Boron Isotopic Fractionation During the First ~50 km of Sediment Subduction in the Nankai Trough, Japan

Abstract:

Subduction zones are the primary locus for recycling of crustal material into the Earth’s mantle, with important implications for mantle and crustal evolution. Subducted sediments contribute volatiles, trace elements, and unique isotopic signatures to arc magmas and some mantle domains. While some elements appear to be conservative during sediment subduction, others may be mobilized and isotopically fractionated during the first several tens of kilometers of subduction - well before reaching sub-arc depths. Characterization of the geochemical processes occurring in this early stage of subduction is relatively limited and largely based on the compositions of fluids expelled from the accretionary prism. In order to better understand the life cycle of boron as it is processed through the subduction system, B concentrations and isotope ratios were measured in a suite of shales from the Shimanto Belt, Shikoku Island, Japan. These shales represent pelagic and neritic sediments from the Nankai Trough that have been partially subducted, underplated, and exhumed. As a counterpoint to the mobile and potentially fractionated boron, Pb isotopes (which are not expected to be fractionated by shallow subduction processes) were used to ensure that the sediments studied were derived from a homogeneous source. Peak temperatures of 140-280˚C are constrained by offshore vitrinite reflectance studies. We find that δ¹¹B in the subducted sediment ranges from -6.6 to -9.9‰, with a negative correlation between δ¹¹B and temperature. In contrast, B concentrations show no systematic relationship with temperature. Measured δ¹¹B of -9.7 in the Sanbagawa schist, possibly a high-pressure-temperature analog of the Shimanto shale, is consistent with previous studies. However, ²⁰⁸Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb ratios indicate that the pelitic schist analyzed in this study was not derived from the same source rock as the Shimanto shale. Further work is needed to determine if there exists an appropriate analog within the Sanbagawa schist for high-temperature (300-500˚C) evolution of the Shimanto shale.