Melt– and Fluid–Rock Interaction in Supra-Subduction Lithospheric Mantle: Evidence from Andesite-hosted Veined Peridotite Xenoliths

We report petrographic, major and trace element data for xenoliths from the andesitic Avacha volcano (Kamchatka), which host orthopyroxene (opx)-rich veins of mantle origin formed either by rapid crystallization of intruded melts or by their interaction with the host harzburgite. Studies of such veins may give better insights into sub-arc mantle processes (in particular on a millimetre to centimetre scale) than those of (1) arc xenoliths that do not preserve solidified initial metasomatizing agents, (2) massif peridotites, probably modified during their emplacement, or (3) arc magmatic rocks, which provide indirect information. We seek to trace the evolution of these agents as they react with the host peridotite and to assess their impact on the wall-rocks. The veins cross-cut spinel harzburgite and consist mainly of opx with minor olivine, clinopyroxene (cpx) and/or amphibole. We identify 'rapidly crystallized' veins that cut wall-rock olivine without petrographic evidence of reaction, and 'reactive' veins subdivided into 'thick' (0*5-1 mm) and 'thin' (<€0*5 mm). Minerals in the rapidly crystallized veins are depleted in rare earth elements (REE) and high field strength elements (HFSE) and enriched in fluid-mobile elements relative to REE. Minerals in the reactive veins have higher Ti, Al, Cr and alkalis than minerals in the rapidly crystallized veins, as well as highly variable trace element abundances, especially in reaction zones, thin veins and related metasomatic pockets in the host peridotite. They commonly show U-shaped REE patterns and positive Zr and Hf spikes in normalized trace element patterns. Our data, supported by recent reports, show that the rapidly crystallized veins formed between 1200°C and 900°C from a liquid derived by fluid-fluxed melting of a refractory (harzburgitic) mantle source depleted in heavy REE. The reactive veins formed via 'fractionation-reactive percolation' from fractionated hydrous derivatives of the melts that precipitated the rapidly crystallized veins. These liquids re-equilibrated with the host through diffusion and fluid-assisted dissolution-precipitation reactions, whose end-products are thin reactive veins and metasomatic pockets with distinctive U-shaped REE patterns and Zr-Hf spikes. Some Avacha xenoliths contain veins of Fe-rich amphibole deposited from the host magma that penetrated fractures in the peridotite fragments during their transport to the surface. These products of contamination were mistakenly attributed to mantle metasomatism in previous studies of other suites of Avacha mantle xenoliths. Trace element abundances in such veins are higher than for reactive veins of mantle origin, but both have similar trace element patterns (U-shaped REE patterns and Zr-Hf spikes) suggesting that 'fractionation-reactive percolation' also took place during their formation and is common during interaction of refractory peridotites with percolating melts and fluids. Metasomatic pockets of cpx and amphibole replacing coarse opx and spinel in the host peridotites commonly occur in the vicinity of fractures that lead to reactive opx-rich veins. The cpx and amphiboles in the pockets show progressive depletion in middle REE and HFSE at constant light REE and/or large ion lithophile elements away from the veins towards the host. This indicates that residual hydrous fluids expelled from the source veins enriched the wall-rock peridotites in incompatible elements but were progressively modified by reaction with the host with increasing percolation distance. This process produces disseminated pockets of metasomatic minerals with a broad range of compositions from a single initial liquid and strongly affects the trace element budgets of the harzburgite xenoliths from Avacha. We show that melts and fluids are likely to undergo profound transformation as they travel through and react with the refractory host mantle, even on a millimetre to centimetre scale. The composition of the initial metasomatizing agents can only be inferred from the composition of the metasomatic phases in mantle rocks combined with appropriate partition coefficients if these phases come from well-equilibrated mineral assemblages located close to melt and/or fluid sources.