CHEMICAL AND THERMAL ANALYSIS OF ZIRCONS FROM THE CERRO TOLEDO RHYOLITE, NEW MEXICO

The zircon record of temperature and trace element changes in a magma chamber(s) that persisted between two supervolcano eruptions (>400 km³) in northern New Mexico is being investigated. The Cerro Toledo Rhyolite of Valles Caldera (CTR) is a series of high-silica rhyolites that erupted intermittently between the voluminous Lower Bandelier (1.61 Ma) and Upper Bandelier (~1.25 Ma) eruptions. Since intercaldera events like the CTR are often lost due to erosion, the Cerro Toledo unit presents a rare opportunity to track magma evolution between large scale eruptions. Previous work on the CTR revealed that quartz grains in the early CTR have Ti-poor cores mantled by relatively Ti-rich rims. This relationship is interpreted to reflect a rimward increase in the temperature of quartz growth and the injection of hot magma into the system, triggering eruption (Campbell et al., 2009). This study seeks to confirm this interpretation by examining the temperature and chemical conditions during growth of coexisting zircon.

Pumice clasts and ash from the end of the LBT unit and from three separate pumice beds in the lowermost portion of the CTR were collected from Los Alamos Canyon near Los Alamos, New Mexico. Quartz crystals separated from these tephra show rims that are relatively bright in catholuminescence images, consistent with the presence of Ti-rich rims. Zircons separated from the same tephra range to >150 microns in size. Trace element results for zircon rims and for zircon depth profiles extending to 6 microns below the rims have been obtained on the ion microprobe at UCLA. These data are being used to determine the Ti-in-zircon temperatures of rim growth and, in addition, the chemical changes that occurred in the lead-up to eruption of the earliest CTR.