RECENT EXPLOSIVE ERUPTIONS AT THE VALLES CALDERA, NEW MEXICO: INSIGHT INTO DYNAMICS FROM PUMICE TEXTURES

Calderas can remain ‘restless’ for long periods of time following their climactic episodes, leading to smaller scale inter-caldera eruptions. The Valles Caldera, NM, experienced two caldera forming eruptions ~1.6 Ma and ~1.2 Ma, while its most recent small-scale eruption occurred ~74 ka, producing the El Cajete (EC) pyroclastic deposits. The EC deposits are petrographically and compositionally different from the caldera-forming eruptions indicating a new batch of magma was beneath the caldera at the time of the eruption. Prior work hypothesizes that recent volcanic activity at the Valles Caldera was the result of magma mixing which has left the system in an eruptible state, with additional geophysical evidence placing the body at 5 – 15 km depth with >10% melt fraction. Pumice clast characterization of the Lower and Upper I Units of the EC fall unit shows a wide range in densities (360 - 2800 kg/m³, with peak occurrence near the low-end of the range), which suggests that the gas available to drive the eruptions was significant. Here we present new thin section analysis of the clasts at a range of densities under a petrographic microscope and SEM that document the crystal phases, textures, and bubble morphologies present. Bubble textures correlate with pumice densities as denser clasts show elongated stretched bubble textures while lower density clasts show more rounded bubbles, potentially reflecting differences between clasts derived from near the conduit walls versus those derived from near the conduit central axis. Images also reveal highly sheared zones and a glassy matrix nearly devoid of microlites, both indicative of very high ascent rates, and consistent with field-based reconstructions of plume heights and mass eruption rates. Zone variability in mineralogy as well as sharp interfaces between zones support the hypothesis that magma mixing triggered the eruption. Next steps will use bubble characteristics to quantitatively constrain decompression rates and characterize strain rates.