HR: 1340h
AN: V53A-0623    [Abstracts]
TI: Origin of Thermal and Compositional Zoning in the Bishop Magma Reservoir: Insights from Zoned Quartz Phenocrysts
AU: * Wark, D A
EM: warkd@rpi.edu
AF: Rensselaer Polytechnic Institute, Dept of Earth and Environmental Sciences, 110 8th Street, Troy, NY 12180 United States
AU: Hildreth, W
EM: hildreth@usgs.gov
AF: Volcano Hazards Team, U.S. Geological Survey, 345 Middlefield Rd, MS-910, Menlo Park, CA 94025 United States
AU: Watson, E B
EM: watsoe@rpi.edu
AF: Rensselaer Polytechnic Institute, Dept of Earth and Environmental Sciences, 110 8th Street, Troy, NY 12180 United States
AU: Cherniak, D J
EM: chernd@rpi.edu
AF: Rensselaer Polytechnic Institute, Dept of Earth and Environmental Sciences, 110 8th Street, Troy, NY 12180 United States
AB: Since they were first described roughly 25 yrs ago, variations in melt composition and in eruption temperature among samples of the 0.76 Ma Bishop Tuff have been documented by many workers. Although generally accepted that these variations reflect thermal and compositional gradients in the pre-eruption magma chamber, the origin and longevity of these gradients have remained controversial. We propose that valuable insight into these issues can be gained by examination of Ti content and zoning patterns in quartz phenocrysts, in combination with the TITANiQ (Titanium-in-Quartz) thermometer (Wark & Watson; 2004 Goldschmidt). As first shown by Peppard et al. (2001; Am. Min.), quartz phenocrysts from late-erupted Bishop pumice display rims with stronger cathodoluminescence (CL) intensity than cores. Our examination of quartz from the entire Bishop sequence reveals a similar pattern: rims of quartz in most pumices are brighter than cores using CL, with the brightest rims in quartz from pumices with the highest FeTi oxide temperatures. Mean Ti contents of quartz phenocryst rims range from about $\sim$40 to 100 ppm, indicating crystallization temperatures (using TITANiQ and assuming {\it a}TiO$_{2}$=0.6) ranging from $\sim$720 to $820\deg$C, closely matching FeTi oxide temperatures from the same pumices. In contrast, the cores of most quartz phenocrysts have lower Ti contents ($\sim$30 to 50 ppm) than rims, apparently preserving a record of quartz crystallization at temperatures of $\sim$680 to $740\deg$C, significantly lower than the eruption temperatures recorded by rims and by FeTi oxides. Where visible, contacts between dark CL (low temperature) cores and bright CL (high temperature) rims are abrupt, and in many cases are discordant to recognizable CL zones in the core, probably reflecting a dissolution event. Based on estimated Ti diffusivities in quartz, the abrupt steps in Ti content would likely have been eliminated by diffusive re-equilibration in less than 1000 yrs at $\sim$$800\deg$C. Together, these observations are tentatively interpreted to indicate that the thermal and melt-composition gradients recorded by minerals and glass at the time of Bishop tuff eruption may have been short lived. Temperatures were apparently lower (by 40 to $80\deg$C) and thermal gradients were shallower at some earlier stage of magma evolution. A thermal pulse - presumably associated with influx of mafic melt - then caused partial resorption of quartz and other phases, with crystallization re-commencing at the elevated temperatures recorded by quartz rims and by FeTi oxides. Release of volatiles from the recharge melt, combined with partial resorption of a deep, "cumulate" phenocryst assemblage may also explain the relatively high concentrations of Ba, Sr, Ti, and CO$_{2}$ in melt inclusions within some quartz rims.
DE: 8499 General or miscellaneous
DE: 5480 Volcanism (8450)
DE: 3640 Igneous petrology
DE: 3670 Minor and trace element composition
DE: 1065 Trace elements (3670)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting