HR: 11:05h
AN: V52B-04    [Abstracts]
TI: Laser Ablation Pb Isotopes as Tracers of Rhyolite Evolution and Crystal Residence Times, Long Valley, CA
AU: * Simon, J I
EM: jisimon@ucla.edu
AF: Department of Earth and Space Sciences, UCLA, Los Angeles, CA 90095 United States
AU: Reid, M R
EM: mary.reid@nau.edu
AF: Department of Earth and Space Sciences, UCLA, Los Angeles, CA 90095 United States
AU: Reid, M R
EM: mary.reid@nau.edu
AF: Department of Geology, NAU, Flagstaff, AZ 86011 United States
AU: Young, E D
EM: eyoung@ess.ucla.edu
AF: Department of Earth and Space Sciences, UCLA, Los Angeles, CA 90095 United States
AU: Young, E D
EM: eyoung@ess.ucla.edu
AF: IGPP, UCLA, Los Angeles, CA 90095 United States
AB: At Long Valley (LV) the Sr isotopic composition of phenocrysts from the caldera-related Bishop Tuff (BT) imply that some crystals are $>$1 Myr older than eruption. In our studies of LV rhyolites, we find that zircon U-Pb ages are generally only hundreds of ky older than eruption, resulting in weak to strong disparities with the Rb/Sr mineral-melt model ages. Since Rb/Sr model ages depend on host glass compositions, these differences could result from crystal-liquid exchange between magmas or, given the relatively low Sr contents ($<$2 ppm) of the glasses, post-crystallization magma modification. To test the isotopic affinities of the various phenocryst populations to both their host melts and to secular changes in the Pb isotopic composition of the LV magma system, we performed {\it in situ} Pb isotopic analyses of feldspars and their host glasses. Lead contents are relatively high (10's ppm) in rhyolitic glass and feldspar and Pb isotope compositions are not expected to vary appreciably due to intrinsic contamination or radiogenic ingrowth ($^{207}$Pb/$^{206}$Pb decreases $\sim$0.00006 per Myr), but may vary if the contributions from different sources change with time. Lead isotopes were obtained for 8 host glasses and 113 feldspar phenocrysts by UV (213 nm) laser ablation MC-ICPMS. We use a sample-standard comparison approach for mass fractionation correction (rather than Tl-doping) during analyses. Measurements are age-corrected to initial ($^{207}$Pb/$^{206}$Pb, $^{208}$Pb/$^{206}$Pb, $^{206}$Pb/$^{204}$Pb) values. With this approach we obtain accurate results with an external precision of \pm0.00016 (2 s.d.) for the $^{207}$Pb/$^{206}$Pb ratio of the NIST SRM 612 glass, comparable to TIMS double-spike methods. The Pb isotope data for feldspars and their host glasses lie along a mixing line between regional mantle ($^{207}$Pb/$^{206}Pb$=0.81366) and crust ($^{207}$Pb/$^{206}$Pb=0.82698) and span $\sim$10 % of that isotopic difference. Differences between the Pb isotope compositions of older ($\sim$1.7-2.2 Ma) precaldera Glass Mountain (GM) rhyolites (e.g., $^{207}$Pb/$^{206}$Pb$_{o}$=0.81856) and younger LV rhyolites, including the BT ($^{207}$Pb/$^{206}$Pb$_{o}$=0.81745), are found. Considering also existing Nd and O isotope data for the same rhyolites, the Pb isotope data record a progressively greater mantle contribution to the LV rhyolites over time. Most feldspars have Pb isotope compositions that are broadly similar to their host glasses. Notably, the strong contrast between the isotopic composition of the BT and the older GM magmas is mirrored in the feldspars they contain. Thus, there is no indication that BT feldspars grew from older GM-like magmas. A number of grains from several GM rhyolites do, however, have distinct Pb isotopic compositions (e.g., $^{207}$Pb/$^{206}$Pb $>$+0.0005) that indicate contamination from a granitic country rock or possibly LV-related intrusions. Moreover, the Pb isotope heterogeneity within individual GM rhyolites suggests that some magmas experienced open system processes that may lead to inaccurate model ages. For this reason, the absolute age constraints we have obtained by U-Pb dating are better guides to interpretations about the timescales of magmagenesis than Rb/Sr model ages. Collectively, the distinct Pb isotope signatures for individual rhyolites and their feldspars imply that LV volcanism did not erupt from a single long-lived magma chamber but rather tapped a number of transient and distinct magma bodies.
DE: 8439 Physics and chemistry of magma bodies
DE: 8499 General or miscellaneous
DE: 3640 Igneous petrology
DE: 1040 Isotopic composition/chemistry
DE: 1094 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting