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 ex-humed 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 δδ 18 O VPDB and δδ 13 C 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 δδ 13 C 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 δδ 13 C VPDB values. This suggests that crack-seal veins acted as conduits for fluid flow at various stages, and calcite samples with the lowest δδ 13 C VPDB values in an area were deposited during the peak tem-perature stage. The fluid source of calcite may be inferred from δδ 18 O SMOW values and depositional temperature. It is revealed that the calcite veins might be deposited from metamorphic fluid. This im-plies 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.