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

Abstract

Abstract-In this study, we present 87 Rb/ 86 Sr and 230 Th/ 238 U isotope analyses of glasses and phenocrysts from postcaldera rhyolites erupted between 150 to 100 ka from the Long Valley magmatic system. Both isotope systems indicate complex magma evolution with preeruptive mineral crystallization and magma fractionation, followed by extended storage in a silicic magma reservoir. Glass analyses yield a Rb-Sr isochron of 257 Ϯ 39 ka, which can be explained by a feldspar-fractionation event ϳ150 ky before eruption. Individual feldspar-glass pairs confirm this age result. A mineral 230 Th-238 U isochron in a low-silica rhyolite from the Deer Mountain Dome defines an age of 236 Ϯ 1 ka, but the glass and whole rock do not lie on the isochron. U-Th fractionation of the rocks is controlled by the accessory minerals zircon and probably allanite, which crystallized at 250 Ϯ 3 ka and 187 Ϯ 9 ka, respectively. All major mineral phases contain accessory mineral phases; therefore, the mineral isochron represents a mixture of zircon and allanite populations. A precision of Ϯ1 ka for the mixing array implies that the minor phases must have crystallized within this timescale. Longer periods of crystal growth would cause the mixing array to be less well defined. U-series data from other lowand high-silica rhyolites indicate younger accessory mineral crystallization events at ϳ200 and 140 ka, probably related to the thermal evolution of the magma reservoir. These crystallization events are, however, only documented by the accessory minerals and had no further influence on bulk magma compositions. We interpret the indistinguishable age results from both isotope systems (ϳ250 ka) to record the fractionation of small magma batches by filter pressing from a much larger underlying magma volume, followed by physical isolation and extended storage at the top of the magma reservoir for up to 150 ky.

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

“…Comparing U-Th-Pb and 40 Ar/ 39 Ar data sets such as those presented herein and reported previously (Crowley et al 2007;Heumann et al 2002;Reid and Coath 2000;Reid et al 1997;Rivera et al 2011;Sarna-Wojcicki et al 2000;Simon and Reid 2005;Simon et al 2007Simon et al , 2008b) provides a more direct and arguably a more accurate means for determining the subvolcanic magma residence time span as compared to estimates based on the repose interval between eruptions (e.g., Spera and Crisp 1981) or those based on 40 Ar/ 39 Ar and Rb/Sr model ages (e.g., Christensen and DePaolo 1993;van den Bogaard and Schirnick 1995). At Long Valley, when compared to magma residence times estimated from 40 Ar/ 39 Ar contained in melt inclusions or Rb/Sr crystal model ages of 100's to 1,000 ka, the zircon-based estimates are markedly shorter (i.e., B100's of ka and not C500 ka).…”

“…3). 40 Ar dating of rhyolites reported by Davies et al (1994), Heumann et al (2002), Miller (1985, Simon et al (2007), and Simon et al (2008b). We used the same mass spectrometer at the Berkeley Geochronology Center, methods, and irradiations and carried out the experiments at essentially the same time as those published previously by Simon et al (2007Simon et al ( , 2008b.…”

“…At Long Valley, zircon age populations have been reported for a number of rhyolite extrusions in addition to the Bishop Tuff (Heumann et al 2002;Reid et al 1997;Simon and Reid 2005;Simon et al 2007Simon et al , 2008b. Zircon ages have been measured in five of the precaldera Glass Mountain domes and show no evidence for inheritance of zircons older than the Long Valley magmatic system (Simon and Reid 2005).…”

“…Fig. 7 of Heumann et al 2002). It follows that the mean Th/U (melt) value of 2.81 ± 0.16 (1 s) used by Crowley et al (2007) from a subset of the published data for quartz-hosted melt inclusions (Anderson et al 2000) may not represent the melt composition from which the dated zircon grew.…”

Accessory mineral U–Th–Pb ages and 40Ar/39Ar eruption chronology, and their bearing on rhyolitic magma evolution in the Pleistocene Coso volcanic field, California

“…TIMS ages reported by Crowley et al 2007), whereas protracted ages like that of the Devils Kitchen are more common (i.e., among ion probe studies summarized in Simon et al 2008). Although the volcanic history of Coso is distinct from many long-lived silicic volcanic centers, the diversity of pre-eruption zircon and allanite age spans appears analogous to that reported for individual rhyolites at Long Valley (Heumann et al 2002;Reid and Coath 2000;Reid et al 1997;Simon and Reid 2005;Simon et al 2007). …”