MAGMA RESERVOIR CONDITIONS AND VOLATILE CONCENTRATIONS FOR INTER-CALDERA EL CAJETE SEQUENCE AT VALLES CALDERA, NEW MEXICO, USA

The Valles Caldera, New Mexico, was created in the Jemez Mountains Volcanic Field by two caldera-forming eruptions at 1.6 and 1.2 Myr. Post-caldera activity consists of lava domes, lava flows, large explosive phases, and an active hydrothermal system. The El Cajete sequence was emplaced ~74 ka and resulted from a Plinian eruption that produced pyroclastic density currents (PDCs), including pyroclastic surges, and a complex pumice-rich fall deposit. Such a series of eruptions of varying styles and dynamics is not uncommon in post-caldera volcanism, but the corresponding magma reservoir conditions are not fully understood. Our study addresses how magmatic source conditions may have controlled the eruption style of El Cajete. To do this, we are studying crystals and melt inclusions (MIs) from surge beds, PDCs, and pyroclastic fall to characterize the magmatic source conditions throughout the El Cajete sequence.

The dominant mineral phase in El Cajete pyroclastics is quartz and is thus the focus of the present study. Cathodoluminesce detection of zoning patterns reveals oscillatory patterns with no correlation between MI location and relative zone brightness. Some zoning patterns are cut off, and visual inspection of the crystal morphology suggests that quartz crystals tend to be fractured, obscuring what is inferred to be pyramidal, equant, or rounded crystal morphology. In the complex Plinian fall in particular, quartz-hosted MIs are common and range from 20 to 160 microns in diameter. Vapor bubbles in the MIs are less common, present in <40% of crystals, and range from 5 to 15 microns in diameter. Nearly 90% of the imaged MIs were faceted, and two-thirds of crystals had resorption patterns or overgrowths.

In terms of composition, morphology, and vapor proportion, there is too little complexity in the MI data to explain the complexity observed in the products of the Plinian eruption. This explanation may reasonably be extended to the early pyroclastic surges. Ongoing work to further characterize the host crystals and melt inclusions will help to further refine the petrologic model of the Valles plumbing system.