Isotopic variation of syn-tectonic calcite veins formed in stress regimes located in subducted plates boundary-implication of geofluids evolution with seismic cycle in Mugi mélange

Abstract

In seismic cycle, the stress state changes with geofluids evolution under the variation of P-T condition, geofluid sources participation, elements composition or physical-chemical properties, etc.The Mugi mélange is one of the most well studied on-land tectonic mélanges along the interface of a subducting plate. Hosokawa and Hashimoto, (2022) firstly revealed different tectonic stress regimes with fluid pressure ratio (λ) in seismic cycles where multiple calcite-quartz veins are located in. In this study, we analyzed the carbon-oxygen isotope of calcite veins collected from varied tectonic stress regimes (Hosokawa and Hashimoto, 2022) in the mélange side near the boundary between mélange and altered basalt. It shows carbon isotope ratio of those three type veins are in constant range (-14.5‰ - -17‰, V-PDB), but obvious difference range in oxygen isotope ratio between reverse stress vein (16‰-17.5‰, SMOW, λ~1.1) and normal stress vein (17.5‰-19‰, SMOW, λ~0.9). Moreover, C-O isotopic ratios of the veins all have positive correlation trends with slope about 0.6. The carbon specie in geofluids where the multiple calcite veins precipitated from was H₂CO₃ (CO₂) and the C-O isotopic evolution trend were caused only by temperature with homogeneous geofluid. The variation of C-O isotopic ratios indicates that isotopic composition of geofluids was different between the stress states, suggesting that the multiple periods of geofluids in the seismic cycle are not only contributed by local fluids but also the joint of external geofluids. Patterns of rare earth elements with Eu anomaly in those veins were obvious in the previous study, which implies that the external fluid could be with higher temperature (over 200 ℃) and different pH or f_O2 properties than local fluids. Moreover, the external geofluids join in the stress evolution process may be a factor which caused overpressure in seismic cycles in Mugi mélange. Δ47 of calcite and oxygen isotope of contemporaneous quartz can be effective for further understandings of geofluid evolution in seismic cycle with tectonic stress change.References: Hosokawa and Hashimoto. 2022. Scientific Reports, 12, 14789.