine mantle source has been active in the Irpinia area (southern Italy) for at least 1 Ma based on the post trapping process [ABSTRACT FROM AUTHOR]

From core observations of shales of the Niutitang formation in the northern part of Guizhou Province, China, calcite was often found to act as a natural fracture filler and affect the extension of fractures in hydraulic fracturing. Therefore, it is crucial to understand the tensile mechanical behavior of shales by calcite veins. In this paper, by computed tomography scanning of shales containing calcite veins of different dip angles from the Niutitang formation, a three‐dimensional numerical model reflecting the internal fine structure of the shale was constructed. Direct tensile numerical simulations were carried out to investigate the effect of calcite veins at different angles on the fine‐scale damage process and mechanical properties of shale. The experimental results show that the tensile capacity of the shale increases with the increase in calcite veins. Depending on the dip angle of the calcite vein, the damage pattern of the shale is divided into three types of damage: pull‐off damage along the calcite vein, jagged damage, and horizontal damage perpendicular to the loading method. At high dip angles, the shale damage is more intense and the fracture network more complex. The temporal and spatial characteristics of the acoustic emissions provide a good indication of the microscopic behavior of the shale specimens during the damage process. The box dimension method was used to calculate the fractal dimension of the shale specimens at the final damage, and it was found that the damage was more intense and the fracture network was more complex at high dip angles, and there was an angular threshold. [ABSTRACT FROM AUTHOR]

We applied field structural data and isotope geochemical (δ13C, δ18O and 87Sr/86Sr) analyses to understand the relationship among calcite veins, fault damage zones and carbonate host rocks in a thrust fault damage zone in the Achado quarry, Irecê Basin in the São Francisco Craton, NE Brazil. Our results reveal three hydrological packages with different rheological behaviours in a stratified carbonate succession. The upper package includes the Achado fault damage zone that is characterised by interlayered dolomitised grainstones and mudstones. These rocks display high positive δ13C values (10‰–13‰), negative δ18O values (mean −6.34‰) and radiogenic (87Sr/86Sr) isotope values (0.70885–0.71519). A second package is marked by a cataclastic brittle shear zone lateral parallel to dolograinstones bedding. These rocks show low to positive δ13C values (−3.41‰ to +8.85‰), more positive δ18O values (mean −3.73‰) and radiogenic (87Sr/86Sr) isotope values (0.71039–0.71373). The lower package is characterised by well‐preserved pristine limestone succession that shows δ13C values ranging between −0.46‰ and +3.17‰, mean δ18O = −5.41‰ and less radiogenic 87Sr/86Sr values (0.70762–0.70818). In contrast to the upper and intermediate packages, rocks from the lower one exhibit very low permeability and behaved as a seal for fluid migration. Fluid flow occurred several times during basin evolution, for example along syn‐rift fault damage zones, bedding‐parallel carbonate breccia, thrust faults, cataclastic shear zones, synorogenic conjugate shear fractures or joints and opening mode I fracture‐fill calcite veins. These fractures allowed pervasive fluid flow in the porous intermediate cataclastic shear zone where fluids flowed and formed veins, as diffuse fluid flow in randomly oriented fracture swarms, or channelised fluid flow in aligned fracture corridors. They record significant centimetre‐scale to km‐scale hydrological behaviour within carbonate layers. Most carbonates that are associated with veins, fault damage zones and hydraulic breccia were formed by fluids of the same origin with low δ13C (−6.0 to −2.0‰) and δ18O (−6.0 to −8.5‰) values, and more radiogenic 87Sr/86Sr values compared to the carbonate host rocks. [ABSTRACT FROM AUTHOR]

