Fluid source of seismogenic accretionary body based on calcite isotope ratio, Shimanto complex, Shikoku, southwest Japan

Abstract

The Shimanto accretionary complex in Shikoku, which was exhumed from seismogenic depths, contains many calcite veins. These veins may be studied to better understand the origin of various fluids in the deep sections of a subduction zone. For example, fluid sources may be inferred from comparison of the isotopic ratio of the calcite and its depositional temperature. This study obtained δ¹⁸O_VPDB and δ¹³C_VPDB measurements from 74 samples of calcite veins in the Yokonami Melange and surrounding rocks, which had been exposed to peak temperatures of 140°C to 250°C, comparable to seismogenic thermal conditions in the Nankai subduction zone.

The study results show that the isotopic ratio of calcite is related to the paleo-temperature of the host rock. The lowest δ¹³C_VPDB value for calcite from several measurements obtained for a single location is dependent on the paleo-temperature of the host rock. In addition, the calcite samples from the higher paleo-temperature areas have the lowest δ¹³C_VPDB values. This suggests that crack-seal veins acted as conduits for fluid flow at various stages, and calcite samples with the lowest δ¹³C_VPDB values in an area were deposited during the peak temperature stage. The fluid source of calcite may be inferred from δ¹⁸O_SMOW values and depositional temperature. It is revealed that the calcite veins might be deposited from metamorphic fluid. This implies that in-situ fluid flows within the main body of the accretionary prism, free of exotic fluid from shallow or deep sections along the plate boundary fault.