The redistribution of B concentration and its isotopes during low-grade metamorphism: Observations in metapelites from the Central Range, Taiwan

Abstract

The Plio-Pleistocene orogeny resulted in the exhumation of a regional metamorphic belt, comprising low- to high-grade metapelites in the Central Range, Taiwan. In this study, we analyze the distributions of B isotopes along a prograde metamorphic profile (argillite to greenschist) in the western Central Range to constrain the effects of metamorphic fluids generated during dehydration on the redistribution of the mobile elements. From west to east, peak metamorphic temperatures increase from ~200 °C to 320 °C. B concentrations and δ11B values generally decrease to the east with ~50% loss of B, and the δ11B values roughly decrease from −5‰ to −15‰. The X-ray diffraction (XRD) analysis results of these mineral compositions, except for calcite, show no correlation with B contents and δ11B. And the mixing monitoring of mineralogical compositions cannot account for the observed B contents and δ11B coherent and W-E distribution trend. As the B isotopes may be fractionated between rock and fluid during dehydration, a Rayleigh distillation equation was applied into a model under similar conditions. The results indicate that the measured δ11B values are consistent with the modeled equilibrium of fluid-rock B isotope fractionation within the constrained temperature ranges, thereby supporting the hypothesis that dehydration processes act as a major control on the distribution of B isotopes in the low-grade metapelites. This model also estimates a δ11B value of 5.8‰ for the integrated expelled fluids when getting totally 50% loss of B. Any B sequestering by tourmalines in meta-sediment is an important factor to retain B to a greater depth in subducting slabs. However, our results suggest this factor is not significant for the low-grade metapelites. This study constrains the effect of dehydration on B flux and B isotope distributions during low-grade metamorphism and how the heterogenetic mineralogical compositions may affect its variations, and may contribute to a better understanding of B mass cycling in subduction environments.