HR: 0800h
AN: V31A-1419    [Abstracts]
TI: Limited Boron Isotopic Variation Between Caldera-Forming and Post-Caldera Low-$\delta$$^{18}$O Rhyolites from Yellowstone Caldera
AU: * Schmitt, A K
EM: axel@argon.ess.ucla.edu
AF: Department of Earth and Space Sciences, University of California Los Angeles, 595 Charles Young Dr. E, Los Angeles, CA 90095-1567 United States
AB: Post-collapse rhyolites from within Yellowstone caldera show prominent excursions to $\delta$$^{18}$O values (VSMOW) as low as $\sim$0 $\permil$ that provide strong evidence for significant non-lithospheric oxygen input into magmas, presumably via infiltration of meteoric waters (1). Very little, however, is known about the behavior of other stable isotope systems, such as boron, in low $\delta$$^{18}$O rhyolites and their potential for unraveling the mechanisms by which brines and magmas interact. Ion microprobe measurements of quartz-hosted melt inclusions from two low-$\delta$$^{18}$O rhyolite flows (South Biscuit Basin SBB and Middle Biscuit Basin MBB) yielded average $\delta$$^{11}$B values (NIST SRM 951) of -1.5 and -0.9 $\permil$, respectively. These values overlap within error with those for glassy obsidian matrix from SBB and MBB. Melt inclusions from two caldera-forming tuff eruptions (Lava Creek Tuff LCT and Huckleberry Ridge Tuff HRT), known to have 'normal' oxygen isotopic compositions, also yielded indistinguishable $\delta$$^{11}$B values of -1.8 $\permil$. Recent petrologic studies (1) suggested that bulk remelting of hydrothermally altered volcanic rocks, specifically HRT, in the down-dropped roof of the magma chamber produced the low-$\delta$$^{18}$O magmas. The lack of strong boron isotopic variations (within $\pm$2$\permil$) between 'normal' and low-$\delta$$^{18}$O rhyolites, however, contrasts with published evidence for strong $^{11}$B-depletion in hydrothermal altered rhyolite encountered in Yellowstone drill-wells ($\delta$$^{11}$B = -9.7 $\permil$; 2). This implies that boron isotopic fractionation due to interaction with hydrothermal fluids was either absent in the source region of the SBB and MBB magmas, or became masked due to subsequent processes. From preliminary mixing calculations it is concluded that assimilation of $^{11}$B- and $^{18}$O-depleted rocks by fresh rhyolite recharge could be a compositionally and thermally viable alterative to bulk remelting. (1) I. N. Bindeman and J. W. Valley (2001) J Petrol 42, 1491-1517; (2) M. R. Palmer and N. C. Sturchio (1990) Geochim Cosmochim Acta 54, 2811-2815
DE: 8424 Hydrothermal systems (8135)
DE: 3640 Igneous petrology
DE: 1030 Geochemical cycles (0330)
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting