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

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