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The role of carbon dioxide, CO, as oxidizing agent at high pressures and temperatures is evaluated by studying its chemical reactivity with three transition metals: Au, Pt, and Re. We report systematic X-ray diffraction measurements up to 48 GPa and 2400 K using synchrotron radiation and laser-heating diamond-anvil cells. No evidence of reaction was found in Au and Pt samples in this pressure-temperature range. In the Re + CO system, however, a strongly-driven redox reaction occurs at P > 8 GPa and T > 1500 K, and orthorhombic β-ReO is formed. This rhenium oxide phase is stable at least up to 48 GPa and 2400 K and was recovered at ambient conditions. Raman spectroscopy data confirm graphite as a reaction product. Ab-initio total-energy structural and compressibility data of the β-ReO phase shows an excellent agreement with experiments, altogether accurately confirming CO reduction P-T conditions in the presence of rhenium metal and the β-ReO equation of state.

a b s t r a c tFor more than half a century, thought about granite genesis and crustal evolution has been guided by the concept of partial melting in the lower crust. In this model, granitic magmas produced at depth are lost to shallow levels, leaving behind a more mafic, volatile poor residue that is depleted in incompatible components (H 2 O, alkalis, and heat-producing elements). Although granite extraction must be the dominant process by which crust is modified over time, the preferred model of granite genesis triggered by metamorphic dehydration reactions (dehydration melting) does not adequately explain important aspects of granite formation. The temperatures required for voluminous granite production by dehydration melting need heat and mass input to the crust from mantle-derived mafic magmas. In addition, prediction of the H 2 O contents of granitic liquids by extrapolation from low-pressure experiments to deep-crustal pressures (P) and temperatures (T) implies that the H 2 O resident in hydrous minerals is insufficient to account for large granite volumes, such as anorogenic granite batholiths in continental interiors. To test this, we conducted new experiments on the H 2 O contents of simple granitic liquids at 10 kbar and 800-950• C. We confirm previous extrapolations from lower P and T indicating that a minimum of 3-4 wt% H 2 O is present at the studied P and T in a granitic liquid in equilibrium with quartz and feldspars. For large-scale melting, this is much more than could have been supplied by the H 2 O resident in biotite and amphibole by dehydration melting at these conditions, unless lower-crustal temperatures were higher than generally inferred. Another problem with the dehydration-melting model is that the crystal chemistry of the large-ion lithophile elements (LILE) does not favor their partitioning into granitic liquids; rather, U, Th, Rb and the rare earth elements (REE) would more likely be concentrated in the postulated mafic residues. Finally, observations of migmatite complexes reveal many features that can not be satisfied by a simple dehydration-melting model.We suggest that the volatile components CO 2 and Cl are important agents in deep-crustal metamorphism and anatexis. They induce crystallization and outgassing of basalt magmas at lower-crustal levels, where the combination of latent heat and liberated H 2 O may contribute to granite production, leading to larger melt fractions than for simple dehydration-melting models. Since the Cl and CO 2 are very insoluble in granite liquids, granite generation leads naturally to production or separation of a coexisting metamorphic fluid with low H 2 O activity. Such a fluid could coexist with granulite-facies assemblages and yet be capable of dehydration, alkali exchange and LILE extraction to explain many chemical processes of deep-crustal metamorphism not readily explainable by dehydration melting.