International Ocean Discovery Program Expeditions 352 and 351 drilled into the Western Pacific Izu‐Bonin forearc and rear arc. The drill cores revealed that the forearc is composed of forearc basalts (FAB) and boninites and the rear arc consists of FAB‐like rocks. These rocks are pervaded by calcite veins. Blocky vein microtextures enclosing host rock fragments dominate in all locations and suggest hydrofracturing and advective fluid flow. Significant diffusion‐fed and crystallization pressure‐driven antitaxial veining is restricted to the rear arc. The lack of faults and presence of an Eocene sedimentary cover in the rear arc facilitated antitaxial veining. Rare earth element and isotopic (δ18O, δ13C, 87Sr/86Sr, and Δ47) tracers indicate varying parental fluid compositions ranging from pristine to variably modified seawater. The most pristine seawater signatures are recorded by FAB‐hosted low‐T (<30 °C) vein calcites. Their 87Sr/86Sr ratios intersect the 87Sr/86Sr seawater curve at ~35–33 and ~22 Ma. These intersections are interpreted as precipitation ages, which concur with Pacific slab rollback. Boninite‐hosted low‐T (<30 °C) vein calcites precipitated from seawater that was modified by fluid‐rock interactions. Mixing calculations yield a mixture of >95% seawater and <5% basaltic 87Sr/86Sr. In the rear arc, low‐T rock alteration lowered the circulating seawater in δ18O and 87Sr/86Sr. Thus, vein calcites precipitated from modified seawater with up to 20–30% basaltic 87Sr/86Sr at temperatures up to 74 ± 12 °C. These results show how the local geology and vein growth dynamics affect microtextures and geochemical compositions of vein precipitates. Plain Language Summary: Our knowledge of the oceanic crust relies on crustal pieces uplifted above sea level and sampling by dredging, diving, and drilling expeditions. Since uplift may involve the alteration and destruction of original geological features, International Ocean Discovery Program Expeditions 352 and 351 drilled into the Izu‐Bonin forearc and rear arc crust (Western Pacific), revealing rocks unaffected by these processes. The volcanic rocks in these drill cores are characterized by fractures through which seawater percolated from which calcium carbonate (calcite) precipitated (veining). By analyzing crystal shapes, elemental and isotopic compositions of vein calcites, this research project investigated the causes of fracturing, the geochemical composition of the percolating fluids, and the calcite formation temperatures. In the forearc calcites formed at temperatures <30 °C in fractures that opened due to high fluid pressures. These calcites precipitated from fluids ranging from unmodified seawater to seawater that was weakly modified by fluid‐rock interaction. Most seawater‐derived calcites formed between ~35 and ~33 million years. In the rear arc calcites precipitated at temperatures up to 74 °C from seawater that was modified by low‐temperature fluid‐rock interaction during which elements and isotopes were exchanged. The rear arc geology facilitated fluid diffusion and fracture‐independent veining. Key Points: In the Izu‐Bonin forearc vein calcites precipitated at <30 °C from pristine to weakly modified seawater in hydrofracturesCalcite veining (max. 74 °C) in the rear arc is characterized by weak to moderate fluid‐rock interaction, diffusive and advective fluid flowA sedimentary cover and the lack of faults facilitated diffusion‐fed and crystallization pressure‐driven antitaxial veining in the rear arc [ABSTRACT FROM AUTHOR]

Calcite veins hosted in pillow lavas of the Late Cretaceous Troodos suprasubduction zone ophiolite provide insights into the timing and physicochemical environment of postmagmatic fracturing and fluid circulation through oceanic crust. This study presents rare earth element and yttrium (REE+Y) concentrations, δ13C, δ18O, 87Sr/86Sr, and clumped isotopic (Δ47) compositions of vein calcites in order to investigate their fluid sources, formation temperatures, and precipitation ages. These geochemical data are combined with microtextural analyses. Intersections of 87Sr/86Sr ratios of vein calcites with the Sr isotope seawater curve suggest two distinct calcite veining phases. Major calcite veining within an interval of ~10 Myr after crust formation is characterized by microtextures that point to extensional fracturing related to crack and sealing, host rock brecciation, and advective fluid flow. These vein calcites show REE+Y characteristics, 87Sr/86Sr ratios, and clumped isotopic compositions indicative of precipitation from seawater at <50 °C. Extended fluid residence times intensified fluid‐rock interactions and lowered Y/Ho ratios of some blocky vein calcites, whereas crack and sealing resulted in pristine seawater signatures. Low 87Sr/86Sr ratios of localized high‐temperature blocky vein calcites point to the involvement of hydrothermal fluids. These calcites show Mn‐controlled oscillatory growth zonations that probably developed in a closed system out of equilibrium. Later calcite veining (<75 Ma) may have coincided with rotation and/or uplift of the Troodos ophiolite. Microtextures of these vein calcites indicate fluid diffusion and fracture‐independent crystallization pressure‐driven veining. Their variably modified seawater signatures resulted from diffusion‐related fluid interaction with hydrothermal sediments. Plain Language Summary: The Troodos ophiolite (Cyprus) formed as oceanic crust 92 million years ago along a mid‐ocean ridge above a young subduction zone before it was uplifted to its present position 2,000 m above sea‐level. Therefore, the Troodos ophiolite constitutes a suitable research object to understand the formation and alteration of oceanic crust. Subsequent to its formation, the oceanic crust underwent structural and mineralogical changes. This investigation explores the age, chemical and physical conditions of these changes using an elemental and isotopic approach. Seawater entered the oceanic crust through fractures and in cases exchanged elements and isotopes with ambient rocks. Calcium carbonate (calcite) precipitated from these waters at temperatures mostly <50 °C and filled fractures. These structures are termed veins. A few vein calcites formed at temperatures up to ~220 °C and show distinguishable zones that developed automatically without external input. The principal mineralization finished ~10 million years after the onset of oceanic crust formation. Later calcite mineralization, less than 75 million years ago, is characterized by diffusive fluid flow and fibrous calcites. The crystallization of these calcite fibers pushed the rock aside. As a result of their different origins, calcites have specific crystal shapes, distinct elemental and isotopic characteristics. Key Points: The Troodos oceanic crust experienced a 10‐Myr‐long major phase of predominantly low‐temperature blocky and syntaxial calcite veiningOscillatory growth‐zoned high‐temperature blocky vein calcites precipitated from hydrothermal fluids in a closed system out of equilibriumLow‐temperature diffusion‐fed antitaxial vein calcites precipitated after the major veining phase from seawater that interacted with umbers [ABSTRACT FROM AUTHOR]

