Origin of Compositional Gradients with Temperature in the High-SiO2 Rhyolite Portion of the Bishop Tuff: Constraints on Mineral–Melt–Fluid Reactions in the Parental Mush

Abstract The Bishop Tuff (BT), erupted from the Long Valley caldera in California, displays two types of geochemical gradients with temperature: one is related to magma mixing, whereas other found high-SiO2 rhyolite portion and characterized by twofold or lower concentration variations minor trace elements that are strongly correlated temperature. It proposed latter zonation, which preceded phenocryst growth, developed as a result mineral–melt partitioning between interstitial melt surrounding crystals parental mush, variable fractions were segregated. To test this hypothesis, trends increasing vs decreasing element concentrations temperature (as proxy for fraction), obtained published data on single-clast pumice samples Tuff, used infer their relative degrees incompatibility compatibility mush. Relative values (RCVi) all i, defined slope divided average concentration, shown be linearly respective bulk partition coefficients (bulk Di). Mineral–melt literature constrain stoichiometry crystallization/melting reaction mush: 32 % quartz + 34 plagioclase 31 K-feldspar 1·60 biotite 0·42 titanomagnetite 0·34 ilmenite 0·093 allanite 0·024 zircon 0·025 apatite = 100 liquid. proportions tectosilicates (i.e. location eutectic) consistent depths segregation ~400–550 MPa an activity H2O ~0·4–0·6. Temperatures <770–780 °C constrained reaction. Evidence fluid phase was present mush seen versus CO2 contents segregated formed Tuff. presence excess phase, partitions melt, required explain compatible behavior CO2, abundance must have been low incompatible H2O. Calculated degassing paths melts, ascended shallower grow phenocrysts, match volatile analyses quartz-hosted inclusions abundances ≤1 wt%. source volatiles irrespective whether it crystallization partial melting, primarily associated basalts, granitoid crust too poor. Approximately twice much basalt needed provide requisite volatiles. determination Di several gives composition leucogranitic shows distinct Mesozoic Sierran arc granitoids, expected. Collectively, results study new constraints models complex, multi-stage processes throughout Plio-Quaternary, involving both mantle-derived pre-existing crust, led origin body