CORRELATING MAGMATIC TEMPERATURES AND MELT INCLUSION ISOTOPIC COMPOSITIONS IN SINGLE QUARTZ CRYSTALS HOSTED IN HIGH SILICA RHYOLITES, VALLES CALDERA, NEW MEXICO

Melt inclusion major, trace element, and isotope signatures in quartz crystals from pumice deposits from ~400,000 years of volcanism record the magmatic transition between volcanic, super eruptions (1.60 Ma and 1.25 Ma) at Valles caldera, New Mexico. Mineral compositions and mineral isotopic signatures vary as do melt inclusion compositions. Quartz crystals from these deposits retain Ti zoning that includes Ti-rich cores and rims and Ti-poor cores and rims. If assumed Ti activities are accurate, these rims and cores record a variety of temperatures (~700°C to 750°C) during which quartz crystallization occurred. As such, melt inclusions captured during growth of quartz from magmas that varied in temperature record the geochemical characteristics of these magmas. The major and trace element characteristics of these individual melt inclusions can be used to identify single crystals or pool multiple crystals for analyses that require critical amounts of analyte such as for Sr and Pb isotope analyses. We use major and trace element characteristics to isolate melt inclusions from “hot” and “cold” regions of quartz crystals and analyze the Sr and Pb isotopes of these inclusions to track magmatic evolution during the crystallization of quartz and other minerals. Results indicate that as magmas cooled, they assimilated more crustal material that generated uniform ⁸⁷Sr/⁸⁶Sr ratios but highly variable Pb isotope ratios. Pb isotopes vary in “hot” regions of quartz towards more radiogenic, mantle like signatures while “cold” regions have less radiogenic, more crustal signatures. All melt inclusions are silica-rich, suggesting the involvement of highly evolved melts without any presence of more mafic magmas. We demonstrate the utility of obtaining isotope results of correlated temperature and isotope signatures of melt inclusions to identify and constrain the liquids and conditions in which high silica rhyolite magmas interact and evolve.