Collaborative Research: Investigation of Volatile Gradients in Silicic Magma Chambers

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Two examples of material from chemically zoned magma chambers are the Bishop Tuff in California and the Lower Bandelier Tuff in New Mexico. In the Bishop Tuff, the eruption appears to have overturned the contents of the magma chamber, preserving a stratigraphic record of the chemistry of the chamber. The eruption of the Lower Bandelier Tuff did not preserve the chemical layering which has been deduced to have been present in the chamber. The difference between the two eruptions suggests that the Bishop Tuff chamber had a density (volatile) gradient while the Lower Bandelier magma chamber was more homogenous with respect to volatiles. This project will quantify the volatile content in the two chambers by analyzing trapped melt inclusions in phenocrysts which represent different stratigraphic horizons. Ion and electron microprobe techniques will be used to determine the content of H, F, Cl and S in the melt inclusions as a fuction of depth in the original magma chamber, thereby determining the magnitude of the volatile gradient. Electron probe analyses for major elements will help determine the unaltered nature of melt inclusions and ion probe microanalyses of selected lithophile trace elements will help determine the original stratigraphic position of each phenocryst. The PI will do the ion microprobe work. He is collaborating with Dr. P. Kyle of of the New Mexico Institute of Mining and Technology, who will do the rest. ***