HR: 09:00h
AN: V51A-05    [PDF]
TI: Boron Isotopic Variations in Hydrous Rhyolitic Glasses From Caldera-Forming Ignimbrite and Post-Caldera Domes at Long Valley
AU: * Schmitt, A K
EM: axel@argon.ess.ucla.edu
AF: Department of Earth and Space Sciences, University of California Los Angeles, 595 Charles Young Dr. E, Los Angeles, CA 90095-1567 United States
AU: Simon, J I
EM: jisimon@ucla.edu
AF: Department of Earth and Space Sciences, University of California Los Angeles, 595 Charles Young Dr. E, Los Angeles, CA 90095-1567 United States
AB: Long Valley, located in central California, is ideal to explore trace element and volatile element systematics in continental silicic magma systems, due to its youthful, accessible and extensive eruptive record. Here we explore boron variations in volcanic glasses from the compositionally and thermally zoned Bishop Tuff and from small-volume post-caldera rhyolites. Ion microprobe measurements of undegassed melt inclusions from early and late erupted Bishop Tuff pumice yielded average $\delta$$^{11}$B values of +3.5$\pm$0.9$\permil$ and +5.0$\pm$0.9$\permil$, resp. (2 standard errors; MSWD $>$1.5). These values overlap with a positive thermal ionization mass spectrometry result for vesiculated groundmass glass from late Bishop Tuff ($\delta$$^{11}$B = +5.4$\pm$0.9$\permil$; 2 standard deviations). Boron concentrations in melt inclusions continuously increase from late ($\sim$48 ppm) to early Bishop Tuff ($\sim$60 ppm). Bishop Tuff results are consistent with experimental evidence that predict only minor $^{11}$B depletion in hydrous melts undergoing gas-saturated fractional crystallization. By contrast, undegassed melt inclusions from two crystal-rich post-caldera lavas (Deer Mountain and South Deadman Dome) are comparatively boron-rich (max. 90 ppm B) and have lower $\delta$$^{11}$B (average +2.2$\pm$0.8 and -0.4$\pm$1.0$\permil$, resp.; MSWD $>$1.1). These values are also distinct from those in coerupted crystal-poor obsidian (average 27 ppm B; $\delta$$^{11}$B = +5.7$\pm$0.8$\permil$; ; MSWD = 0.4). Degassing and/or post-eruptive alteration are unlikely to have caused these variations. Furthermore, variable degrees of country-rock contamination would be expected to cause some correlation between $\delta$$^{11}$B and radiogenic isotope compositions (e.g., $^{143}$Nd/$^{144}$Nd) and this is not observed. Therefore, we favor assimilation of $^{11}$B depleted, but otherwise compositionally similar, chilled magma chamber margin rocks in the genesis of crystal-rich post-caldera rhyolites. Boron isotopes thus complement evidence from low-$\delta$$^{18}$O rhyolitic suites described for other silicic magma systems and trace the recycling of hydrothermally altered rocks that so far has gone undetected for Long Valley.
DE: 1030 Geochemical cycles (0330)
DE: 1040 Isotopic composition/chemistry
DE: 3640 Igneous petrology
DE: 8404 Ash deposits
DE: 8424 Hydrothermal systems (8135)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting