PETROLOGIC CONSTRAINTS ON ERUPTION TRIGGERING AND MAGMA ASCENT AT MAMMOTH MOUNTAIN, CALIFORNIA

In this study, we present results that constrain the timescales of eruption triggering and magma ascent at Mammoth Mountain, a trachydacitic to rhyodacitic dome cluster located on the southwestern rim of Long Valley Caldera in eastern California. Field-based observations and analytical evidence show that many Mammoth Mountain dome magmas were repeatedly intruded by batches of mafic magma prior to eruption based on the presence of mafic enclaves, derivative xenocrysts, and disquilibria textures suggesting that eruptions from Mammoth Mountain may be triggered by mafic replenishment events. Electron microprobe analysis touching Fe-Ti oxide pairs analyzed from 8 domes (including Dragon's Back) show a range in temperatures from 748 to 945C (Log fO2 -10.71 to -16.65) for core compositions, which are taken to represent a pre-mixing temperature of the magma, and 862 to 989C (Log fO2 -10.15 to -18.26) for rim compositions, which are interpreted to represent a post-mixing temperature. Diffusion profiles in Fe-Ti oxides suggest that mixing preceded eruption of trachydacites over timescales of approximately 1 month. Constraining the timescales and styles of magma ascent was performed by investigating the thickness, mineralogy, and texture of amphibole and biotite reaction rims found in Mammoth Mountain domes. Hornblende phenocrysts are angular surrounded by reaction rims composed of plagioclase, pyroxene and Fe-Ti oxides. Though all samples are similar in mineralogy, rim microlites range in size from 2 to 13 um and rim thicknesses range from 2.80-271.60 um in diameter, indicating a highly variable eruption pathway. Rim textures of samples taken from Dragon's Back (located on the southwestern periphery of the dome complex) are texturally zoned, where the rim adjacent to groundmass is coarser relative to rim microlites adjacent to the phenocryst. Rim textures of samples taken from elsewhere on the Mammoth edifice are homogenous with respect to rim microlite grain size. Aspect ratios of the reaction rim microlites from all samples range from 0.9 to 5.9 with a maximum crystal length of 12.7um.