Publication Date:
2021-10-22
Description:
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 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 due to 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 (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, 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
Print ISSN:
0022-3530
Electronic ISSN:
1460-2415
Topics:
Geosciences
