New Mexico Geological Society Annual Spring Meeting — Abstracts

Isotopic signatures and major and trace element compositions of single quartz crystals with melt inclusions from the Cerro Toledo Rhyolite of Valles caldera (New Mexico) have never been correlated with crystallization temperatures of the same crystals. Combining the compositions of melt inclusions and the localized temperatures of host quartz crystals offers insight into how high silica rhyolite magma systems evolve.

Two major ignimbrite units were erupted from Valles caldera, the Otowi (1.61 Ma) and Tshirege (1.22 Ma) Members of the Bandelier Tuff. Between the Otowi and Tshirege eruptions, much smaller-scale eruptions of Cerro Toledo Rhyolite were sporadically erupted. Secondary Ionization Mass Spectrometry was used to measure titanium concentrations for titanium-in-quartz geothermometry and trace element compositions within melt inclusions. Trace element compositions in melt inclusions are variable within the Cerro Toledo eruption deposits and crystals. Strontium concentrations are ~1 ppm within melt inclusions, with crystals from one Cerro Toledo eruption deposit ranging from ~7-11 ppm. Such low strontium concentrations must result from fractionation of feldspar. Niobium concentrations range from ~60-180 ppm indicating quartz crystals form in highly fractionated melts. Future isotopic studies will be undertaken to help characterize distinct melts with unique isotopic signatures, or whether mixing can be recognized between isotopically distinct melts for the same Cerro Toledo quartz-hosted melt inclusions.

Titanium in quartz geothermometry was originally calibrated at 10 kbar. Thomas et al. (2010) added pressure, or depth dependence that affected the solubility of titanium in quartz. Using this additional pressure constraint at ~3 kbar, Cerro Toledo Rhyolite quartz crystals yield temperatures that range from ~460-560 °C, which are not realistic for quartz formed in a chamber which eventually formed a caldera. Thus, either this titanium in quartz geothermometer does not yield accurate temperatures for quartz associated with caldera forming eruptions, or these quartz crystals were formed at or near 10 kbar depth (~680 °C), which seems highly unlikely.