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纪四万十带（Shimanto Belt）的5处混杂岩（mélange）单元，以及美国阿拉斯加科迪亚克增生杂岩（Kodiak Accretionary Complex）中的1处混杂岩单元内的矿物脉体，对其碳、氧同位素组成及相关温度估算展开了系统研究。本研究采用二次离子质谱仪（Secondary Ion Mass Spectrometer，SIMS）对脉体中的石英与方解石完成了原位同位素测试；后续通过该测试获得的δ¹⁸O值，用于计算石英-方解石矿物对最后达到同位素平衡时的温度（即脉体矿物的形成温度）。此外，本研究还对从脉体中提取的碳酸盐样品开展了团簇同位素分析（clumped isotope analyses），并基于碳酸盐同位素团簇数据（Δ47）推导得到相应温度值。值得注意的是，尽管脉体类型存在差异，且各混杂岩单元的最高温度记录各不相同，但通过两种地质温标（geothermometer）得到的温度估算值均处于100~250℃的区间内。该研究结果表明，脉体形成于温度区间小于150℃的脱水过程中，且释出的流体在促进质量转移方面发挥了关键作用。