Quartz-hosted inclusions and embayments reveal storage, fluxing, and ascent of the Mesa Falls Tuff, Yellowstone

Abstract

Quartz-hosted glasses from the Mesa Falls Tuff provide a geochemical window into the pre-eruptive magmatic system from one of Yellowstone's largest-volume caldera-forming eruptions. H2O and CO2 concentrations, along with major and trace elements, were measured in both fully enclosed glass inclusions and partially enclosed embayments in the same quartz crystals. Major elements are largely consistent between the inclusions and embayments, except for K2O and Na2O. Of note, K2O is enriched by ∼1 wt.% in embayment interiors relative to inclusions. Most trace elements are also enriched in the embayment interiors compared to inclusions from the same crystals. Fractionation trends of trace elements are consistent with ∼30-60% crystallization. Quartz-hosted glass inclusions preserve 3.1±0.9 wt.% H2O and 493±227 ppm CO2 whereas embayment interiors have 0.9±0.1 wt.% H2O and 399±229 ppm CO2. The CO2 is roughly similar, but the distinct ∼2 wt.% discrepancy between inclusion and embayment interior H2O contents may have been produced by CO2 fluxing sourced from underplated Yellowstone basalts. H2O gradients within embayments are flat in their interiors and modified by sharp positive gradients near embayment exteriors which were produced by post-eruptive rehydration. CO2 gradients occur as gently sloping concentration gradients that extend inward 150 to 250 μm from the embayment exterior. Finite-difference 1D and 2D diffusion modeling indicates the distribution of H2O and CO2 in embayments was produced by slow, fluid-saturated decompression that preceded rapid ascent during the caldera-forming eruption.