Master of Science (MS)
Jonathan . Miller
Geochemistry; Geology; Petrology
The Pleistocene to Holocene Mono Craters in eastern California comprise at least 28 high-silica rhyolite domes, lava flows and tephra cones. To examine the geochemical evolution of the magma system, trace elements were analyzed on crystal rims of zircons separated from juvenile pumice clasts from 5 ashes in the correlative Wilson Creek Formation (spanning from 62 to 21 ka) and from 3 rhyolite domes (21, 12.5, and 7 ka). Rare earth element (REE) patterns show similar overall shapes with prominent negative Eu anomalies, indicative of similar geochemical environments of zircon crystallization, but variation in key trace elements and ratios indicate that eruptions tapped discrete magma bodies within a larger rhyolite system. Ti-in-zircon temperatures (TTi,zrc) are predominantly 650°C to 750°C; similar to zircon-saturation temperatures. Zircons were stored at relatively cool but not necessarily subsolidus temperatures. Comparison of zircon rims with a subset of sectioned interiors for the 12.5 and 7 ka domes show decreasing Hf and increasing Eu/Eu*, Ti, and Th/U, consistent with a temperature increase prior to eruption. These observations indicate that heat from mafic injection helped maintain the magma system and possibly triggered eruptions. Zircon-melt partition coefficients (KDs) were also obtained by analyzing coexisting rhyolite glasses from each unit. The values are generally consistent with published partition coefficients and thus may be useful for modeling melt compositions for zircons.
Baker, Natalie D., "Zircon Trace Element Variation from Mono Craters (California) Ashes and Domes as Recorders of Temporal Changes in the Rhyolite Magma System" (2019). Master's Theses. 5056.