HR: 0800h
AN: V31C-0628    [Abstracts]
TI: Adakite Induced Metasomatism of the Mantle Wedge: a Systematic Experimental Study at 1.6 GPa
AU: * Rapp, R P
EM: rrapp@notes.cc.sunysb.edu
AF: Observatoire de Physique du Globe de Clermont-Ferrand, Laboratoire Magmas et Volcans, 5 rue Kessler, Clermont-Ferrand ced, 63038 France
AU: Laporte, D
EM: D.Laporte@opgc.univ-bpclermont.fr
AF: Observatoire de Physique du Globe de Clermont-Ferrand, Laboratoire Magmas et Volcans, 5 rue Kessler, Clermont-Ferrand ced, 63038 France
AU: Martin, H
EM: h.martin@opgc.univ-bpclermont.fr
AF: Observatoire de Physique du Globe de Clermont-Ferrand, Laboratoire Magmas et Volcans, 5 rue Kessler, Clermont-Ferrand ced, 63038 France
AB: In order to better understand the origin and nature of the slab-derived agent in subduction zones, and how the arc geochemical signature is imparted to the mantle wedge, it is necessary to have specific geochemical parameters to differentiate between fluids and partial melts generated by dehydration of the crustal component (MORB) of the downgoing oceanic lithosphere. ``Adakite'' magmatism in subduction zones is generally attributed to partial melting of basaltic oceanic crust during prograde metamorphism and dehydration of the subducting slab. In this study, we focus on adakites or ``slab melts'' as the agent of metasomatism of the overlying mantle wedge, armed beforehand with relatively comprehensive major- and trace-element constraints on the geochemical nature of these melts from our previous work. We have conducted a series of melt-rock reaction experiments in the piston-cylinder apparatus at 1.6 GPa and temperatures of 1000-1250°C in which we systematically vary the proportion of adakite melt relative to mantle peridotite. Our starting materials consist of mechanical mixtures of an adakite glass containing ~8 wt% H₂O (pre-hydrated in an internally-heated pressure vessel at 900°C and 2.5 kbar for 3-4 hours), and a natural depleted peridotite powder made from a xenolith from the Kamchatkan sub-arc mantle. Experiments were conducted in thick-walled, pressure-welded gold capsules at temperatures below 1100°C, and in Ag-Pd capsules at higher temeperatures; the adakite melt:peridotite rock ratio has been varied from 2:1 to 1:4, with the mixtures allowed to equilibrate over the course of 5-10 days, depending on run temperature. Our results indicate that the reaction between adakite melt and peridotite is dominated by the consumption of olivine and crystallization of orthopyroxene and amphibole, and takes the general form: melts₁ (pristine adakite) + olivine Ø melt₂ (hybridized adakite) + orthopyroxene + amphibole. The chemical characteristics of metasomatic amphibole in the reaction assemblages progressively shifts towards ``peridotitic clinopyroxene-like'' compositions with increasing temperature, and amphibole appears to be stable to temperatures approaching 1200°C (i.e., temperatures considerably higher than its stability in either basaltic or peridotitic systems). The Mg-number of orthopyroxene in the hybridized, adakite metasomatized assemblages increases with increasing temperature, and decreases as the melt:rock ratio increases. At melt:rock ratios less than 1:2, only small amounts of low Mg-number (0.20-0.30), high SiO₂ content (>70 wt%) melts remain after modal metasomatic reactions that form Mg-rich orthopyroxene and amphibole. It is only after the melt:rock ratio increases above 1:1 that substantial quantities of high Mg-number (>0.60), intermediate SiO₂ liquids (~58-60 wt%) exist in equilibrium with pyroxenite (opx ± cpx ± amph) reaction residues. In terms of their major element characteristics, these liquids resemble natural high-Mg andesites possessing the adakite trace-element geochemical signature. Partial melting of adakite-metasomatized peridotite implies amphibole breakdown at relatively high (i.e., >1200°C) temperatures.
DE: 1012 Reactions and phase equilibria (3612, 8412)
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1065 Major and trace element geochemistry
DE: 3619 Magma genesis and partial melting (1037)
DE: 3630 Experimental mineralogy and petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005