Metamorphic olivine records external fluid infiltration during serpentinite dehydration
E De C Clarke, Jan C.M. De Hoog, Linda A. Kirstein, Jason Harvey, B.P.R. Debret
Abstract
We used boron (B) isotope systematics of co-existing olivine and serpentine to study deep fluid flow in subduction zones. Metamorphic olivine produced by serpentine dehydration at sub-arc conditions from high pressure ophiolites in the Western Alps contains significant concentrations of B (2-30 g/g) with a high 11 B values (9 to 28 ), whilst co-existing serpentine has 2-50 g/g B with 11 B = 6 to 24 . Boron isotope fractionation between olivine and its precursor serpentine ( 11 B ol-srp = 11 B ol - 11 B srp ) is highly variable, which indicates significant isotopic disequilibrium between these minerals. Importantly, samples with B-enriched olivine have low 11 B ol-srp (down to -9 ), evidence that olivine grew in the presence of a mixture of serpentine-derived fluids and external fluids with 11 B of ca. 6 to 15 . The composition of these external fluids is consistent with those from subducting sediments and altered oceanic crust at 50-80 km depth, and at least 15-45 % fluid addition. Our work shows that large scale slab fluid infiltration and fluid-mobile element transport accompanies serpentinite dehydration in subduction zones.