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GEOCHEMISTRY GEOPHYSICS GEOSYSTEMS, VOL. 6, Q04J15, doi:10.1029/2004GC000777, 2005

Geochemistry of serpentinized peridotites from the Mariana Forearc Conical Seamount, ODP Leg 125: Implications for the elemental recycling at subduction zones

Ivan P. Savov

Geology Department, University of South Florida, 4202 East Fowler Avenue, Tampa, Florida, SCA 528, USA


Jeffrey G. Ryan

Geology Department, University of South Florida, 4202 East Fowler Avenue, Tampa, Florida, SCA 528, USA


Massimo D'Antonio

Dipartimento di Scienze della Terra, University Federico II of Napoli, Napoli, Italy
Osservatorio Vesuviano, INGV, Napoli, Italy


Katherine Kelley

Department of Earth Sciences, Boston University, Boston, Massachusetts, USA


Patrick Mattie

Geology Department, University of South Florida, 4202 East Fowler Avenue, Tampa, Florida, SCA 528, USA
Advanced Environmental Technologies, Inc., Albany, Georgia, USA


Abstract

Recent examinations of the chemical fluxes through convergent plate margins suggest the existence of significant mass imbalances for many key species: only 20–30% of the to-the-trench inventory of large-ion lithophile elements (LILE) can be accounted for by the magmatic outputs of volcanic arcs. Active serpentinite mud volcanism in the shallow forearc region of the Mariana convergent margin presents a unique opportunity to study a new outflux: the products of shallow-level exchanges between the upper mantle and slab-derived fluids. ODP Leg 125 recovered serpentinized harzburgites and dunites from three sites on the crests and flanks of the active Conical Seamount. These serpentinites have U-shaped rare earth element (REE) patterns, resembling those of boninites. U, Th, and the high field strength elements (HFSE) are highly depleted and vary in concentration by up to 2 orders of magnitude. The low U contents and positive Eu anomalies indicate that fluids from the subducting Pacific slab were probably reducing in nature. On the basis of substantial enrichments of fluid-mobile elements in serpentinized peridotites, we calculated very large slab inventory depletions of B (79%), Cs (32%), Li (18%), As (17%), and Sb (12%). Such highly enriched serpentinized peridotites dragged down to depths of arc magma generation may represent an unexplored reservoir that could help balance the input-output deficit of these elements as observed by Plank and Langmuir (1993, 1998) and others. Surprisingly, many species thought to be mobile in fluids, such as U, Ba, Rb, and to a lesser extent Sr and Pb, are not enriched in the rocks relative to the depleted mantle peridotites, and we estimate that only 1–2% of these elements leave the subducting slabs at depths of 10 to 40 km. Enrichments of these elements in volcanic front and behind-the-front arc lavas point to changes in slab fluid composition at greater depths.

Received 15 June 2004; accepted 21 January 2005; published 7 April 2005.

Keywords: serpentinite; forearc; mantle; Marianas; subduction; Ocean Drilling Program.

Index Terms: 1031 Geochemistry: Subduction zone processes (3060, 3613, 8170, 8413); 1025 Geochemistry: Composition of the mantle; 3036 Marine Geology and Geophysics: Ocean drilling; 1030 Geochemistry: Geochemical cycles (0330); 8426 Volcanology: Mud volcanism.

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Citation: Savov, I. P., J. G. Ryan, M. D'Antonio, K. Kelley, and P. Mattie (2005), Geochemistry of serpentinized peridotites from the Mariana Forearc Conical Seamount, ODP Leg 125: Implications for the elemental recycling at subduction zones, Geochem. Geophys. Geosyst., 6, Q04J15, doi:10.1029/2004GC000777.