Boron isotopic fractionation in subducted oceanic crust

Subducted oceanic crust plays an important role in controlling the chemical budget of the crust and mantle and in the composition of arc lavas. Boron isotopes, ¹⁰B and ¹¹B, are strongly fractionated during oceanic-crust dehydration in subduction zones. The temperature-dependent equilibrium boron isotope fractionation depends on boron coordination in the B-hosting minerals and fluids. Two approaches can be employed to quantify boron isotopic fractionation in subducted oceanic crust: modeling based on boron coordination in minerals and fluids, and direct measurement of the boron isotope budget of the devolatilized slab. To address this, simultaneous measurements of major and trace elements, as well as boron isotope ratios were conducted using a split-stream LA-SF-ICPMS setup at the Frankfurt Isotope & Element Research Center (FIERCE) at Goethe Universität Frankfurt.

We investigated the in-situ boron isotope compositions of minerals from reaction zone rocks from the high-pressure (HP) mélange on the island of Syros, which formed at approximately 0.7 Gpa, 415  ±15 °C. The paragenesis tourmaline + phengite + omphacite + glaucophane are in textural equilibrium, which offers the opportunity to determine equilibrium B isotope fractionation among these minerals. The proportions of trigonally and tetrahedrally coordinated B in omphacite and glaucophane can be then estimated from the respective boron isotope fractionation against tourmaline and phengite. It is 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.

Bulk rock elemental abundances and boron isotopic compositions of oceanic metamorphic rocks, from Raspas Complex, Zambezi Belt, Cabo Ortegal Complex, Syros Island, and Tian Shan were measured as well. The boron isotopic composition of almost all samples (approximately -10 to +5 ‰) ranges from δ¹¹B values close to that of fresh MORB to that of typical altered oceanic crust. Our results, thus, demonstrate that B isotopic fractionation in subducted oceanic crust is much smaller than predicted in previous studies.