Fluids and melts liberated from subducting oceanic crust recycle lithophile elements back into the mantle wedge, facilitate melting and ultimately lead to prolific subduction-zone arc volcanism. The nature and composition of the mobile phases generated in the subducting slab at high pressures have, however, remained largely unknown. Here we report direct LA-ICPMS measurements of the composition of fluids and melts equilibrated with a basaltic eclogite at pressures equivalent to depths in the Earth of 120-180 km and temperatures of 700-1,200 degrees C. The resultant liquid/mineral partition coefficients constrain the recycling rates of key elements. The dichotomy of dehydration versus melting at 120 km depth is expressed through contrasting behaviour of many trace elements (U/Th, Sr, Ba, Be and the light rare-earth elements). At pressures equivalent to 180 km depth, however, a supercritical liquid with melt-like solubilities for the investigated trace elements is observed, even at low temperatures. This mobilizes most of the key trace elements (except the heavy rare-earth elements, Y and Sc) and thus limits fluid-phase transfer of geochemical signatures in subduction zones to pressures less than 6 GPa.

Results of high-pressure experiments on samples of hydrated mantle rocks show that the serpentine mineral antigorite is stable to approximately 720 degrees C at 2 gigapascals, to approximately 690 degrees C at 3 gigapascals, and to approximately 620 degrees C at 5 gigapascals. The breakdown of antigorite to forsterite plus enstatite under these conditions produces 13 percent H(2)O by weight to depths of 150 to 200 kilometers in subduction zones. This H(2)O is in an ideal position for ascent into the hotter, overlying mantle where it can cause partial melting in the source region for calc-alkaline magmas at a depth of 100 to 130 kilometers and a temperature of approximately 1300 degrees C. The breakdown of antigorite in hydrated mantle produces an order of magnitude more H(2)O than does the dehydration of altered oceanic crust.

No abstract

Differentiation of mantle-derived, hydrous, basaltic magmas is a fundamental process to produce evolved intermediate to SiO 2-rich magmas that form the bulk of the middle to shallow continental and island arc crust. This study reports the results of fractional crystallization experiments conducted in a piston cylinder apparatus at 0.7 GPa for hydrous, calc-alkaline to arc tholeiitic magmas. Fractional crystallization was approached by synthesis of starting materials representing the liquid composition of the previous, higher temperature experiment. Temperatures ranged from near-liquidus at 1,170°C to near-solidus conditions at 700°C. H 2 O contents varied from 3.0 to more than 10 wt%. The liquid line of descent covers the entire compositional range from olivine-tholeiite (1,170°C) to high-silica rhyolite (700°C) and evolves from metaluminous to peraluminous compositions. The following crystallization sequence has been established: olivine ? clinopyroxene ? plagioclase, spinel ? orthopyroxene, amphibole, titanomagnetite ? apatite ? quartz, biotite. Anorthite-rich plagioclase and spinel are responsible for a marked increase in SiO 2-content (from 51 to 53 wt%) at 1,040°C. At lower temperatures, fractionation of amphibole, plagioclase and Fe-Ti oxide over a temperature interval of 280°C drives the SiO 2 content continuously from 53 to 78 wt%. Largest crystallization steps were recorded around 1,040°C and at 700°C. About 40 % of ultramafic plutonic rocks have to crystallize to generate basaltic-andesitic liquids, and an additional 40 % of amphibole-gabbroic cumulate to produce granitic melts. Andesitic liquids with a liquidus temperature of 1,010°C only crystallize 50 % over an 280°C wide range to 730°C implying that such liquids form mobile crystal mushes (\50 % crystals) in long-lived magmatic systems in the middle crust, allowing for extensive fractionation, assimilation and hybridization with periodic replenishment of more mafic magmas from deeper magma reservoirs. Keywords Liquid line of descent Á Fractional crystallization Á Calc-alkaline magmas Á Mid-crustal magma reservoirs Communicated by T. L. Grove.

