Thermal History of the Bandelier Magmatic System: Evidence for Magmatic Injection and Recharge at 1.61 Ma as Revealed by Cathodoluminescence and Titanium Geothermometry

Abstract

The rhyolitic Valles caldera complex, New Mexico, is one of the type examples of resurgent calderas and has experienced two well‐studied caldera‐forming eruptions. The first formed the Lower Bandelier Tuff (LBT) at 1.61 Ma, and the second emplaced the Upper Bandelier Tuff (UBT) at 1.22–1.26 Ma. During the time between the LBT and the UBT, the much smaller‐scale Cerro Toledo Rhyolite (CTR) was sporadically erupted. Quartz crystals from these stages of activity were imaged with cathodoluminescence microscopy, and growth zones in certain quartzes, due to varying Ti content, were revealed. Crystallization temperatures were obtained with a titanium‐in‐quartz geothermometer. The LBT quartzes are unzoned, with temperatures clustering between 660° and 715°C when a calculated $$a_{\mathrm{TiO}\,_{2}}$$ of 0.4 is applied to the system. These near‐solidus temperatures imply that the LBT magma chamber was highly crystalline at one point. However, the low crystal content and the widespread presence of resorption features in LBT crystals require that pervasive melting affected the LBT magma chamber at some point before eruption. This melting is hypothesized to result from a hot magmatic injection into the system, with the injection also being a likely trigger of the cataclysmic LBT volcanism. The earliest‐erupted CTR units contain many zoned quartz crystals. Inner zones are usually rounded and invariably reveal cold (∼660°–700°C) cores and hot (∼750°–825°C) rims. We interpret these results as thermal evidence of magmatic recharge, whereby new magma mixed vigorously with leftover magma and high‐temperature rims crystallized around low‐temperature restitic quartz cores. Thermal data for the rest of the CTR record the continuing cooling and evolution of this mixture of magma, while results for the culminating UBT reveal generally unzoned quartz crystals with a roughly constant temperature of 685°–725°C. Altogether, these results present an unprecedented glimpse into the thermal history of the Bandelier magma system, as well as strong evidence concerning the timing and overall importance of magmatic injections in silicic magma systems.