Boron Isotope Constraints on Fluid-Rock Interactions in the Shallow Megathrust at the Japan Trench

Abstract:

IODP Expedition 343 (JFAST) drilled three holes through the plate boundary near the Japan Trench to investigate the cause of very large fault slip during the 2011 Tohoku-Oki earthquake. Interstitial fluids and rocks within and around the plate-boundary fault were recovered from Hole C0019E. Chemical characteristics of these fluids and rocks provide useful information for understanding fluid-related processes that occurred in the shallow megathrust fault zone at the Japan Trench. In this paper, we report concentrations and isotope ratios of boron determined for JFAST fluids and rocks, as well as for sediments from DSDP site 436, which is a nearby input site.

Depth profiles of B content and B isotope ratio (δ11B value) for the interstitial fluids show a clear minimum and a maximum, respectively, around the plate boundary fault. Fluids from the vicinity of the fault are characterized by lower B content and higher δ11B value compared with seawater. The B contents and δ11B values of the plate-boundary fault rocks are indistinguishable from those of smectite-rich sediments from DPDP site 436, which is consistent with observations for other trace element compositions.

In the systems composed of seawater-like fluid and sediment, both B concentrations and δ11B values in the fluid and solid phases are temperature-sensitive, and higher temperatures result in higher B and lower δ11B in the fluid phase and lower B and lower δ11B in the solid phase. Actually, interstitial fluids from ODP site 808 (Nankai Trough) showed a clear increase and a decrease of B and δ11B, respectively, with increasing depth at temperatures higher than 50 deg. C (You et al., 1995). The ODP site 808 rocks also showed distinct decreases of B and δ11B at the depths with temperatures higher than 100 deg. C (You et al., 1995). The observed B and δ11B characteristics of the JFAST fluids and rocks thus indicate that fluids and rocks within and around the plate-boundary fault have no clear record for fluid-rock interactions at high temperatures, at least for samples analyzed in this study. This provides constraints on the nature of fluid-rock interactions in the shallow megathrust fault zone during the Tohoku-Oki earthquake.

Reference: You et al. (1995) Geology, 23, 37-40.