H2O and Ultrahigh-Pressure Subsolidus Phase Relations for Mafic and Ultramafic Systems |
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| Abstract: | Experimental studies show that aqueous fluid-mediated mineralogic solution/redeposition mechanisms are orders faster than solid-solid transformations; hence the presence of a separate aqueous fluid markedly enhances reaction rates, whereas its total absence impedes mineralogic transformations. Where does this volatile component come from? For typical subduction-zone P-T trajectories, amphibole constitutes the major OH-bearing phase in most deep-seated metamorphic rocks of basaltic composition; other hydrous minerals are of minor abundance. Clinoamphiboles dehydrate at pressures of-2.0 to 2.4 GPa, but devolatilization may be delayed slightly by pressure overstepping; thus mafic blueschists and barroisitic amphibolites expel H2O at arc melt-generation depths of ～100 km, and commonly achieve the stable eclogitic phase configuration. Serpentinized mantle beneath the oceanic crust devolatilizes at comparable conditions. Only where metagabbroic rocks are completely dry and coarse grained are low-pressure assemblages metastably preserved. For realistic subduction-zone geothermal gradients, white micas ± biotites remain stable in sialic crust to pressures exceeding 3.5 GPa. Accordingly, under conditions attending descent to great depths, mica-rich quartzofeldspathic schists and gneisses that constitute the continental crust fail to evolve substantial amounts of H2O, and transform incompletely to stable eclogite-facies assemblages. The deep underflow of partly hydrated oceanic lithosphere thus generates most of the deep-seated volatile flux—and consequent partial melting to produce the calc-alkaline suite along and above a subduction zone; where large volumes of micaceous sialic materials are carried down to extreme depths, volatile flux severely diminishes. |
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