LEAD ISOTOPIC (204, 206, 207, 208) COMPOSITION OF INDIVIDUAL MELT INCLUSIONS FROM THE BISHOP TUFF, LONG VALLEY, CA

The Bishop Tuff (BT) is a classic large-volume, high-silica rhyolite ash flow tuff that was erupted 760,000 years ago. The major element composition of the BT is remarkably homogeneous (75.5 to 77.6 wt% SiO2) but trace element concentrations are strongly zoned from early to late in the eruptive sequence. The origin of the magmatic system has been ascribed to magma mixing of crust and mantle melt components but the nature of these components and their distribution within the plumbing system are poorly defined. We have analysed Pb isotopes in 29 melt inclusions (MI) in quartz and plagioclase phenocrysts from a sample of the early plinian phase of the BT collected from the Chalfant Quarry near Bishop, CA, using 193 nm excimer laser ablation (LA)-multiple ion counter (MIC)-inductively coupled plasma-mass spectrometry (ICPMS). The MIC array on the NEPTUNE ICPMS instrument allowed, for the first time, simultaneous collection of all four lead isotopes (204Pb, 206Pb, 207Pb, 208Pb) at total lead concentrations levels of only 25 to 30 ppm, in high-silica rhyolite MI. In order to assess the reproducibility, replicate analyses of two large (>100 micron) inclusions, RE-3-20B (n=5) and RE-3-27A (n=7) were made using 30 and 40 micron spots. The scatter of analyses of 208,207,206Pb/204Pb ranges from 0.58% to 1.16% for MI RE-3-20B and 0.61% - 1.47% for MI RE-3-27A. These ranges are significantly smaller than the total range of measured lead isotope ratios determined in all BT inclusions of this study (~2.2% - 2.54%). The MI and plagioclase data define lead isotope mixing arrays between the upper mantle and old upper crust sources. Preservation of lead isotope heterogeneities within MI suggests that melts from discrete mantle and upper crustal sources coexisted at early stages of BT magmatic evolution and only became homogenized during or just prior to eruption.