Boron isotope fractionation during oceanic-crust dehydration in subduction zones: boron coordination in omphacite

Boron has two stable isotopes, ¹⁰B and ¹¹B, which are strongly fractionated during geological processes. They have been widely used to trace fluids in subduction zones. The temperature-dependent equilibrium boron isotope fractionation depends on boron coordination in the B-hosting minerals and fluids. In blueschist- and eclogite-facies high-pressure metamorphic rocks, omphacite (Cpx), amphibole and white mica are the dominant hosts of B. Yet, different crystallographic mechanisms of B substitution in Cpx have been proposed with first-order implications for B isotope fractionation during slab dehydration and eclogite formation. Hence, clarification of B coordination in clinopyroxene is desired, but a direct determination of boron coordination in silicates at the trace-element level is not technically possible. In this study, we have thus determined the B coordination in omphacite, glaucophane and mica by indirect means through the investigation of the B isotope fractionation in natural rocks.

We investigated a set of six different tourmaline-bearing reaction zone rocks from the high-pressure (HP) mélange on the island of Syros formed at approximately 0.7 GPa, 430 °C. The rocks show the paragenesis tourmaline + phengite + omphacite + glaucophane 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, glaucophane and phengite was then estimated from the respective boron isotope fractionation against tourmaline. The B isotope fractionation between phengite and tourmaline is -14.7 ±0.6 ‰, and -12.4 ±0.8 ‰ between omphacite and tourmaline. B isotope composition in omphacite is 2.5 ±1.6 ‰ heavier than in phengite. No significant difference was found between glaucophane and phengite. From these results, we conclude that boron in omphacite is dominantly in tetrahedral coordination (84 ±6 % of the total B) with a minor amount of B in trigonal coordination (16 ±6 %).