11B-rich fluids in subduction zones

the role of antigorite dehydration in subducting slabs and boron isotope heterogeneity in the mantle

Jason Harvey*, Carlos J. Garrido, Ivan Savov, Samuele Agostini, Jose Alberto Padron-Navarta, Claudio Marchesi, Vicente Lopez Sanchez-Vizcaino, Maria Teresa Gomez-Pugnaire

*Corresponding author for this work

Research output: Contribution to journalArticle

39 Citations (Scopus)

Abstract

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, distinguish fluid contributions from crustal versus serpentinite sources. Despite 85% of boron hosted within abyssal peridotite being lost at the onset of subduction at the lizardite-to-antigorite transition, a sufficient cargo of boron to account for the composition of island arc magma is retained (c. 7μgg -1, with a δ 11B of +22‰) until the down-going slab reaches the antigorite-out isograd. At this point a 11B-rich fluid, capable of providing the distinctive δ 11B signature of island arc basalts, is released. Beyond the uniquely preserved antigorite-out isograd in serpentinites from Cerro del Almirez, Betic Cordillera, Spain, the prograde lithologies (antigorite-chlorite-orthopyroxene-olivine serpentinite, granofels-texture chlorite-harzburgite and spinifex-texture chlorite-harzburgite) have very different boron isotope signatures (δ 11B=-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. 11B-rich fluid, which at least partly equilibrated with pelagic sediments, is implicated in the composition of these prograde lithologies, which dehydrated under open-system conditions. 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.

Original languageEnglish
Pages (from-to)20-30
Number of pages11
JournalChemical Geology
Volume376
DOIs
Publication statusPublished - 29 May 2014
Externally publishedYes

Keywords

  • Boron
  • Isotopes
  • Subduction
  • Antigorite dehydration
  • Island arc volcanics
  • Serpentinite
  • MID-ATLANTIC RIDGE
  • CERRO-DEL-ALMIREZ
  • NEVADO-FILABRIDE COMPLEX
  • MARIANA FORE-ARC
  • HIGH-PRESSURE DEHYDRATION
  • ODP LEG 209
  • MOBILE ELEMENTS
  • OCEANIC-CRUST
  • SE SPAIN
  • ULTRAMAFIC ROCKS

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