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

Abstract

We determined Ar/Ar eruption ages of eight extrusions from the Pleistocene Coso volcanic field, a longlived series of small volume rhyolitic domes in eastern California. Combined with ion-microprobe dating of crystal ages of zircon and allanite from these lavas and from granophyre geothermal well cuttings, we were able to track the range of magma-production rates over the past 650 ka at Coso. In B230 ka rhyolites we find no evidence of protracted magma residence or recycled zircon (or allanite) from Pleistocene predecessors. A significant subset of zircon in the *85 ka rhyolites yielded ages between *100 and 200 Ma, requiring that generation of at least some rhyolites involves material from Mesozoic basement. Similar zircon xenocrysts are found in an *200 ka granophyre. The new age constraints imply that magma evolution at Coso can occur rapidly as demonstrated by significant changes in rhyolite composition over short time intervals (B10's to 100's ka). In conjunction with radioisotopic age constraints from other young silicic volcanic fields, dating of Coso rhyolites highlights the fact that at least some (and often the more voluminous) rhyolites are produced relatively rapidly, but that many small-volume rhyolites likely represent separation from long-lived mushy magma bodies.

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

“…These studies conclude that either two distinct magma systems existed or portions of a large stratified body evolved independently at that time (Heumann and Davies 1997). In allanite-saturated rhyolites from Toba (Sumatra) and Coso (California), compositional shifts toward higher La/Nd have been related to input of more mafic magma (e.g., Vazquez and Reid 2004;Simon et al 2009). Addition of evolved magma and/or intrusions where allanite saturation has not occurred could also drive up La/Nd values, i.e., that possibly exists among the Mesozoic granitoids (Fig.…”

“…Eruption temperatures (i.e., derived from coexisting Fe-Ti oxide pairs) are expected to be equal to or less than crystallization temperatures, although reversals are reported elsewhere (e.g., Simon et al 2009). In general, there are limitations to establishing a robust geothermometry based on coexisting mineral assemblages for Long Valley rhyolites (e.g., Evans and Bachmann, 2013), in part because of the crystal-poor nature of the rocks, the possible exceptions being the Bishop Tuff (Hildreth 1977) and a short list of precaldera rhyolites (Metz and Mahood 1991).…”

“…Obsidian samples were crushed to produce fragments of black glass, free of phenocrysts, and cleaned in distilled water with an ultrasonic bath. Two irradiations were used, the same ones used by Simon et al (2009). The rhyolite materials, along with 91 Alder Creek feldspars (1,193 ± 1 ka; Nomade et al 2005), used to monitor the neutron fluence, were irradiated at the Oregon State University TRIGA reactor in the Cdlined CLICIT facility for 30 min within a series of stacked aluminum disks, permitting precise monitoring of both vertical and lateral (negligible) flux gradients (see supplementary Appendix for specifics).…”

“…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.…”

Uranium Series, Volcanic Rocks

“…As with other isochron methods, dating precision is maximized when minerals or rocks exhibit a large contrast in parent/daughter ratios. For example, isochrons pairing accessory minerals such as zircon and allanite, whose U/Th ratios vary by orders of magnitude, may yield late Pleistocene ages with <5 % precision (Simon et al 2009). To date volcanic eruptions, the crystallization or fractionation responsible for U-series isochrons must occur shortly before eruption.…”