Prelude to the Tshirege caldera-forming eruption of the Bandelier magmatic system, New Mexico, USA

Meszaros, Nicholas Franklin
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Two caldera-forming eruptions have occurred in the Bandelier magmatic system of the Jemez Mountains of northern New Mexico, the latter of which formed Valles caldera during the Tshirege eruption at 1232 ka. Despite no eruptions at Valles in the last ca. 70 kyr, the presence of partially molten magma bodies at depth evokes the question of how the system might evolve prior to a return of catastrophic volcanism. To address this question, I utilize mineral and glass compositions, phase equilibrium experiments, and ⁴⁰Ar/³⁹Ar sanidine geochronology to identify changes in the eruption rate and crystal cargo of the inter-caldera Valle Toledo Member rhyolites. Following the Otowi caldera-forming eruption of fayalite-bearing rhyolite at 1605.4 ± 2.3 ka, post-caldera rhyolites are mostly either biotite-bearing lava or pyroxene and/or hornblende-bearing tephra. During the first 8.1 ± 6.8 kyr following caldera collapse the average eruption recurrence interval is ~ 1.4 ± 1.1 kyr. For the next 239.8 ± 34.5 kyr, eruption frequency decreased to an average recurrence of 8.0 ± 1.2 kyr between events. The decrease in activity concluded in a hiatus of 117 ± 32 kyr that ended at 1240.5 ± 2.1 kyr with the first eruptions of fayalite rhyolite in the Bandelier system since the Otowi eruption, as well as a return to heightened eruption frequency (avg. recurrence 1.0 ± 0.4 kyr). These fayalite-bearing rhyolites are stored at nearly identical temperature (695-735°C) and pressure (75-160 MPa) conditions as biotite-bearing rhyolites, with fayalite only appearing in magma stored at lower oxygen fugacity. As evident by FeO and Cl concentration variation between the two rhyolite-types, fayalite likely crystallized as a recharge-associated reducing supercritical fluid fluxed through the system and resorbed biotite. During this magma recharge event, clinopyroxene antecrysts that crystallized prior to the hiatus were entrained into the post-hiatus Tshirege reservoir and underwent additional growth that recorded changes in magmatic conditions during their entrainment. The late pre-caldera rejuvenation initiated between 10³ and 10⁴ years prior to the Tshirege event, as constrained by the first Valle Toledo Member fayalite rhyolite eruption and absence of evidence for any diffusive partial re-equilibration of clinopyroxene core and rim compositions.