Fluid-mobile Element Systematics of Ultramafic Xenoliths from the Izu-Bonin-Mariana Forearc:Implications for the Chemical Cycling in Subduction Zones.

ODP Leg 125 recovered serpentinized harzburgites and dunites from a total of three sites on the crest and flanks of the Conical Seamount in the Mariana forearc region. The geochemistry of these ultramafic rocks reveal fluid-mediated mass transport occurring in the shallow mantle wedge beneath convergent margins.

We report here geochemical results on highly serpentinized ultramafic clasts from Leg 125, Site 779A. Our new data demonstrate that mobilization of B, Cs, Rb, Li and Pb via low-temperature slab derived hydrous fluids is greatly enhanced relative to other incompatible lithophile elements (LILE’s; K, Sr, LREE, Th, U; Be). This pattern points to a minimal role for complexation processes involving Cl, F or S, at least at forearc depths. The elements enriched in the Hole 779A serpentinites seem exclusively to be from the suite of "fluid-mobile" elements, all of which show abundance variations in arc volcanic rocks and subduction complex metamorphic massifs that are strongly related to slab temperature.

The observed elemental enrichments in the Mariana Forearc serpentinites (B ~ 17 ppm, Cs ~ 0.14 ppm, Rb ~ 0.33 ppm, Li ~ 3.5 ppm, and Pb ~ O.16 ppm, all highly elevated relative to normal mantle values) demonstrate that these elements are not efficiently subducted to the depths of the volcanic roots beneath arcs. Our data, along with existing O, H, and B isotope data (Benton et al, 2000) confirm that the serpentinizing fluids rising in the Conical Seamount are slab derived, and that a significant return flux exists through the Mariana forearc mantle wedge. In the case of the boron cycle, it may account for more than half of the subducted inventory. Especially for the fluid-mobile elements it is impossible to model the chemical cycle of subduction zones without accounting for potentially large forearc elemental fluxes.