Volatiles H2O, CO2, and CO in Silicate Melt Inclusions in Quartz Phenocrysts from the Rhyolitic Bandelier Air-Fall and Ash-Flow Tuff, New Mexico

Abstract

Silicate melt inclusions in quartz phenocrysts from air-fall and ash-flow tuff were studied from a section of Bandolier Tuff in Sandia Canyon, Jemez Mountains, New Mexico. My aim was to estimate the concentration of some major volatile species (H2O, CO2, and CO) in the magma before explosive eruption. On a microscope heating stage, I determined the melting points of the inclusions and, with an electron microprobe, determined the bulk chemical composition. I estimated total H2O, CO2, and CO by difference on the basis of the microprobe analyses and measured H2O, CO2, and CO separately by quantitative mass spectrometry. I found several textural varieties of modified inclusions that correlate with field occurrence and cooling rate: devitrivication, zoning, and resorption of the host mineral are evidence of slow cooling. The modified inclusions are unsuitable for chemical analysis using the microprobe. Unmodified inclusions in the quartz phenocrysts from air-fall pumice are clear and glassy, and presumably contain the intratelluric assemblage of volatiles. Bulk chemical analysis of these clear glassy inclusions by electron microprobe revealed a rhyolitic composition of the glass consistent with petrography. Electron microprobe analysis by difference techniques were used to determine volatile contents in the glass. Oxide totals range from 2.4 to 7.0 ± 2.5% weight percent volatiles (average 5.4 wt.%). Using the melting point determinations of the inclusions (average 935°C ± 10°C), quantitative monitoring mass spectrometry revealed an average volatile content of the inclusions to be 3.6 ± 0.9 wt.%. The principal volatile species were found to be H2O 91.9%, CO2 2.7%, and CO 4.8% (in percent of total volatiles). Utilizing the temperature-compositional data, I used the melt inclusions as geobarometers, which revealed a depth to chamber of about 5 km that is consistent with geologic estimates.