SSZ Ophiolite Peridotites as Analogues for Supra-Subduction Zone Mantle

Abstract:

Studies of non-accreting forearcs show that their characteritic rock assemblages are the same as those found many ophiolites. These forearcs are thought to form during subduction initiation, as are supra-subduction zone (SSZ) ophiolites. Thus, the lithospheric mantle that underlies SSZ crust provides in situ samples of the mantle wedge, and allows us to study directly geochemical flux in the mantle wedge during subduction, which is critical to our understanding of the subduction factory process and arc volcanism.

Most peridotites in SSZ ophiolites are harzburgites; lherzolites are important but much less common. The harzburgites are thought to represent >20% hydrous partial melting. Trace element data confirm this, and document REE patterns that are consistent with residues produced by boninite melting. These data also require an early stage of garnet phase melting, followed by continued melting in the spinel field. Lherzolites form by lower degrees of melting (<7%). Trace element data are consistent with spinel-field melting, although minor garnet field melting is permitted. Lherzolites may reflect trapped oceanic lithosphere, but we suggest most were parental to forearc basalts, and escaped continued melting as they moved upward to subsolidus depths.

Because most SSZ peridotites are partially serpentinized, assessing geochemical flux in the mantle wedge requires precision analyses of residual phases by EMPA and high-precision laser ablation ICP-MS. We derived an alogrithm that may be used to calculate the enrichment of FME:

C_wr,add=[C_cpx-obs/[[D_cpx/(D_bulk–PF)]*[1– (PF/D_bulk)]^(1/P)]]–[C_0,wr]

Where C_wr,add = concentration of FME added to mantle wedge, C_cpx-obs= observed pyroxene, D_cpx and D_bulk = mineral and bulk partition coefficients, P=melt proportion, and F=melt fraction.

We use MREE-HREE to assess melt extraction, HFSE to assess melt enrichment, and fluid mobile elements (FME) to assess fluid flux. Our results show that high concentrations of fluid-mobile elements in supra-subduction peridotites result from a continuous flux of aqueous fluid or fluid-rich melt phase derived from the subducting slab, and allow us to quantify the composition of the fluid added. We also show that most lherzolites were exposed to a similar fluid flux, despite their low degree of melting.