Experimental constraints on rutile saturation during partial melting of metabasalt at the amphibolite to eclogite transition, with applications to TTG genesis

摘要

TiO₂ solubility in rutile-saturated felsic melts and coexisting minerals was determined at 1.5–3.5 GPa, 750–1250 °C, and 5–30 wt% H₂O. TiO₂ 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 TiO₂ solubility model was obtained and is expressed as: ln(TiO₂)_melt = ln(TiO₂)_rutile + 1.701 – (9041/T) – 0.173P + 0.348FM + 0.016H₂O, where TiO₂ and H₂O 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)]/Al, in which the chemical symbols represent cation fractions. TiO₂ 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 TiO₂ content required for rutile saturation during partial melting of hydrous metabasalt at the amphibolite to eclogite transition. The results show that only 0.8–1.0 wt% TiO₂ is required for rutile saturation during low-degree (<20%) melting. Rutile is stable up to ~1150 °C with 1.6 wt% TiO₂ in the protolith and 30–40% melting for dehydration melting and up to ~1050 °C and 50–60% melting for fluid-present melting. The data also show that 0.7–0.8 wt% TiO₂ in the protolith is needed for rutile saturation during subsolidus dehydration. Therefore, nearly all basaltic protoliths in deep-crustal settings and subduction zones will be saturated with rutile during subsolidus dehydration and low-degree melting at hydrous conditions.