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.