Pb isotope variations among Bandelier Tuff feldspars: No evidence for a long-lived silicic magma chamber

Abstract

Research Article| June 01, 2003 Pb isotope variations among Bandelier Tuff feldspars: No evidence for a long-lived silicic magma chamber J.A. Wolff; J.A. Wolff 1 Department of Geology, Washington State University, Pullman, Washington 99164, USA Search for other works by this author on: GSW Google Scholar F.C. Ramos F.C. Ramos 1 Department of Geology, Washington State University, Pullman, Washington 99164, USA Search for other works by this author on: GSW Google Scholar Geology (2003) 31 (6): 533–536. https://doi.org/10.1130/0091-7613(2003)031<0533:PIVABT>2.0.CO;2 Article history received: 27 Nov 2002 rev-recd: 21 Feb 2003 accepted: 24 Feb 2003 first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation J.A. Wolff, F.C. Ramos; Pb isotope variations among Bandelier Tuff feldspars: No evidence for a long-lived silicic magma chamber. Geology 2003;; 31 (6): 533–536. doi: https://doi.org/10.1130/0091-7613(2003)031<0533:PIVABT>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract We report, for the first time, high-precision Pb isotope data from a high-silica rhyolite. Prior work on Sr isotopes in the 1.6 Ma Otowi Member of the Bandelier Tuff (Valles caldera, New Mexico) established that large 87Sr/86Sr variations exist among Otowi glasses and sanidine phenocrysts. While the glasses display unequivocal evidence for wall-rock contamination of the Otowi magma following sanidine growth, a positive correlation between 87Sr/86Sri and 87Rb/86Sr among the feldspars could be interpreted as either a mixing line or an in situ magmatic isochron dating a differentiation event ∼270 k.y. prior to eruption. The 206Pb/204Pb and 87Sr/86Sr ranges for Otowi sanidines are 17.790 ± 0.002 to 17.831 ± 0.002 and 0.7074–0.7052, respectively. This Pb isotope range cannot be produced by radiogenic ingrowth at the U/Pb ratios of the host magma on any geologically reasonable time scale, and hence is unequivocal evidence for open-system behavior of the Otowi magma prior to and/or concurrent with feldspar growth. Open-system behavior is predicted to control Sr isotope variations due to much higher concentrations of Sr, relative to Pb, in the country rock than in the magma. These observations therefore undermine any age significance of the Rb-Sr isotope variations. In the absence of supporting data, Rb-Sr relations alone do not impart any information about residence times of high-silica rhyolite magmas with subchondritic concentrations of Sr. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.