Boron isotope geochemistry of metasedimentary rocks and tourmalines in a subduction zone metamorphic suite

Abstract

In order to understand the behavior of boron (B) and its isotope fractionation during subduction zone metamorphism, B contents and isotopic compositions together with major element compositions were determined for metasedimentary rocks and tourmalines from the Sambagawa Metamorphic Belt, central Shikoku, Japan. No systematic changes in whole-rock B content and isotope composition of the metasediments were observed among the different metamorphic grades, indicating the lack of a bulk fluid-rock B isotope fractionation as a result of devolatilization. Both modal abundance and grain size of tourmaline increase with increasing metamorphic grade. In contrast, B contents in muscovite and chlorite decrease with increasing metamorphic grade. These observations combined with mass balance calculations of B suggest the formation of tourmaline during progressive metamorphism from metamorphic fluids containing B mainly derived from muscovite and subordinately from chlorite without allowing significant net removal of B from the metasedimentary rocks. Tourmalines in the higher-grade metasedimentary rocks have zonal structure of B isotope and major element composition with decreasing δ 11 B and increasing Mg/(Mg+Fe) from the inner rim (core) to the outer rim. The change of Mg/(Mg+Fe) in the tourmalines with increasing grade is paralleled by similar variation in chlorite. These observations suggest that the growing tourmalines record the progressive evolution of the B isotopic composition of the metamorphic fluid , in the outermost rims preserving the isotope signature of peak metamorphic P– T-fluid conditions. Based on the above observations, the δ 11 B of the tourmaline is thought to have been nearly identical to that of the metamorphic fluid resulting in the “apparent” B isotopic fractionation factor between metamorphic fluid and whole-rock ( α=( 11 B/ 10 B) fluid /( 11 B/ 10 B) whole- rock ) which decreases from 1.007±0.003 to 1.001±0.003 from chlorite to biotite zone metamorphism. Such results together with the formation of tourmaline from (and sequestering of) B in metamorphic fluids may lead to less B isotopic fractionation as a result of subduction zone devolatilization than noted in suites containing less tourmaline. This, therefore, makes it possible to transport B isotopic signatures, which ultimately reflect Earth’s surface materials, to the deep mantle, perhaps resulting in mantle B isotope anomalies near convergent margins.