University of Cambridge > > Department of Earth Sciences Seminars (downtown) > Hydrated peridotite as a sink for boron: implications for mantle heterogeneity and arc volcanism

Hydrated peridotite as a sink for boron: implications for mantle heterogeneity and arc volcanism

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Serpentinites form by hydration of mantle peridotite and constitute the largest potential reservoir of fluid-mobile elements entering subduction zones. Isotope ratios of one such element, boron, can be used to distinguish fluid contributions from crustal versus serpentinite sources. Although up to 85 % of boron hosted within abyssal peridotite is lost at the onset of subduction as a result of the lizardite-to-antigorite transition, a sufficient cargo of boron (c. 7 μg g-1, with a δ11B of +22 ‰) to account for the boron systematics of island arc volcanics is retained until the down-going slab reaches the antigorite-out isograd. At this point a 11B-rich fluid is released that is capable of providing the distinctive δ11B signature of island arc volcanics. Beyond the antigorite-out isograd in serpentinites from Cerro del Almirez, Betic Cordillera, Spain, the prograde lithologies (antigorite-chlorite-orthopyroxene-olivine serpentinite, granofelstexture chlorite-harzburgite and spinifex-texture chlorite-harzburgite) have very different boron isotope signatures (δ11B = -3 to +6 ‰), but with no significant difference in boron concentration compared to the antigorite-serpentinite on the low P-T side of the isograd. Serpentinite dehydration-derived 11B-rich fluid, which at least partly equilibrated with pelagic sediments, is implicated in the composition of these prograde lithologies. In addition, serpentinite-hosted boron lost during the early stages of dehydration is readily incorporated into forearc peridotite. This, in turn, may be dragged to sub-arc depths as a result of subduction erosion and incorporated in a mélange comprising forearc serpentinite, altered oceanic crust and pelagic sediment. At the antigorite-out isograd it dehydrates, thus potentially providing an additional source of 11B-rich fluids. The main carrier of boron beyond the antigorite-out isograd is fluid inclusion-rich prograde olivine, which remains stable to depths far in excess of the sub-arc. This provides a possible mechanism for the delivery of isotopically heterogeneous boron into the convecting mantle, where it may play a role in accounting for the marked differences between the δ11B of the asthenospheric mantle and ocean island basalts

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