U-Pb dating of zircon in the Bishop Tuff at the millennial scale

• Published in: Geology (Boulder) 2007-12, Vol.35 (12), p.1123-1126
• Main Authors: Crowley, J. L, Schoene, B, Bowring, S. A
• Format: Article
• Language: English
• Subjects: absolute age; Bishop Tuff; California; Cenozoic; Crystallization; dates; equilibrium; eruptions; experimental studies; Geochronology; grains; igneous rocks; Isotopes; Lead; Magma; magma chambers; magmas; mass spectra; millenial variations; Mono County California; nesosilicates; orthosilicates; Owens River gorge; Pleistocene; pyroclastics; Quaternary; Quaternary geology; Rocks; sampling; silicates; spectra; thermal ionization mass spectra; tuff; U/Pb; United States; Uranium; volcanic rocks; volcanism; zircon; zircon group; Zirconium
• ISSN: 0091-7613; 1943-2682
• DOI: 10.1130/G24017A.1

Zircon from the Bishop Tuff (eastern California) was dated by the U-Pb isotope dilution thermal ionization mass spectrometry (ID-TIMS) method to evaluate time scales of magmatic evolution and applicability of the method to rocks younger than 1 Ma. The ...Pb/...U dates from 17 of 19 grains are equivalent with a weighted mean of 767.1 ± 0.9 ka that overlaps with the published ...Ar/...Ar sanidine age, requiring that most zircon crystallized immediately before eruption. Dates from two grains that are 6 and 12 k.y. older indicate minimum times that a small portion of the grains resided in the magma. These findings are consistent with chemical zoning patterns in zircon that suggest a relatively simple crystallization history, yet contrast with previous results of ion microprobe dating of zircon that show 50-80 k.y. of magma residence. The weighted mean ...Pb/...U date is ... 52 k.y. older than the ...Pb/...U date, a difference that is attributed to excess ...Pb from initial Pa/U disequilibrium. The results demonstrate that high-precision ID-TIMS geochronology can resolve magma chamber dynamics of