Boron coordination in omphacite and glaucophane derived from inter-mineral B isotope fractionation in natural rocks

Abstract Boron coordination in omphacite and glaucophane was determined by indirect means through the B isotope fractionation among coexisting minerals. Samples from tourmaline-bearing reaction zones in the high-pressure metamorphic mélange on the island of Syros were investigated. These metasomatic zones formed at ∼0.7 GPa, 415 ± 15 °C. Equilibrium B isotope fractionations among paragenetic dravite, phengite, omphacite, and glaucophane were determined by in situ B isotope analyses using laser ablation multi-collector ICP-MS. The proportions of trigonally and tetrahedrally coordinated B in omphacite and glaucophane were estimated by comparing their boron isotope fractionation relative to dravite (94.6% [3]B and 5.4% [4]B) and phengite (100% [4]B). The B isotope fractionation of omphacite and phengite against dravite are −12.0 ± 1.2‰ and −13.4 ± 2.2‰, respectively, where δ11B values of omphacite are 1.9 ± 1.6‰ higher than in coexisting phengite. No significant isotopic difference was observed between glaucophane and phengite. Consequently, we concluded that in clinopyroxene (omphacite), 88 ± 9% of boron is incorporated in tetrahedral coordination, for example via the B(F,OH)Si−1O−1 substitution, with the remaining 12 ± 9% entering by replacement of SiO4 tetrahedra with BO3 triangles. In contrast, B in glaucophane is exclusively incorporated in the tetrahedrally coordinated sites. The boron coordination in high-pressure minerals that host boron in subducting rocks, such as clinopyroxene, amphibole, and mica, is relevant for modeling B isotope fractionation during slab dehydration in subduction zones.