Development of the Long Valley rhyolitic magma system: strontium and neodymium isotope evidence from glasses and individual phenocrysts

Abstract

Abstract-Pre-caldera high-silica rhyolites of Glass Mountain and the voluminous, zoned rhyolitic Bishop Tuff record the evolution of a magma system from initiation at 2

Crystallization history of rhyolites at Long Valley, California, inferred from combined U-series and Rb-Sr isotope systematics

“…Contamination must also be restricted to events occurring shortly before eruption because diffusive processes are comparatively fast for biotite, forcing isotopic reequilibration with the melt within a few months (Freer, 1981;Giletti, 1990). Similar conclusions on the isotope compositions of biotites have been drawn from studies on Long Valley precaldera rhyolites (Christensen and DePaolo, 1993;Davies et al, 1994;Davies and Halliday, 1998). It is also noteworthy that the biotites display minor ( 234 U/ 238 U) disequilibrium, suggestive of a more complex history.…”

“…A key to understanding the magmatic processes that generated the rhyolites at Long Valley lies in the long-term temporal evolution of the system associated with the rapid and episodic nature of rhyolite production (this study; Halliday et al, 1989;Christensen and DePaolo, 1993;Davies et al, 1994;Reid et al, 1997;Davies and Halliday, 1998). The rhyolites at Long Valley have been shown to be volatile rich (e.g., Wallace et al, 1999).…”

“…To explain the linear trend in Figure 5, two-component mixing would require an end member with a relatively high Sr concentration and a low 87 Sr/ 86 Sr ratio and an end member with a low Sr concentration and a higher 87 Sr/ 86 Sr ratio. As explained above, assimilation of local country rocks (e.g., Duffield et al, 1995;Knesel et al, 1999) does not seem to account for this second component (see detailed discussion in Davies and Halliday, 1998). Preferential assimilation of biotite breakdown products has been proposed as a possible mechanism (Knesel and Davidson, 1997), but it is hard to see how such a physical mechanism could operate in a relatively low-temperature magmatic system the size of Long Valley, which has biotite on the liquidus.…”

Assimilation of preexisting Pleistocene intrusions at Long Valley by periodic magma recharge accelerates rhyolite generation: rethinking the remelting model

“…The new 40 Ar/ 39 Ar ages help strengthen previously reported temporal geochemical trends (cf. Davies and Halliday 1998, reference therein, and this study) and a lower age limit with which to compare published U-Pb zircon ages (obtained by both high spatial resolution and bulk crystal techniques). Unless stated otherwise all reported uncertainties are 1r.…”

“…Our goal is to understand how the caldera-forming Bishop Tuff magmas relate to those erupted as pre-and postcaldera rhyolites. This is accomplished primarily by integrating new Hf and Nd isotopic data and compositional data with literature Nd isotopic data (Davies and Halliday 1998;Davies et al 1994;Heumann and Davies 1997) and new and published 40 Ar/ 39 Ar eruption ages and U-Pb zircon crystallization ages (Mankinen et al 1986;Reid et al 1997;Simon and Reid 2005;Simon et al 2007Simon et al , 2008bSimon et al , 2009. New data are reported for precaldera rhyolite samples collected for previous investigations (Simon and Reid 2005;Simon et al 2007) and from new samples collected from postcaldera rhyolites and the Bishop Tuff specifically for this study.…”