Volatile and Sr Isotope Analysis of Melt Inclusions from the Bandelier Tuff, Valles Caldera, New Mexico: Insights into Pre-Eruptive Magma Conditions

Abstract

The Valles Caldera, located in northwest New Mexico, is one of three caldera volcanoes in the United States that is still active today. Calderas have the largest, most cataclysmic eruptions that result in large bowl-shaped depressions. Calderas are still not completely understood so studying their byproducts can not only give insights into pre-eruptive magma conditions of past eruptions but also provide insights into the behavior of future eruptions. An extremely useful tool to utilize for such studies is melt inclusions. Melt inclusions are small amounts of melt that were trapped at magmatic temperatures and pressures and potentially preserve pre-eruptive conditions. In order to gain more insights into the pre-eruptive magma conditions of the Valles Caldera, this study analyzed melt inclusions from the Lower and Upper Bandelier Tuff, which resulted from the two caldera forming events at 1.6 Ma and 1.2 Ma. Volatile contents (S and H2O), major elements and 87Sr/86Sr ratios of melt inclusions were measured. 87Sr/86Sr ratios revealed that melt inclusions represent bulk magma compositions and suggest a mantle-derived magma recharge event between the Lower and Upper Bandelier Tuff eruptions. Electron microprobe analysis results show high H2O content and low S content for the melt inclusions in pumices from the base of the Lower Bandelier and the base of the Upper Bandelier. These volatile contents indicate that both Bandelier eruptions were very explosive as a result of high H2O contents, but did not have much effect on global cooling as suggested by their low S contents. As for future eruptions, if these volatile trends (high H2O and low S) continue, then the behavior of such eruptions would behave in a similar fashion.