Temperatures and water contents of Long Valley, CA basalts: Application of olivine–melt thermometry and hygrometry at the liquidus

Abstract

The Plio-Quaternary Long Valley (LV) basalts, derived from subduction-modified lithosphere, are found in close proximity to the Long Valley caldera in California, from which hydrous high-SiO2 rhyolites were erupted, including the climactic Bishop Tuff (>600 km3). To better understand the processes by which the influx of basalts into the crustal column beneath Long Valley caldera led to the generation of voluminous high-SiO2 rhyolite, it is necessary to determine the range of temperatures and water contents of the LV basalts. In this study, olivine–melt thermometry and hygrometry are applied to several, high-MgO (~7–10 wt%) LV basalts which contain phenocrysts of olivine and clinopyroxene, many of which display diffusion-limited, rapid-growth textures. Histograms of analyzed olivine compositions display a continuous, unimodal population in each sample. When the most Mg-rich olivine in each sample is paired with a liquid composition that matches that for the whole-rock (and a Fe3+/FeT ratio of 0.24), Fe2+−MgKD (olivine–melt) values of 0.32–0.36 are obtained, consistent with preservation of the first olivine composition to crystallize near the liquidus due to rapid phenocryst growth during ascent. Both a Mg- and Ni-based olivine–melt thermometer from the literature, calibrated on the same experimental dataset, were applied to the most Mg-rich olivine paired with the whole-rock melt composition. Because the Ni-thermometer is independent of water content, it provides the actual temperature at the onset of olivine crystallization in each of the LV basalts (1198–1053 °C), whereas the Mg-thermometer gives the temperature under anhydrous conditions and thus allows ∆T (= TMg–TNi = depression of liquidus due to water) to be obtained. The average ∆T for all samples is 86 (±27) °C, which literature models show is consistent with an average melt water content of ~4.3 (±1.5) wt%.