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In the deep, chemically reducing parts of Earth's mantle (1), hydrous fluids contain significant amounts of molecular hydrogen ([H.sub.2]). Thermodynamic models of fluids in Earth's mantle so far have always assumed that molecular hydrogen and water are completely miscible. Here we show experimental evidence that water and hydrogen can coexist as two separate, immiscible phases. Immiscibility between water and hydrogen may be the cause of the formation of enigmatic, ultra-reducing domains in the mantle that contain moissanite (SiC) and other phases indicative of extremely reducing conditions (2,3). Moreover, the immiscibility between water and hydrogen may provide a mechanism for the rapid oxidation of Earth's upper mantle immediately following core formation (4)., The oxygen fugacity of Earth's mantle controls the composition of volcanic gases and is therefore a key parameter in the evolution of Earth's atmosphere (5,6). In general, it decreases from [...]

Ti[O.sub.2] solubility in rutile-saturated felsic melts and coexisting minerals was determined at 1.5-3.5 GPa, 750-1250[degrees]C, and 5-30 wt% [H.sub.2]O. Ti[O.sub.2] solubility in the melt primarily increases with temperature and melt basicity; it increases slightly with water content in the melt, and it decreases with pressure. A general Ti[O.sub.2] solubility model was obtained and is expressed as: ln[(Ti[O.sub.2]).sub.melt] = ln[(Ti[O.sub.2]).sub.rutile] + 1.701 - (9041/T) - 0.173P + 0.348FM + 0.016[H.sub.2]O, where Ti[O.sub.2] and [H.sub.2]O are in wt%, T is in Kelvin, P in GPa, and FM is the melt composition parameter given by FM = (1/Si)*[Na + K + 2(Ca + Fe + Mn + Mg)]/ A1, in which the chemical symbols represent cation fractions. Ti[O.sub.2] solubility in amphibole, garnet, and clinopyroxene also increases with temperature and empirical equations describing this temperature dependence were derived. These data were used to assess the protolith Ti[O.sub.2] content required for futile saturation during partial melting of hydrous metabasalt at the amphibolite to eclogite transition. The results show that only 0.8-1.0 wt% Ti[O.sub.2] is required for rutile saturation during low-degree ( Archean tonalites-trondhjemites-granites (TTG) are widely accepted to be the products of low-degree melting of metabasalts at the amphibolite to eclogite transition, with rutile being present in the residue. Comparison of natural TTG compositions with our experimental rutile solubility data indicates that the dominant TTG magmas were produced at temperatures of 750-950[degrees]C, which requires that the partial melting occurred at hydrous conditions. Models involving melting at the base of oceanic plateaus are inadequate to explain TTG genesis because the plateau root zones are likely dominated by anhydrous cumulates. A slab-melting model satisfies the requirement of a hydrous metabasalt, which during subduction would melt to produce voluminous TTG melts under high Archean geothermal gradients. The geothermal gradients responsible are estimated to be between 10 and 19[degrees]C/km based on a pressure range of 1.5-2.5 GPa for the amphibolite to eclogite transition. Keywords: Rutile, amphibolite to eclogite transition, partial melting, TTG, Archean subduction

A new synthesis technique allows large fluid inclusions to be produced at conditions under which one normally would obtain only small inclusions, with the additional advantage that the timing of fluid entrapment can be controlled. In the first step, primary fluid inclusions are grown at P-T conditions under which it is relatively easy to produce large inclusions. In the second step, which can be performed either during the same experiment or during a separate experiment after a desired time of pre-equilibration, some of the primary inclusions produced during the first step are re-opened by in-situ fracturing, causing partial replacement of the inclusion content. The sample is then left at high pressure and temperature until the cracks leading to the re-opened inclusions are healed. To test the method and quantify the efficiency of fluid replacement in the re-opened inclusions, we produced primary inclusions at 700[degrees]C/200 MPa and re-opened them during a second experiment in the presence of a compositionally different fluid at 500[degrees]C/70 MPa. Laser-ablation ICP-MS (LA-ICP-MS) analyses of eight primary and 11 refilled inclusions demonstrate that in the latter more than 97% of the original fluid was replaced by new fluid. Thus, the refilled inclusions are representative of the surrounding fluid at the time of in situ fracturing. Keywords: Synthetic fluid inclusions, experimental technique, LA-ICP-MS, [H.sub.2]O-salt

Research on quartz crystals found in Mole Granite, Australia reveal information regarding post-entrapment changes regarding fluid inclusions. The authors find that these modifications can be attributed to deformation processes, and are more common in phenocrysts and vein quartz than free-standing crystals quartz.

Porphyry Cu deposits are commonly thought to have formed by magmas that were unusually rich in metal and/or sulfur. In this study, we test this assumption by reconstructing the metal and sulfur content of an ore-related latite magma at Bingham Canyon and comparing it with that of intermediate magmas in several other arc magma systems. The ore-related latite magma at Bingham Canyon records strong evidence for magma mixing and has a major to trace element composition that can successfully he modeled by a mixture of ∼40 wt % mafic magma, which was similar to the most mafic rock found at Bingham Canyon (a melanephelinite containing 45 wt % Si02), and ∼60 wt % felsic magma of rhyolitic composition. Based on the modal abundance of 0.19 ± 0.01 vol % sulfides and laser ablation-inductively coupled plasma-mass spectrometry analyses of unaltered sulfide inclusions preserved within hornblende and plagioclase phenocrysts, the latite magma contained 50 to 90 ppm Cu, 0.8 to 2.0 ppb Au, 2 to 3 ppm Mo, and ∼0.12 to 0.14 wt % S. Whole-rock and melt and sulfide inclusion data suggest that the bulk of copper and Au in the latite magma was derived from the mafic end member, whereas significant amounts of sulfur were also provided by the felsic end member. A rough, independent estimate of the amount of Cu present in die mixed magma can be obtained by taking the Cu content of mafic, sulfide-undersaturated silicate melt inclusions and multiplying it with the mass fraction of mafic magma involved in the magma mixing. Applying this latter approach to two other poqrhyry Cu-mineralized magma systems (Santa Rita, USA; Bajo de la Alumbrera, Argentina) and several modern arc magma systems suggests that ore-forming intermediate magmas in mineralized systems were not unusually Cu rich. Whether or not they were unusually sulfur rich could not be answered with the available data. If the sulfur contents of mineralizing magmas prove to be normal, then the most distinctive feature of fertile magma systems may be the formation of large, long-lived magma chambers at 5- to 15-km depth and the development of vent structures that enable focused fluid flow. [ABSTRACT FROM AUTHOR]