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The damage behavior of limestone rock masses containing calcite mineral filling under uniaxial compression experimental conditions is unclear, and the fracture mechanism of the rock masses needs to be further explored. In this study, uniaxial compression tests were conducted on limestone rock specimens containing veined calcite by combining acoustic emission and digital image correlation techniques. The effects of veined calcite on the generation and development of cracks on the surface of the specimens until the formation of macroscopic penetration and the strength properties of the rock mass were analyzed. The results showed that the transversely distributed veined calcite caused significant stress concentrations in the rock specimens. The longitudinally distributed veined calcite caused cracks in the specimens or influenced the expansion path of the longitudinal principal cracks. The final damage pattern of the specimens didn’t differ significantly from that of conventional rock masses due to the presence of veined calcite. The presence of the veined calcite had effect on the uniaxial compressive capacity of the rock, but the load variation process of the specimen with time still conformed to the load variation pattern during the uniaxial compressive test of conventional rocks. [ABSTRACT FROM AUTHOR]

Based on the published data of pillow lava-hosted mineralized veins, this study compares post-magmatic fracturing, fluid flow, and secondary mineralization processes in the Troodos and Izu–Bonin supra-subduction zone (SSZ) and discusses the crucial factors for the development of distinct vein types. Thin section and cathodoluminescence petrography, Raman spectroscopy, fluid inclusion microthermometry, and trace element and isotope (87Sr/86Sr, δ18O, δ13C, Δ47) geochemistry indicate that most veins consist of calcite that precipitated from pristine to slightly modified seawater at temperatures < 50 °C. In response to the mode of fracturing, fluid supply, and mineral growth dynamics, calcites developed distinct blocky (precipitation into fluid-filled fractures), syntaxial (crack and sealing), and antitaxial (diffusion-fed displacive growth) vein microtextures with vein type-specific geochemical signatures. Blocky veins predominate in all study areas, whereas syntaxial veins represent subordinate structures. Antitaxial veins occur in all study areas but are particularly abundant in the Izu–Bonin rear arc where the local geological setting was conducive of antitaxial veining. The temporal framework of major calcite veining coincides with the onset of extensional faulting in the respective areas and points to a tectonic control on veining. Thus, major calcite veining in the Troodos SSZ began contemporaneously with volcanic activity and extensional faulting and completed within ~ 10–20 Myr. This enabled deep seawater downflow and hydrothermal fluid upflow. In the Izu–Bonin forearc, reliable ages of vein calcites point to vein formation > 15 Myr after subduction initiation. Therefore, high-T mineralization (calcite, quartz, analcime) up to 230 °C is restricted to the Troodos SSZ. [ABSTRACT FROM AUTHOR]

Serpentinization and calcite precipitation of mantle peridotites exhumed along detachment faults at the slow- to ultraslow-spreading centers can provide important clues to the hydrothermal alteration processes. The Tianxiu hydrothermal field is a new-found active and ultramafichosted hydrothermal vent site along the Carlsberg Ridge, Northwest Indian Ocean. Two types of calcite veins are recognized in serpentinized harzburgite samples collected from the seafloor at the water depth of 3 500 m (3.67°N/63.83°E) and 400 m north of Tianxiu hydrothermal field. Calcite veins I occur in the fractures that cut through mesh texture in the highly serpentinized harzburgite, while calcite veins II precipitate within the mesh texture in the relatively weaker serpentinized harzburgite. Both veins show similar δ13CPDB (+0.54‰ and +0.58‰) but different δ18OPDB(−16.67‰ and +4.46‰) values, suggesting that they were derived from the same carbon source but precipitated at different temperatures. Taking the deep seawater temperature of 2 °C as the precipitation temperature of the calcite veins I, the equilibrium δ18OV-SMOW of calcite-precipitating fluid was calculated to be 1.78‰, which is close to the average δ18OV-SMOW value (1.74‰) of vent fluid samples from the ultramafic-hosted hydrothermal systems worldwide. The formation temperature of calcite veins II is inferred to be approximately 134 °C, based on the calculated δ18OV-SMOW above. The temperature differences of calcite precipitation probably resulted from the fluid cooling conductively and mixing with seawater along the presumed fractures during slow upflow. The low-temperature calcite postdates the mesh texture, while the high-temperature calcite may precipitate under relatively low water/rock ratios, alkaline and reduced conditions among the mesh texture, which is revealed by the geochemical models. Therefore, it is suggested that they both have been influenced by hydrothermal fluids and the sampling site is near the discharge zone of hydrothermal circulation. [ABSTRACT FROM AUTHOR]

