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HR: 11:35h
TI: Isotopic and Elemental Signatures of the Forearc, and Impacts on Subduction Recycling: Evidence from the Marianas
AU: * Ryan, J G
EM: ryan@chuma.cas.usf.edu
AF: Department of Geology, University of South Florida, 4202 East Fowler Ave., Tampa, FL 33620
AU: Benton, L
EM: lbenton@exponent.com
AF: Department of Terrestrial Magnetism, 5241 Broad Branch Rd, NW, Washington, DC 20015;
Exponent, Inc., 15375 SE 30th Place, Bellevue, WA 98007
AU: Savov, I P
EM: savovip@yahoo.com
AF: Department of Geology, University of South Florida, 4202 East Fowler Ave., Tampa, FL 33620

Trace element and B and Li isotope systematics of serpentinites from the Conical Seamount in the Mariana forearc indicate significant geochemical changes occur in the slab and mantle wedge in the earliest stages of subduction. Elevated K, Rb, Cs, B, Li and possibly Pb and As contents in fluids, clasts, and/or muds from Conical indicate transport of H2O-soluble species off the slab at ~<$30 km depths. Boron contents in Conical clasts correlate broadly with LOI, suggesting continuous inputs of B with progressive hydration. Abundances of Li, Rb and Cs covary mutually, but do not correlate with LOI or other elemental concentrations in Conical clasts. The alkalies may thus reflect a different (later?) stage of slab inputs in which a broader menu of species are mobilized. Boron isotopic systematics of Conical serpentines point to additions of fluid with uniform \delta$^{11}$B, irrespective of fluid-rock ratio. This fluid, at \delta$^{11}$B of +13\permil, is $>10‰ heavier than any subducted component, indicating fractionation of B isotopes during slab fluid releases beneath forearcs. The temperatures of shallow slabs must be cool enough for B isotope fractionations to occur, which is consistent with the observed, low T serpentine mineralogy in Conical samples. The fraction of slab B recycled in the forearc depends on the model for developing high 11B arc lavas: either slab fluids with elevated 11B are released beneath arcs, or a hotter (and presumably lower 11B) fluids from the slab mix with some portion of hydrated, high 11B mantle convected in from shallower depths. Li isotopes in Conical serpentinites indicate more modest, if variable, slab inputs. 7Li variations in these samples (from -0.5 to +11‰) are greater than those observed in probable slab materials, or in most arc lavas, and may indicate the development of Li isotopic heterogeneities in the Mariana forearc mantle, like those suggested by Li isotope variations in "hot" arcs (Tomascak et al., 2000). 7Li in Mariana arc lavas (at +1 to +8.5‰), are more diverse than observed in other arcs, but show less variation than Mariana forearc rocks, supporting contentions that Li from the mantle wedge dilutes the slab input during arc melting.