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

Abstract

[1] 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.

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“…No significant site-to-site or seamount-to-seamount variability was evident in these major elements or in the variations in Al 2 O 3 , which range from 0.1 to 1.7 wt% with a mean value of ≈0.6 wt% (Table T3). This range is similar to the ranges reported by Savov et al (2005aSavov et al ( , 2005bSavov et al ( , 2007 for serpentinized ultramafic clasts from Conical Seamount and for mudflow matrix and clasts from South Chamorro Seamount.…”

Mantle hydration and Cl-rich fluids in the subduction forearc

“…Serpentinites interacting with large amounts of dehydration fluids close to the subduction interface should have low Cl content (20-100 ppm) while those of the forearc mantle should have a range of concentrations up to several thousand ppm. Obtaining Cl concentrations in serpentinites from in situ forearc context (Savov, et al 2005) or from exhumed forearc (Deschamps, et al 2010) could help resolve this issue.…”

Geochemistry of metasomatized peridotites above subducting slab: a case study of hydrous metaperidotites from Happo-O'ne complex, central Japan

“…3b). These characteristics are due to the mantle metasomatism by slab -derived fluids (e.g., Scambelluri et al, 2004;Savov et al, 2005;Marocchi et al, 2007). It is well known that HFSEs are relatively immobile in aque-Geochemistry of metasomatized peridotites above subducting slab ous fluids but more mobile in slab melts (Audétat and Keppler, 2005 and references therein).…”