MAGMATIC RECHARGE DURING FORMATION OF VALLES CALDERA, NEW MEXICO: EVIDENCE FROM QUARTZ COMPOSITIONAL ZONING AND GEOTHERMOMETRY
Since the 1960's, the Valles caldera complex has exemplified a model of the resurgent caldera. A key question is how are such large volumes of magma thermally sustained over several million years. We have examined thermal conditions prevalent during (1) the cataclysmic eruption of the voluminous 400 km3 Tshirege Member, Bandelier Tuff (UBT), (2) resurgence-related magmatism and (3) initial lava dome emplacement within the ring-fracture zone of the caldera. These events span a relatively short time period of ~150 ka, from 1.256 Ma to ~1.11 Ma. We have identified a signature of continual magmatic recharge throughout this time period, through application of quartz cathodoluminescence imaging and TitaniQ geothermometry. Pre-UBT eruptive temperatures of 647-696 °C are indicated by low-Ti quartz crystals from the initial plinian fallout deposit and units 1 and 2 of the ignimbrite, which together comprise ~80% of the total erupted volume of the UBT. Within the mid to late-erupted UBT unit 3, there is an abrupt occurrence of compositionally zoned quartz crystals, showing high-Ti rims encompassing low-Ti rounded cores, which record temperature increases of 30-100 °C. Quartz crystals from the remaining ignimbrite units 4 and 5 exhibit similarly elevated temperatures of 730-805 °C. Modelling of titanium diffusion rates in quartz suggest magmatic recharge commenced <7400 years prior to eruption of the UBT. We have also discovered thermal heterogeneity within post-collapse rhyolite lavas. There is a strong spatial control on eruption of “hot” crystal-poor (unzoned quartz), versus “cool” porphyritic (zoned quartz, high-Ti rims) lavas of the Deer Canyon Rhyolite onto the resurgent dome. This scenario may be a product of localised crystal mush rejuvenation by new magma entering the system. Quartz crystals from the Cerro del Medio ring fracture rhyolite indicate isothermal conditions but at significantly elevated temperatures than pre-UBT. They may be derived from small-volume, crystal-poor magma batches expelled from the crystal mush. The magma responsible for initial heating of the UBT chamber as well as that erupted following caldera collapse was probably sourced from an underlying magma body of more mafic composition. Intrusion of this magma into a residual UBT mush zone may ultimately be a driving mechanism for resurgence.