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

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• Jason Harvey, University of Leeds
• Tuesday 11 February 2014, 16:30-17:30
• Harker 1 seminar room, Department of Earth Sciences.

If you have a question about this talk, please contact John Maclennan.

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

This talk is part of the Department of Earth Sciences Seminars (downtown) series.

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