Origin of Ti-rich rims in quartz phenocrysts from the Upper Bandelier Tuff and the Tunnel Spring Tuff, southwestern USA

Abstract

Four compositionally zoned, melt inclusion-bearing quartz phenocrysts from the Upper Bandelier Tuff, New Mexico and the Tunnel Spring Tuff, Utah were examined in detail by cathodoluminescence, laser-ablation ICP-MS, and titanium-in-quartz (TitaniQ) thermobarometry in order to reconstruct the conditions of quartz crystallization. Temperature and TiO2 activity (aTiO2) in the magma were estimated from melt inclusion compositions by means of zircon saturation thermometry and an experimentally calibrated TiO2 solubility model, whereas pressure was estimated with the most recent calibration of the TitaniQ thermobarometer on the host quartz next to the analyzed melt inclusions.All four investigated quartz phenocrysts display a low-luminescent, Ti-poor core overgrown by a high-luminescent, Ti-rich rim. Reconstructed P–T–aTiO2 conditions suggest that the rimward increase in the Ti content of the two quartz crystals from the Upper Bandelier Tuff is due to a combination of increasing temperature, increasing aTiO2 and decreasing pressure, with the temperature effect being dominant in one crystal and the aTiO2 effect being dominant in the other crystal. In the two crystals from the Tunnel Spring Tuff the rimward increase in Ti content is mostly due to increasing temperature, but in one of them a negative pressure change played an equally important role.This study demonstrates that in rapidly cooled, peraluminous or metaluminous rhyolites the crystallization conditions of quartz phenocrysts can be reconstructed at the scale of individual growth zones, which allows to investigate the origin and evolution of such magmas in great detail.