The continent is the second largest carbon sink on Earth's surface. With the diversification of vascular land plants in the late Paleozoic, terrestrial organic carbon burial is represented by massive coal formation, while the development of soil profiles would account for both organic and inorganic carbon burial. As compared with soil organic carbon, inorganic carbon burial, collectively known as the soil carbonate, would have a greater impact on the long-term carbon cycle. Soil carbonate would have multiple carbon sources, including dissolution of host calcareous rocks, dissolved inorganic carbon from freshwater, and oxidation of organic matter, but the host calcareous rock dissolution would not cause atmospheric CO2 drawdown. Thus, to evaluate the potential effect of soil carbonate formation on the atmospheric pCO2 level, different carbon sources of soil carbonate should be quantitatively differentiated. In this study, we analyzed the carbon and magnesium isotopes of pedogenic calcite veins developed in a heavily weathered outcrop, consisting of limestone of the early Paleogene Guanzhuang Group in North China. Based on the C and Mg isotope data, we developed a numerical model to quantify the carbon source of calcite veins. The modeling results indicate that 4–37 wt% of carbon in these calcite veins was derived from atmospheric CO2. The low contribution from atmospheric CO2 might be attributed to the host limestone that might have diluted the atmospheric CO2 sink. Nevertheless, taking this value into consideration, it is estimated that soil carbonate formation would lower 1 ppm atmospheric CO2 within 2000 years, i.e., soil carbonate alone would sequester all atmospheric CO2 within 1 million years. Finally, our study suggests the C–Mg isotope system might be a better tool in quantifying the carbon source of soil carbonate. [ABSTRACT FROM AUTHOR]

Calcite veins in carbonate fracture have been investigated by petrographic, fluid inclusion, geochemical analyses and coupled with basin modeling techniques to provide useful insights into fluid activity and deformation conditions of the Cambrian to Triassic Shizhu synclinorium from the western region of Mid-Yangtze, central China. The results of the fluid inclusion microthermometry show a wide range of homogenization temperatures (78.6-215.5 °C) and salinities (0.18-23.11 wt.% NaCl equivalent), indicating the formation under diverse fluid conditions. All the calcite veins have negative Ce anomalies, which are the typical characteristic of marine carbonate sediments; it is therefore plausible that calcite veins were precipitated from the marine basin fluid. The stable carbon iso-topic compositions of calcites ( δ C=-2.5‰-4.26‰) and host limestones ( δ C=-3.56‰-5.80‰) are very similar with a correlation coefficient of 0.86, however, four calcites from the Lower Permian and Lower Triassic show lower δ C values relative to the host limestones, and they are depleted in total REE concentrations (ΣREE ratio varies from 0.74 to 2.06), suggesting the derivation of dissolved carbon from marine carbonates hosting the calcite veins and, less commonly, from the degradation of organic matter. Calculated δ O of the fluids-precipitating calcites ( δ O=-0.41‰-14.42‰), Sr/Sr ratios varying in the range of coeval seawater and the distinct REE pattern simultaneously suggest calcite-forming fluids in each stratigraphic unit could have formed from the involvement of fluids that originated from coeval seawater and evolved through different degrees of water rock interaction. However, the presence of more radiogenic Sr/Sr ratios than coeval seawater and pronounced positive Eu anomalies in calcites of Lower to Middle Ordovician rocks indicate that terrestrial input from upper strata mudstone and siliciclastic rocks could be involved in the precipitation of the Ordovician calcite. Fluid-inclusion data combined with burial and thermal history modeling indicate there was large-scale flow of evolved basinal fluids through the carbonate formation fractures spanning a time frame from 135 to 50 Ma (Early Cretaceous-Eocene). Therefore, the geochemical characteristics of calcite veins can provide the basis for deformation events in Late Yanshanian and Early Himalayan orogeny. [ABSTRACT FROM AUTHOR]

Calcite veins are a common product of hydrothermal fluid circulation. Clumped-isotope palaeothermometry is a promising technique for fingerprinting the temperature of hydrothermal fluids, but clumped-isotope systematics can be reset at temperatures of > ca. 100 °C. To model whether the reconstructed temperatures represent calcite precipitation or closed-system resetting, the precipitation age must be known. LA-ICP-MS U–Pb dating of calcite is a recently developed approach to direct dating of calcite and can provide precipitation ages for modelling clumped-isotope systematics in calcite veins. In this study, clumped-isotope and LA-ICP-MS U–Pb calcite analyses were combined in basalt-hosted calcite veins from three settings in Scotland. Samples from all three localities yielded precipitation temperatures of ca. 75–115 °C from clumped-isotope analysis, but veins from only two of the sites were dateable, yielding precipitation ages of 224 ± 8 Ma and 291 ± 33 Ma (2σ). Modelling from the dated samples enabled confident interpretation that no closed-system resetting had occurred in these samples. However, the lack of a precipitation age from the third location meant that a range of possible thermal histories had to be modelled meaning that confidence that resetting had not occurred was lower. This highlights the importance of coupling clumped-isotope thermometry and LA-ICP-MS U–Pb calcite dating in determining the temperature of hydrothermal fluids recorded in calcite veins. This paired approach is shown to be robust in constraining the timing and precipitation temperature of calcite formation, and thus for tracking hydrothermal processes. [ABSTRACT FROM AUTHOR]