Ten Thousand Years of Magma Storage Preceding the Last Caldera-Forming Eruption of the Bandelier Magmatic System, New Mexico, USA.

This article focuses on the pre-eruptive magma storage conditions and processes that preceded the 1.232 million-year-old (ka) caldera-forming eruption of the Bandelier magmatic system in the Jemez Mountains Volcanic Field. Using high-precision ^40Ar/^39Ar sanidine dating, phase equilibrium experiments, and geothermobarometry, the study identifies at least eight rhyolite eruptions within ~8.6 ± 3.4 thousand years (kyr) before the ~400 km³ Tshirege Member eruption, distinguishing two mineralogically distinct rhyolite types: fayalite-bearing and biotite-bearing. Both types were stored at similar temperatures (~695–750°C) and pressures (~70–190 MPa), but differed in oxygen fugacity (fO₂), with fayalite-bearing rhyolites forming under more reducing conditions (≤ quartz-fayalite-magnetite buffer) likely due to interaction with a reducing, Cl-bearing, H₂O-rich supercritical fluid associated with magmatic recharge. This fluid fluxing is proposed as a key mechanism for magmatic rejuvenation that enabled the generation of large volumes of eruptible, reduced rhyolite prior to the caldera collapse, and the presence of fayalite-bearing rhyolite is interpreted as a potential indicator of imminent large-volume explosive eruptions in this volcanic system.