EXAMINING TIMELINES OF HYDROTHERMAL EXCHANGE AND PARTIAL MELTING USING SIMS IN-SITU δ18O MEASUREMENTS IN GRANODIORITE EJECTED IN THE CLIMACTIC ERUPTION OF MOUNT MAZAMA, CRATER LAKE, OREGON

Interaction with the upper crust is likely an important component of the development of large silicic magma chambers associated with caldera-forming eruptions. At Crater Lake, Oregon, evidence for this interaction is indicated by the presence of granodiorite blocks in pyroclastic deposits of the climactic, caldera-forming eruption of Mt. Mazama (7.7 ka). The granodiorite has been interpreted to represent wall rocks of the climactic magma chamber that, prior to being erupted, experienced variable amounts of exchange with meteoric hydrothermal fluids and subsequent partial melting, and provide an opportunity to examine the timeline(s) of hydrothermal circulation and heating associated with the caldera-forming magma chamber. Because of the large fractionation of oxygen isotopes between igneous rocks and meteoric water, and the sensitivity of fractionation to temperature, oxygen isotopes are particularly well suited for tracing hydrothermal alteration and upper-crustal assimilation processes in volcanic rocks. In this study, we collected SIMS in-situ measurements of oxygen isotope ratios in the cores and rims of plagioclase and quartz in rocks that have experienced from 0 to 50% melting in order to determine if core to rim variation is present, and, if so, calculate the time necessary to produce said zonation. Previously measured bulk plagioclase and quartz separates from these samples have δ¹⁸O values that range from +3.3‰ to -3.4‰ and +5.1‰ to -2.2‰ VSMOW, respectively (Bacon et al., 1989, EPSL). We found that oxygen isotope ratios in plagioclase are surprisingly homogenous, with Δ_Core-Rim falling at or within analytical uncertainty (±0.2‰) in most crystals. Oxygen isotopes in quartz, on the other hand, retain core to rim zonations of up to Δ_Core-Rim=1.7 ± 0.2‰, although most are significantly smaller (<0.5‰). We propose that diffusion via hydrothermal fluids, and subsequent heating, resulted in the near-complete homogenization of the plagioclase, but not the quartz, as oxygen diffuses more quickly in plagioclase. Current work is focused on modeling this scenario to determine a more robust estimate of the timing of the onset of hydrothermal circulation associated with the climactic magma chamber, as well as the timing of heating ± partial melting.