HR: 1340h
AN: V53A-0625    [Abstracts]
TI: A Transitional Pumice Clast From the Bishop Tuff
AU: * Anderson, A T
EM: canderso@uchicago.edu
AF: University of Chicago, Dept. of Geophysical Science, 5734 S Ellis Ave,, Chicago, IL 60637
AU: Davis, A M
EM: a-davis@uchicago.edu
AF: University of Chicago, Dept. of Geophysical Science, 5734 S Ellis Ave,, Chicago, IL 60637
AU: Davis, A M
EM: a-davis@uchicago.edu
AF: Enrico Fermi Institute, Univ. of Chicago, 5640 S Ellis Ave,, Chicago, IL 60637
AU: Liu, Y
EM: yangl@uchicago.edu
AF: University of Chicago, Dept. of Geophysical Science, 5734 S Ellis Ave,, Chicago, IL 60637
AU: Steele, I M
EM: steele@geosci.uchicago.edu
AF: University of Chicago, Dept. of Geophysical Science, 5734 S Ellis Ave,, Chicago, IL 60637
AB: A common interpretation is that large bodies of silicic magma like that which erupted to form the Bishop Tuff rhyolite receive episodic injections of basaltic magma that rejuvenate the silicic magma and keep it hot and molten. Lack of mixing, as indicated by the common zoning patterns described below, is difficult to reconcile with injections of basaltic magma and consequent convective heat transfer. Peppard et al (2001) reported brightly cathodoluminescent rims on quartz phenocrysts from northern (late-erupted) units of the Bishop Tuff. Quartz phenocrysts in documented southeastern deposits (early-erupted) lacked bright rims, and this dichotomy in zoning pattern was interpreted to require independent pre-eruptive evolution of separate parts of the magma. Magmas that erupted separately and formed different deposits did not mix together after the formation of the bright rims. We have now discovered a pumice clast from the southeastern Chalfant Quarry deposit that has quartz phenocrysts with brightly cathodoluminescent rims. The new clast is from a later (higher) stratigraphic horizon than any we had formerly studied from the southeast (early-erupted) deposits. The brightly cathodoluminescent rims vary in thickness but are in general relatively thin, especially on big quartz phenocrysts and generally intermediate between the thicker rims on the northern clasts and the rimless earlier-deposited southern clasts. In accord with the observations of Peppard et al., all of the imaged quartz phenocrysts in the new clast are similar in having bright rims. The key observation is that the clast contains a single population of quartz phenocrysts, not a mix of crystals with and without bright rims. Thus the message remains the same: after formation of the bright rims, no mixing occurred between the different parts of the magma. Unlike inclusions in northern-deposited quartz (Anderson et al 2000), there is no correlation between proximity to rim and Ba in these new melt inclusions; however, Ba-rich melt inclusions are more common in the smallest quartz phenocrysts. We suggest that large quartz crystals that initially grew in Ba-poor melt sank into more Ba-rich magma where they continued to grow; smaller ones grew mainly in Ba-rich melt.
DE: 8400 VOLCANOLOGY
DE: 3640 Igneous petrology
DE: 1065 Trace elements (3670)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting