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SUMMARY:Hydrated peridotite as a sink for boron: implications for mantle h
 eterogeneity and arc volcanism - Jason Harvey\, University of Leeds
DTSTART:20140211T163000Z
DTEND:20140211T173000Z
UID:TALK49618@talks.cam.ac.uk
CONTACT:John Maclennan
DESCRIPTION:Serpentinites form by hydration of mantle peridotite and const
 itute the largest potential reservoir of fluid-mobile elements entering su
 bduction zones. Isotope ratios of one such element\, boron\, can be used t
 o 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\nlizardite-to-antigorite trans
 ition\, 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 retai
 ned 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 distin
 ctive δ11B signature of island arc volcanics. Beyond the antigorite-out i
 sograd in serpentinites from Cerro del Almirez\, Betic Cordillera\, Spain\
 , the prograde lithologies (antigorite-chlorite-orthopyroxene-olivine serp
 entinite\, granofelstexture chlorite-harzburgite and spinifex-texture chlo
 rite-harzburgite) have very different boron isotope signatures (δ11B = -3
  to +6 ‰)\, but with no significant difference in boron concentration co
 mpared to the\nantigorite-serpentinite on the low P-T side of the isograd.
  Serpentinite dehydration-derived 11B-rich fluid\, which at least partly e
 quilibrated with pelagic sediments\, is implicated in the composition of t
 hese prograde lithologies. \nIn addition\, serpentinite-hosted boron lost 
 during the early stages of dehydration is readily incorporated into forear
 c peridotite. This\, in turn\, may be dragged to sub-arc depths as a resul
 t of subduction erosion and incorporated in a mélange comprising forearc 
 serpentinite\, altered oceanic crust and pelagic sediment. At the antigori
 te-out isograd it dehydrates\, thus potentially providing an additional so
 urce of 11B-rich fluids. The main carrier of boron beyond the antigorite-o
 ut isograd is fluid inclusion-rich prograde olivine\, which remains stable
  to depths far in excess of the sub-arc. This provides a possible mechanis
 m for the delivery of isotopically heterogeneous boron into the convecting
  mantle\, where it may play a role in accounting for the marked difference
 s between the δ11B of the asthenospheric mantle and ocean island basalts\
 n
LOCATION:Harker 1 seminar room\, Department of Earth Sciences
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