OLIVINE EXIT INTERVIEWS FROM BASALTIC MAGMAS NEAR MAMMOTH MOUNTAIN, CA

Mammoth Mountain, CA, and its peripheral basaltic vents have been active over the last 200 kyr. While the main edifice produces magmas large stored and modified in the crust, peripheral basaltic magmas provide a window into the deeper magmatic processes, prior to shallow crustal storage. Within the basaltic magmas olivine crystals potentially retain information about magma assembly and mixing of different melt compositions, compared to the whole rock or melt composition that integrates over these processes.

Here we present laser-ablation inductively-coupled plasma mass spectrometry data from olivine in three peripheral basalts near Mammoth Mountain. The olivine trace and minor element chemistry supports a range of primitive melt compositions that cannot simply be explained by fractional crystallization. We studied olivine from the basalts of Horseshoe Lake (BHL), Fish Hatchery (BFH), and Casa Diablo Hot Springs (BCD). Olivine varies between Fo70 and Fo88 with BCD being more restricted at higher Fo content (Fo80-88). Different populations of olivine exist in each basalt that imply the presence of different melt pockets of distinct magma chemistry prior to final assembly and eruption. Olivine from BHL has a unique trace element signature with comparably low Cr and Ni concentrations and slightly elevated Sc at any given Fo content. These variations require different parental melts derived from the mantle suggesting that the mantle beneath Mammoth Mountain is compositionally heterogenous over small scales.