Sr and Nd Isotopic Studies of Melt Inclusions in Quartz: Evidence for Multiple Age Components in the Bishop Tuff, California

John N. Christensen Univ. of Michigan, Dept. of Geological Sciences, Ann Arbor, MI 48109, USA

Alex N. Halliday Univ. of Michigan, Dept. of Geological Sciences, Ann Arbor, MI 48109, USA

Melt inclusions in quartz provide direct samples of silicate liquid during the evolution of a magmatic system. Strontium isotopic studies of the high silica rhyolites of the Long Valley system indicate a long (100's kyrs) pre-eruption residence time for magmas (Halliday et al., 1989; Christensen and DePaolo, 1993; Davies et al., 1994). Furthermore, recent 40Ar-39Ar dating of melt inclusion bearing quartz from early erupted BT airfall indicates that quartz had resided in the Bishop Tuff (BT) magma for 1.1 Myr before eruption at 0.76 Ma (van den Bogaard and Schirnick, 1995). In contrast the Bogaard and Schirnick data for melt inclusions from later erupted ignimbrite do not conclusively indicate a ~2 Ma age for those melt inclusion bearing quartz grains. In order to investigate further the age and origin of BT melt inclusions we have determined the Rb-Sr isotopic compositions of melt inclusions in individual quartz crystals and bulk quartz separates from the Bishop Tuff. As a further tracer of melt inclusion origin we have conducted Nd isotopic analyses of melt inclusions from bulk quartz separates. Pumice samples are from the early erupted plinian airfall, an early ignimbrite flow of the Volcanic Table Land and late erupted ignimbrite of the Adobe lobe. Crystals were separated from bulk pumice samples as well as from individual pumice lumps. For the airfall sample individual crystals were selected for analysis as well as bulk quartz separates from a single pumice lump and a bulk pumice separate. In order to calculate apparent Rb-Sr ages for the melt inclusions we use an initial 87Sr/86Sr of 0.7060±1, based on Sr isotopic data for the BT and the pre-caldera Glass Mt.(GM) rhyolites magmas (Halliday et al., 1989; Christensen and DePaolo, 1993; Davies et al., 1994). All errors are quoted at 2s level. Two inclusion bearing quartz crystals from BT airfall have apparent ages of 2.1±0.1 Ma and 2.19±0.15 Ma similar to the Rb-Sr isochron ages of early GM rhyolites (2.047±0.013 and 1.894±0.013 Ma (Davies et al., 1994)). Two other quartz crystals have apparent ages of 2.5±0.2 Ma and 1.45±0.08 Ma. A bulk quartz crystal separate from the same pumice sample has an apparent age of 1.9±0.3 Ma, but a much lower 87Rb/86Sr than the individual crystals (33 vs. 210 to 300), indicating a mixed population. A quartz separate from a single airfall pumice lump has an apparent age of 1.33±0.08 Ma, indistinguishable from the 1.45±0.08 Ma individual quartz crystal. This suggests that in addition to a ~2 Ma component, there may also be younger components in the plinian airfall. This also underlines the importance of analyzing material separated from individual pumice lumps.

Melt inclusions from BT ignimbrite samples have younger apparent ages than those from the early erupted plinian airfall. Pumice from the late erupted Adobe lobe yield melt inclusion bearing quartz with an apparent age of 1.04±0.14 Ma. Two individual pumice lumps from early ignimbrite yield bulk quartz separates with apparent ages of 1.18±0.08 Ma (pumice lump 1) and 1.06±0.11 Ma, 1.08±0.11 Ma (pumice lump 2, two quartz size fractions). Therefore both late and early ignimbrite melt inclusions yield ages indistinguishable from the isochron age for the late GM rhyolites, 1.14±0.08 Ma (Halliday et al., 1989) and consistent with differentiation of the BT magma beginning at ~1.2 Ma (Christensen and DePaolo, 1993). Early GM rhyolites (~2 Ma differentiation age) have an eNd of -3 that contrasts with late GM rhyolites and the BT (eNd=-1). All bulk quartz separates, including the airfall sample, have melt inclusions with eNd= -1, identical to the BT and the late GM rhyolites but distinct from the early GM rhyolites (eNd= -3). This indicates the existence at ~2 Ma of two spatially separate bodies of magma, one that erupted to form the early GM rhyolites and the other reserved until the eruption at 0.76 Ma of the plinian phase of the Bishop Tuff.

Although, the Rb-Sr data support the presence of a 1.89 to 2.3 Ma component in the BT airfall as determined by 40Ar/39Ar on melt inclusions (van den Bogaard and Schirnick, 1995), they also indicate the presence of ~1.1 Ma melt inclusions from early and late ignimbrite representing the bulk of the Bishop Tuff, consistent with earlier studies based on Sr and Nd isotope geochemistry (Halliday et al., 1989; Christensen and DePaolo, 1993).


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