BORON ISOTOPE FRACTIONATION REVEALS THE INTERCONNECTION BETWEEN WATER MOBILITY AND CALCIUM DISTRIBUTION IN SUBDUCTED OCEANIC CRUST FROM ALPINE CORSICA

Water, carbon, and fluid-mobile elements are cycled through subduction zones by means of fluid-rock interactions, including (de)hydration, (de)carbonation, and a variety of fluid transport mechanisms. Boron is a light, fluid-mobile element that is isotopically fractionated during dehydration, making it a useful tracer of fluid-rock interactions during subduction. Although a progressive decrease in boron isotope ratios with metamorphic grade is predicted and broadly observed, few coherent sample sets exist that allow direct observation of coupled dehydration and isotopic evolution along the prograde path in a subducted slab. To quantitatively link dehydration to B isotopic fractionation, we have analyzed a suite of progressively metamorphosed pillow basalts from Alpine Corsica. We find systematic boron loss and isotopic fractionation going from [B] = 24 ppm and δ¹¹B = 5‰ in prehnite-pumpellyite facies pillow cores to [B] = 2 ppm and δ¹¹B = -4‰ in eclogite-facies cores; similar trends are observed in pillow mantles and rims that underwent seafloor alteration prior to metamorphism.

It has been shown that the depths at which slab dehydration reactions occur depend on both pressure-temperature-time path and lithology. In models based on altered oceanic crust (AOC) compositions, the primary host of both water and boron at eclogite-facies conditions is phengite. However, in the Corsican sample suite, we observe two significant major element changes with increasing grade: i) open-system K₂O loss between prehnite-pumpellyite facies and blueschist-facies conditions, resulting in the near-complete loss of phengite from the system, and ii) decarbonation and calcite loss in the pillow interiors resulting in calcium metasomatism, which stabilizes lawsonite and omphacite at the expense of glaucophane. Boron is less readily incorporated in lawsonite than in phengite, which promotes boron loss and isotopic fractionation even as little water is lost from the system. We conclude that prograde K₂O loss, coupled with the presence or absence of calcite in altered oceanic crust protoliths, may impact the relative stability of lawsonite vs. phengite – which in turn will have significant implications for the behavior of water, boron, and other “slab-derived” elements during subduction and dehydration.