Calcite veins at outcrop in the Mesozoic, oil-bearing Wessex Basin, UK, have been studied using field characterization, petrography, fluid inclusions and stable isotopes to help address the extent, timing and spatial and stratigraphic variability of basin-scale fluid flow. The absence of quartz shows that veins formed at low temperature without an influence of hydrothermal fluids. Carbon isotopes suggest that the majority of vein calcite was derived locally from the host rock but up to one quarter of the carbon in the vein calcite came from CO2 from petroleum source rocks. Veins become progressively enriched in source-rock-derived CO2 from the outer margin towards the middle, indicating a growing influence of external CO2. The carbon isotope data suggest large-scale migration of substantial amounts of CO2 around the whole basin. Fluid inclusion salinity data and interpreted water-δ18O data show that meteoric water penetrated deep into the western part of the basin after interacting with halite-rich evaporites in the Triassic section before entering fractured Lower and Middle Jurassic rocks. This large-scale meteoric invasion of the basin probably happened during early Cenozoic uplift. A similar approach was used to reveal that, in the eastern part of the basin close to the area that underwent most uplift, uppermost Jurassic and Cretaceous rocks underwent vein formation in the presence of marine connate water suggesting a closed system. Stratigraphically underlying Upper Jurassic mudstone and Lower Cretaceous sandstone, in the most uplifted part of the basin, contain veins that resulted from intermediate behaviour with input from saline meteoric water and marine connate waters. Thus, while source-rock-derived CO2 seems to have permeated the entire section, water movement has been more restricted. Oil-filled inclusions in vein calcite have been found within dominant E-W trending normal faults, suggesting that these may have facilitated oil migration. [ABSTRACT FROM AUTHOR]

We present a structural, microstructural, and stable isotope study of a calcite vein mesh within the Cretaceous Natih Formation in the Oman Mountains to explore changes in fluid pathways during vein formation. Stage 1 veins form a mesh of steeply dipping crack-seal extension veins confined to a 3.5-m-thick stratigraphic interval. Different strike orientations of Stage 1 veins show mutually crosscutting relationships. Stage 2 veins occur in the dilatant parts of a younger normal fault interpreted to penetrate the stratigraphy below. The δ18O composition of the host rock ranges from 21.8‰ to 23.7‰. The δ13C composition ranges from 1.5‰ to 2.3‰. This range is consistent with regionally developed diagenetic alteration at top of the Natih Formation. The δ18O composition of vein calcite varies from 22.5‰ to 26.2‰, whereas δ13C composition ranges from −0.8‰ to 2.1‰. A first trend observed in Stage 1 veins involves a decrease of δ13C to compositions nearly 1.3‰ lower than the host rock, whereas δ18O remains constant. A second trend observed in Stage 2 calcite has δ18O values up to 3.3‰ higher than the host rock, whereas the δ13C composition is similar. Stable isotope data and microstructures indicate an episodic flow regime for both stages. During Stage 1, formation of a stratabound vein mesh involved bedding-parallel flow, under near-lithostatic fluid pressures. The 18O fluid composition was host rock-buffered, whereas 13C composition was relatively depleted. This may reflect reaction of low 13C CO2 derived by fluid interaction with organic matter in the limestones. Stage 2 vein formation is associated with fault-controlled fluid flow accessing fluids in equilibrium with limestones about 50 m beneath. We highlight how evolution of effective stress states and the growth of faults influence the hydraulic connectivity in fracture networks and we demonstrate the value of stable isotopes in tracking changes in fluid pathways. [ABSTRACT FROM AUTHOR]