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To decipher the petrogenesis of chromitites from the MohoTransition Zone of the Cretaceous Oman ophiolite, we carried out detailed scanning electron microscope and electron microprobe investigations of $500 silicate and chromite inclusions and their chromite hosts, and oxygen isotope measurements of seven chromite and olivine fractions from nodular, disseminated, and stratiform ore bodies and associated host dunites of the Maqsad area, Southern Oman. The results, coupled with laboratory homogenization experiments, allow several multiphase and microcrystal types of the chromite-hosted inclusions to be distinguished. The multiphase inclusions are composed of micron-size (1^50 mm) silicates (with rare sulphides) entrapped in high cr-number [100Cr/(Cr þAl) up to 80] chromite. The high cr-number chromite coronas and inclusions are reduced (oxygen fugacity, f O2 , of $3 log units below the quartz^fayalite^magnetite buffer, QFM). The reduced chromites, which crystallized between 600 and 9508C at subsolidus conditions, were overgrown by more oxidized host chromite (f O2 % QFM) in association with microcrystal inclusions of silicates (plagioclase An 86 , clinopyroxene, and pargasite) that were formed between 950 and 10508C at 200 MPa from a hydrous hybrid mid-ocean ridge basalt (MORB) melt. Chromium concentration profiles through the chromite coronas, inclusions, and host chromites indicate non-equilibrium fractional crystallization of the chromitite system at fast cooling rates (up to $0·18C a À1). Oxygen isotope compositions of the chromite grains imply involvement of a mantle protolith (e.g. serpentinite and serpentinized peridotite) altered by seawater-derived hydrothermal fluids in an oceanic setting. Our findings are consistent with a three-stage model of chromite formation involving (1) mantle protolith alteration by seawater-derived hydrothermal fluids yielding serpentinites and serpentinized harzburgites, which were probably the initial source of chromium, (2) subsolidus crystallization owing to prograde metamorphism, followed by (3) assimilation and fractional

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This contribution provides an overview of available experimental, thermodynamic, and molecular data on Au aqueous speciation, solubility, and partitioning in major types of geological fluids in the Earth's crust, from low-temperature aqueous solution to supercritical hydrothermal-magmatic fluids, vapours, and silicate melts. Critical revisions of these data allow generation of a set of thermodynamic properties of the AuOH, AuCl−2, AuHS, and Au(HS)−2 complexes dominant in aqueous hydrothermal solutions; however, other complexes involving different sulphur forms, chloride, and alkali metals may operate in high-temperature sulphur-rich fluids, vapours, and melts. The large affinity of Au for reduced sulphur is responsible for Au enrichment in S-rich vapours and sulphide melts, which are important gold sources for hydrothermal deposits. Thermodynamic, speciation, and partitioning data, and their comparison with Au and S contents in natural fluid inclusions from magmatic-hydrothermal gold deposits, provide new constraints on the major physical-chemical parameters (temperature, pressure, salinity, acidity, redox) and ubiquitous fluid components (sulphur, carbon dioxide, arsenic) affecting Au concentration, transport, precipitation, and fractionation from other metals in the crust. The availability and speciation of sulphur and their changes with the fluid and melt evolution are the key factors controlling gold behaviour in most geological situations

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samples documents the release of an aqueous fluid enriched in Cl, Na, Ca, Cd, Sb and Zn during the paroxysmal subplinian eruption. The paroxysmal eruption may have been produced by saturating the pre-eruptive basaltic andesite magma with this hybrid aqueous carbonic NaCl-HCl-rich fluid due to bulk assimilation creating high partial pressure of CO 2 at shallow crustal conditions of about 200 MPa. In contrast, mildly explosive block-and-ash flows (typical Merapi-type) may result from incomplete (selective) assimilation of the carbonatebearing xenoliths and lower CO 2 partial pressure that may not lead to explosive degassing.

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