MINERALOGICAL AND TEXTURAL ANALYSIS OF HORNBLENDE REACTION RIMS IN LAVA FLOWS FROM MOUNT SHASTA, SISKIYOU COUNTY, CA

With Mount Shasta’s close proximity to the town of Weed and Mount Shasta City in Northern California and its history of having at least one Plinian eruption, it poses a hazard to those neighboring communities. In this study, we reconstruct magma ascent rates of hornblende-bearing lava flows erupted from Mount Shasta over the past 375ka using the mineralogy and textures of decompression-induced reaction rims surrounding hornblende phenocrysts. The mineralogy of hornblende reaction rims in lavas from the Haystack (~375 ka), Sargents Ridge (~180 ka), Shastina (~10.9 ka), and Black Butte (~10.7 ka) are dominated by plagioclase, with lesser orthopyroxene, clinopyroxene, and Fe-Ti oxides. Scanning Electron Microscopy was used to characterize and measure the widths of hornblende reaction rims. Hornblende reaction rims in lavas from the Haystack range from 277 to 29 microns thick with an average thickness of 77 microns. Reaction rims in lavas from Shastina range from 218 to 70 microns thick with an average thickness of 130 microns. And, reaction rims in lavas from Black Butte range from 45 to 19 microns with an average thickness of 31 microns. These thicknesses are consistent with magma ascent durations of only 7-8 days for Black Butte lavas compared to at least 30 days for Shastina and the Haystack.