Reaction between slab-derived melts and peridotite in the mantle wedge: experimental constraints at 3.8 GPa

Laboratory experiments on natural, hydrous basalts at 1-4 GPa constrain the composition of "unadulterated" partial melts of eclogitized oceanic crust within downgoing lithospheric slabs in subduction zones. We complement the "slab melting" experiments with another set of experiments in which these s ame "adakite" melts are allowed to infiltrate and react with an overlying l ayer of peridotite, simulating melt:rock reaction at the slab-mantle wedge interface. In subduction zones, the effects of reaction between slab-derive d, adakite melts and peridotitic mantle conceivably range from hybridizatio n of the melt, to modal or cryptic metasomatism of the sub-are mantle, depe nding upon the ''effective'' melt:rock ratio. In experiments at 3.8 GPa, as similation of either fertile or depleted peridotite by slab melts at a melt :rock ratio similar to 2:1 produces Mg-rich, high-silica liquids in reactio ns which form pyrope-rich garnet and low-Mg# orthopyroxene, and fully consu me olivine. Analysis of both the pristine and hybridized slab melts for a r ange of trace elements indicates that, although abundances of most trace el ements in the melt increase during assimilation (because melt is consumed), trace element ratios remain relatively constant. In their compositional ra nge, the experimental liquids closely resemble adakite lavas in island-are and continental margin settings, and adakite veins and melt inclusions in m etasomatized peridotite xenoliths from the sub-are mantle. At slightly lowe r melt:rock ratios (similar to 1:1), slab melts are fully consumed, along w ith peridotitic olivine, in modal metasomatic reactions that form sodic amp hibole and high-Mg# orthopyroxene. (C) 1999 Elsevier Science B.V. All right s reserved.