To evaluate the role of garnet and amphibole fractionation at conditions relevant for the crystallization of magmas in the roots of island arcs, a series of experiments were performed on a synthetic andesite at conditions ranging from 0.8 to 1.2 GPa, 800-1,000°C and variable H 2 O contents. At water undersaturated conditions and fO 2 established around QFM, garnet has a wide stability field. At 1.2 GPa garnet ? amphibole are the high-temperature liquidus phases followed by plagioclase at lower temperature. Clinopyroxene reaches its maximal stability at H 2 Ocontents B9 wt% at 950°C and is replaced by amphibole at lower temperature. The slopes of the plagioclase-in boundaries are moderately negative in T-X H 2 O space. At 0.8 GPa, garnet is stable at magmatic H 2 O contents exceeding 8 wt% and is replaced by spinel at decreasing dissolved H 2 O. The liquids formed by crystallization evolve through continuous silica increase from andesite to dacite and rhyolite for the 1.2 GPa series, but show substantial enrichment in FeO/MgO for the 0.8 GPa series related to the contrasting roles of garnet and amphibole in fractionating Fe-Mg in derivative liquids. Our experiments indicate that the stability of igneous garnet increases with increasing dissolved H 2 O in silicate liquids and is thus likely to affect trace element compositions of H 2 O-rich derivative arc volcanic rocks by fractionation. Garnetcontrolled trace element ratios cannot be used as a proxy for 'slab melting', or dehydration melting in the deep arc. Garnet fractionation, either in the deep crust via formation of garnet gabbros, or in the upper mantle via formation of garnet pyroxenites remains an important alternative, despite the rare occurrence of magmatic garnet in volcanic rocks.

No abstract

In order to understand the role of mica-rich rocks as a source of granite magmas, a series of melting experiments was performed on two different starting materials. The first composition is a model biotite gneiss consisting of 30 wt % biotite, 30 wt % plagioclase, and 40 wt % quartz. The second composition is a model two-mica pelites consisting of 15 wt % biotite, 15 wt % muscovite, 30 wt % plagioclase, and 40 wt % quartz. Experiments were performed under vapor-absent conditions at 1.0 GPa and between 750 ø and 950øC. With only biotite in the starting material the volume of melt is always less than 15 vol % below 900øC and reaches 25 vol % at 950øC. In experiments that involve both biotite and muscovite in the starting material, the melt proportion increases up to 28 vol % at 825øC and reaches 60 vol % at 950øC. For the biotite-plagioclase-quartz (BPQ) assemblage, the solidus is located at 800øC at 1.0 GPa. The melting reaction produces a metaluminous granitic liquid and leaves a residuum consisting of garnet + biotite + orthopyroxene + plagioclase + quartz. In addition, the experiments show that at 1.0 GPa biotite can be stable above 950øC. With both micas in the starting material (BPQM), the solidus at 1.0 GPa is located at 750øC. The melting reactions produce a peraluminous granitic liquid and leave a residuum of garnet + sillimanite + biotite + quartz + plagioclase + Kfeldspar in experiments below 900øC. At 950øC the residuum consists of garnet + orthopyroxene + biotite + plagioclase. The melt fraction is determined by the proportions of the hydrous phases and of the amount of feldspar relative to quartz. Mineral modes of the source rocks, particularly the amount of quartz, are at least as important as the amount of available H20 in controlling the melt fraction generated during crustal anatexis. Paper number 95JB00916. 0148-0227/95/95JB-00916505.00 asthenospheric impingement under continental crust following delamination of eclogitic thickened lower crust. Biotite-bearing gneisses are abundant in exposed lower crustal terrain. Their protoliths may have been graywackes and/or compositionally intermediate metavolcanic and plutonic rocks. Vapor-absent melting of biotite in metapelites begins at higher temperature than that of muscovite [Vielzeuf and Holloway, 1988; LeBreton and Thompson, 1988; Patiho-Douce and Johnston, 1991] but at a lower temperature than that of hornblende in amphibolite [Rushruer, 1991], metavolcanics [Conrad et al., 1988], and granodiorite to tonalite [Naney, 1983; Schmidt, 1993]. Fluid-absent melting of muscovite occurs abruptly by virtually univariant reaction because of the restricted variation in composition of this mineral. By contrast, fluid absent-melting of biotite and hornblende takes place over a relatively broad temperature interval because of the large variation in composition of these phases (due to solid solutions). Muscovite-bearing metapelites are extremely fertile because they are sensitive to vapor-absent melting in collision thickened lower crust. For example, trace elem...

No abstract