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

In this study, we present new evidence for changes in magma storage conditions that preceded the 1232 ka caldera-forming eruption of the Bandelier magmatic system in the Jemez Mountains Volcanic Field. Using high precision Ar-40/Ar-39 sanidine dating we determine that at least eight rhyolites erupted within 8.6 +/- 3.4 kyr of the-400 km(3) eruption that formed Valles caldera. Some of those rhyolites contain fayalite with or without biotite, others contain only biotite. An eruption of fayalite-bearing rhyolite at 1240.5 +/- 2.1 ka ended an eruption hiatus of at least 100 kyr. Following that first post-hiatus episode of volcanism, at least four more eruptions of fayalitebearing rhyolite and three eruptions of biotite-bearing rhyolite occurred prior to the caldera-forming eruption. We use phase equilibrium experiments and geothermobarometry to infer the storage conditions and processes that led to these differing crystal cargos and ultimately generated-400 km3 of predominantly fayalite rhyolite ignimbrite (Tshirege Member of the Bandelier Tuff). We find that biotite-bearing rhyolites were stored at 695-750(degrees)C, 75-160 MPa, and at an oxygen fugacity more oxidizing than the quartz-fayalitemagnetite (QFM) buffer reaction. Fayalite-bearing rhyolites were similarly stored at 695-745(degrees)C and 70-190 MPa, but at more reducing conditions (fO(2)<= QFM). We suggest that the reduced, fayalite-bearing rhyolite was most likely produced via interaction of crystal poor rhyolitic magma with a reducing, potentially Cl-bearing, and H2O-rich supercritical fluid phase. This fluid flux event was a key component of the substantial magmatic rejuvenation that enabled the mobilization of-400 km(3 )of mostly fayalite-bearing rhyolite prior to not only the Tshirege event, but the older Otowi event as well.