Determining the chemical variability of Obsidian Dome

Event Center A & B

Obsidian Dome is part of the Inyo Domes magmatic system. Obsidian Dome erupted 650 years ago on the eastern side of the Sierra Nevada Mountains of California. The composition of the dome was examined using a Niton X-ray fluorescence spectrometer to determine the chemical composition of the rocks that comprise the dome, both in the field and in the laboratory. This study is an attempt to characterize the chemical variation shown by the rocks that make up this dome and to try and determine the sequence of eruptive events that formed Obsidian Dome. The fractional crystallization of the magma chamber should result in measurable differences in chemical composition throughout the dome. If the magma erupted in different events then the composition of the melt, and therefore the successive eruptions, will crystallize with different concentrations of elements within the melt. The petrography shows a change in phenocryst percentage and size with subsequent eruptive events. 62 locations around the perimeter and across the top of the dome were analyzed. Samples were also collected to make into petrographic slides to examine variation in phenocrysts in these rocks.