Compositional and thermal zoning within quartz ejected before, during and after a supervolcanic eruption at 1.256 Ma: Valles Caldera, New Mexico, USA

We have examined individual quartz crystals from a large caldera-forming eruption (CFE) at Valles caldera to highlight the potential role of magmatic recharge in the evolution of large-volume, high-silica, rhyolite systems. Valles represents the type example of a resurgent caldera erupting 400 km3 of Upper Bandelier Tuff (UBT) at 1.256 ± 0.010 Ma. Following this eruption, central resurgence of >1000 m proceeded for the next c. 27 ka, contemporaneous with emplacement of small-volume rhyolite lavas and tuffs. Dating of the initial, undeformed, ring fracture rhyolite at 1.229 ± 0.017 Ma (Cerro del Medio rhyolite) places an upper time constraint on the duration of resurgence at Valles (Phillips et al., 2007). Quartz crystals from the UBT, resurgence-related volcanics and Cerro del Medio lavas were analyzed by cathodoluminescence (CL) and TitaniQ geothermometry techniques (Wark & Watson, 2006). Titanium concentrations were determined using LA-ICP-MS and were in the range of 30 to 110 ppm. A crystallisation temperature based upon Ti concentration in quartz can be calculated using TitaniQ, given an estimate of TiO2 activity. Rutile was not found in these samples, but several lines of reasoning suggest an aTiO2 of 0.4 (Campbell et al., 2009). Ti activity was assumed constant during the plinian venting and resurgence magmatism. CL imaging of quartz from the initial plinian fallout unit of the UBT indicates isothermal conditions (i.e. no apparent compositional zoning) at temperatures of ~700 °C using a TiO2 activity of 0.4. Mid-to-late stage ignimbrite units exhibit quartz with complex zoning profiles, indicating thermal disequilibrium during the waning stages of the eruption. The titanium concentrations of bright CL rims on these quartz crystals suggest temperature increases of 70-100 °C compared to dark core regions, which show Ti concentration and thus temperatures similar to the plinian unit. Quartz crystals from the resurgence-related lava units show multiple high-temperature rim overgrowths and dissolution events as suggested by quartz embayments, again with core-to-rim temperature increases of 80-100 °C compared to thermal conditions prevalant during the initial plinian stage. A return to isothermal but hotter conditions (>820 °C, using aTiO2 of 0.4 ) is indicated by quartz crystals from the Cerro Del Medio rhyolite. Our preliminary conclusion from these data is that a magmatic recharge event or perhaps multiple replenishments occurred during the mid-to-late stages of the UBT eruption. Whether this event would have facilitated caldera resurgence is indeterminable at this stage, but nonetheless demonstrates the importance of this process in CFEs. References cited: Campbell et al., 2009. Thermal History of the Bandelier Magmatic System: Evidence for Magmatic Injection and Recharge at 1.61 Ma as Revealed by Cathodoluminescence and Titanium Geothermometry. JG. 117, no. 5. Phillips et al., 2007. The 40Ar/39Ar age constraints on the duration of resurgence at the Valles caldera, New Mexico. JGR. 112:B08201. Wark & Watson, 2006. TitaniQ: a titanium-in-quartz geothermometer. CMP. 152:743.