Temperatures of Vein Formation Associated with Plate Interface Deformation Constrained by Oxygen and Clumped Isotope Thermometry

In this study, we investigate carbon and oxygen isotopic composition and temperature estimates associated with mineral veins within five mélange units of the Cretaceous Shimanto belt in Japan and one within the Kodiak accretionary complex in Alaska. In-situ isotopic measurements of quartz and calcite in the veins were performed using the Secondary Ion Mass Spectrometer (SIMS). The δ18O values obtained from SIMS measurements were subsequently employed to calculate the temperature at which isotopic equilibrium was last achieved between the quartz-calcite pairs (i.e., the temperature at which the vein minerals formed). Additionally, clumped isotope analyses were conducted on carbonate samples extracted from the veins. Temperatures were then derived from carbonate isotopic clumping (Δ47). Remarkably, temperature estimates obtained from both geothermometers fell within the range of 100 to 250°C, despite the variation in vein types and distinct maximum temperature records of each mélange. This finding suggests that veins formed in response to dehydration processes operating over a temperature range of less than 150°C, with liberated fluids playing a crucial role in facilitating mass transfer.本研究针对日本白垩系四万十带（Cretaceous Shimanto belt）的5个混杂岩（mélange）单元，以及阿拉斯加科迪亚克增生复合体（Kodiak accretionary complex）内的1个混杂岩单元中的矿脉，开展了碳、氧同位素组成分析与相关温度估算工作。研究采用二次离子质谱仪（Secondary Ion Mass Spectrometer, SIMS）对矿脉中的石英与方解石完成原位同位素测试，基于测试获得的δ¹⁸O值，计算得到石英-方解石同位素对末次达到平衡时的温度（即矿脉矿物的形成温度）。此外，研究还对从矿脉中提取的碳酸盐样品开展了团簇同位素分析，并通过碳酸盐团簇同位素Δ47推导得到形成温度。值得注意的是，尽管矿脉类型存在差异，且各混杂岩单元的最高温度记录各不相同，但两种地质温标得到的温度估算值均处于100~250℃区间内。这一发现表明，矿脉形成于温度范围小于150℃的脱水作用过程中，释放的流体在促进物质迁移方面发挥了关键作用。