Hydrothermal Fluids, Nebular Vapors and Silicate MELTS As Seem through the Eyes of Minerals

Abstract:

Three cases where advances in mineral thermochemistry provide new insights into geological processes are discussed: (I) Sb-rich fahlores are used to constrain compositions of hydrothermal fluids and processes responsible for Ag-Pb-Zn sulfide ores from three Ag mining districts, Keno Hill, (Yukon, Canada); Julcani, (Angaraes, Peru); Coeur d’Alene, (Idaho, USA), confirming that the Ag initially contained in galena solid solutions was a substantial contributor to the total Ag mined in each of them; (II) Al_8/3O₄-rich MgAl₂O₄-Al_8/3O₄ spinels may have condensed from the primordial solar nebula, with their Al_8/3O₄ annealed out with cooling, as AgSbS₂ is “annealed” out of galena in Ag-Pb-Zn ore deposits. Such spinels may explain inconsistencies between mineral condensation sequences recorded in Ca-Al-rich inclusions (CAIs) in carbonaceous chondrites and those calculated assuming spinel was MgAl₂O₄. A new thermodynamical model and corundum-bearing CAIs, suggest we may anticipate realization of higher temperatures and pressures of canonical condensation than currently in vogue; (III) Fassitic pyroxenes in the composition space CaMgSi₂O₆ (Di) – CaTiAlSiO₆ (Gr) – CaTi_1/2Mg_1/2AlSiO₆ (AlBf) – CaAl₂SiO₆ (CATS) exhibit at least one prominent gap evident in composition data from CAIs. This may be interpreted as due to the near superposition of the extension of the Di– AlBf gap into this fassaite tetrahedron, with an isolated two-phase region formed by the double intersections of the (Gr+CATS) – AlBf critical curve with the surfaces of constant Ti(Al)_-1 exchange potential characteristic of CAIs. A thermodynamic model for fassaites may provide context for critical thinking about CAI petrogenesis, as well as presaging potential refinements to the thermochemical model for pyroxenes in MELTS.