Tracing Late-Stage Fluid Sources and Vein Formation within Ophiolitic Mélanges from the Indus Suture Zone, Ladakh Himalaya
Aditya KharyaHimanshu K. SachanChristopher J. SpencerKoushik SenDivya PrakashShashi RanjanVikash Kumar
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Abstract:
Quartz-calcite veins in the Zildat ophiolitic mélange (ZOM) and Shergol ophiolitic mélange (SOM) of the Indus Suture Zone preserve a diversity of fluid activity in the late stages of ophiolitic mélange formation. This article presents fluid-inclusion and isotope geochemistry of these veins to understand their source and evolution in terms of pressure and temperature. The microstructures of quartz and calcite veins indicate deformation temperatures between 200° and 400°C. The δ13C and δ18O values of calcite veins from the ZOM and SOM are within the mixing hyperbolas of marine and primitive-mantle fields in the mixing model. The Sr and Pb isotopic values of calcite veins from the ZOM suggest a mid-ocean ridge basalt (MORB) fluid source of vein formation that was radiogenically enriched by metasomatism in a suprasubduction zone. For the SOM, fluids may have originated from the enriched-mantle (EM) and the depleted-MORB-mantle rocks. It is inferred that the carbonic fluids were derived from ultramafic lithologies and oceanic crust that formed the ophiolitic mélange rocks, which also host these veins. These source rocks have EM and MORB geochemical signatures that are also obtained in the quartz-calcite veins, as characterized by their C-O-Sr-Pb isotopic ratios. The magmatic saline fluid is inferred to have formed in the early stages of vein formation and to have been subsequently diluted, as exemplified by the presence of low-saline secondary aqueous inclusions. The microthermometry fluid pressure-temperature estimation of veins from the studied sections suggests that the maximum depth of emplacement of veining fluid was about 24.5 MPa (corresponding to ∼2.5 km) at 336°C. The vein-forming fluids (calcareous and siliceous) were introduced into the fractures that developed in the host as a result of deformation.Keywords:
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Stable isotope compositions (δ13C, δ18O) of calcite within the veins hosted by the Sanbagawa metamorphic rocks in the Nagatoro area, Kanto mountains, Japan gave clues on movement of fluids during exhumation. The host rock calcite shows the nearly uniform δ18O values ranging from +14.5 to +16.5‰ (V-SMOW), and wide δ13C variation from −12.1 to +1.3‰ (V-PDB). The δ13C values of calcite within the exhumation-related veins also have wide variation, and the Δhost-vein (= δ13Chost rock-δ13Cvein) value vary depending on vein thickness and textures. Thin stretched crystal veins contain calcite with δ13C value close to that of the host rock calcite (Δhost-vein < ±2‰). In contrast, calcite grains within the thick blocky veins have homogenous δ13C values with large variations with host rock (Δhost-vein = +3.5 - −3.0‰). The latter types of sealed cracks may have played as dominant pathways of regional fluid flow.
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To determine the opening and precipitation history and characteristics of vein‐forming fluids, analyses of oxygen and carbon isotopes and trace elements were carried out on multilayered crack‐seal calcite veins in the Austin Chalk Formation near San Antonio, Texas. The veins developed within the normal fault zones possessing unique chemical and textural characteristics which indicate sequential vein development. They are composed of alternating millimeter‐ to submillimeter‐thick calcite veinlets and host lithons, occasionally crosscut by coarse, equant‐grained secondary calcite veins. Regular changes in δ 18 O (e.g., −2.6 to −5.6‰, Pee Dee belemnite (PDB)) of the calcite veinlets along the length of veins suggest that the individual calcite veinlets were sequentially developed. A systematic δ 18 O decrease in the vein opening direction primarily resulted from a continuous increase in temperature of the ascending fluids delivered to the Austin Chalk. Relatively constant δ 13 C (approximately +1.4±0.4‰, PDB) for the multilayered veins and most secondary veins indicates that the composition of fluids from which the calcite veins precipitated was initially buffered by the bulk chalk. There is no spatial variation in trace element composition of the calcite veinlets along the length of veins. Low Sr concentrations in both calcite veinlets and secondary veins relative to those of the host chalk reflect a low partition coefficient of Sr in calcite during vein formation. Normal faults in the Cretaceous Austin Chalk were conduits to upwardly mobile vein‐forming fluids.
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ABSTRACT Cultures of known species of fungi placed on crystals of Iceland spar calcite resulted in extensive dissolution of the calcite. This organically mediated dissolution produced large patches of spiky calcite within a period of 253 days. The dissolution of the calcite occurred via surface-reaction-controlled kinetic processes that were mediated by the fungi. This occurred despite the lack of vast quantities of fluids undersaturated with respect to calcite. Locally, at least 10 µm of calcite was removed from the original crystal surface.
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