The fate of boron isotopes in olivine after serpentine dehydration and fluid exposure

Abstract

The isotopic ratio of boron in metamorphic and metasomatic olivine can shed light on its origin and fluid sources, but this data cannot be interpreted without understanding the isotopic fractionation of boron between different phases as a function of physical conditions. In this work we use ab-initio Density Functional Theory to calculate relative isotopic fractionation factors for ¹¹B and ¹⁰B in a variety of phases (β_olivine, β_antigorite and β_fluid) at a variety of pressures and temperatures relevant to dehydration of serpentinites, and derive values for Δ¹¹B_{olivine-fluid} and Δ¹¹B_{olivine-antigorite}. We consider new variables (pressure and boron concentration [B]) and new defect sites in olivine and antigorite. We show that fractionation in olivine and serpentine is complex and cannot be considered solely with trigonal/tetrahedral sites and that the effects of pressure and [B] cannot be discarded. For olivine produced by subduction-related antigorite serpentine dehydration we predict that it largely preserves the original B isotopic composition of the solid and that Δ¹¹B_Ol-Srp is generally close to 0. Δ¹¹B_Ol-fluid is large (> −3 to −17‰) and so exposure to metasomatic fluids can overwrite the B isotopic signature of olivine. Variations in these values are controlled primarily by two interrelated key parameters, fluid pH and depth. Boron isotopic systematics in metamorphic olivine are thus predicted to largely be records of internal and external fluid exposures, and the depth of this exposure. Serpentinite material will largely preserve its B isotope signature during subduction dehydration and potentially introduce anomalous material into the deeper mantle, whilst initial isotopic heterogeneities may be reduced by fluid circulation accompanying serpentinite dehydration.