Shallow slab fluid release across and along the Mariana arc-basin system: Insights from geochemistry of serpentinized peridotites from the Mariana fore arc

Abstract

Shallow slab devolatilization is not only witnessed through fluid expulsion at accretionary prisms, but is also evidenced by active serpentinite seamounts in the shallow fore-arc region of the Mariana convergent margin. Ocean Drilling Program (ODP) Leg 195 recovered serpentinized peridotites that present a unique opportunity to study the products of shallow level exchanges between the upper mantle and slab-derived fluids. Similar to samples recovered during ODP Leg 125, the protoliths of these fore-arc serpentinized peridotites are mantle harzburgites that have suffered large volume melt extraction (up to 25%) prior to interactions with fluids released from the downgoing Pacific Plate. Samples recovered from both ODP legs 125 and 195 show U-shaped rare earth element (REE) patterns and very low REE abundances (0.001–0.1 chondrites). Relative to global depleted mantle values these rocks typically have 1–2 orders of magnitude lower high field strength elements, REE, Th, and U contents. Interestingly, all fore-arc rocks thus far examined show extreme enrichments of fluid mobile elements (FME: B, As, Cs, Sb, Li). Because the elemental and B, Li, and Sr isotope systematics in these fore-arc serpentinites point to nonseawater-related processes, studies of elemental excesses and anomalous isotopic signatures allow assessment of how much of the subducted inventory is lost during the initial subduction process between 10 and 40 km. On the basis of similar but substantial enrichments of FME in the Mariana fore-arc samples recovered at ODP legs 125 and 195, we report large slab inventory depletions of B ( 75%), Cs ( 25%), As ( 15%), Li ( 15%), and Sb ( 8%); surprisingly low (generally less than 2%) depletions of Rb, Ba, Pb, U, Sr; and no depletions in REE and the high field strength elements (HFSE). Such slab-metasomatized mantle wedge materials may be dragged to depths of arc magma generation, as proposed by Tatsumi (1986) and Straub and Layne (2002) and thus represent an unexplored class of mantle material, different in its origins, physical properties and geochemical fingerprint from mantle rocks traditionally used in modeling a wide range of subduction zone processes.