The pace of rhyolite differentiation and storage in an ‘archetypical’ silicic magma system, Long Valley, California

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). …”

“…Regardless, demonstrating the possibility that a substantive reservoir existed at certain times in the past does not necessarily indicate that there was a single magma reservoir evolving through time. The Coso magma system is likely to have waxed and waned over time, as suggested for Long Valley by Reid and Coath (2000), Simon and Reid (2005), and Simon et al (2007). Wallace et al (1999) for quartz hosted melt inclusions for Long Valley rhyolites.…”

“…The approach taken here that directly measures the difference between crystallization age and eruption age, which is usually interpreted as a minimum storage time for magma in its reservoir (e.g., Davies et al 1994;Reid et al 1997;van den Bogaard and Schirnick 1995;Vazquez and Reid 2004), assumes that this duration of crystallization places some measurable estimate on the differentiation time and existence of melt in the shallow crust. It remains unclear as how to best translate this evidence into rhyolite production rates (Charlier et al 2005;Christensen and DePaolo 1993;Davies et al 1994;Halliday et al 1989;Jicha et al 2005;Knesel et al 1999;Michaut and Jaupart 2006;Reid and Coath 2000;Simon and Reid 2005;Wolff and Ramos 2003). The existence of a long-recognized relationship between the repose interval between explosive eruptions and their volumes (Smith 1979;Trial and Spera 1990) has led to the long-held view that the duration of silicic magma accumulation is proportional to erupted volume (see inset in Fig.…”

“…Moreover, during this time period we find clear evidence for crustal assimilation and crystal inheritance as indicated by Mesozoic zircon ages. The absence of protracted residence times and of evidence for recycling in the younger (B230 ka) Coso rhyolites contrast sharply with the discrete, i.e., 10's to 100's ka age spans reported to represent pre-eruption crystal residence (''magma residence times'' senso lato) in the older *0.6 Ma Devils Kitchen rhyolite, as well as at other large silicic magma centers, e.g., the Whakamaru Tuff at Taupo Volcano, New Zealand (Brown and Fletcher 1999), some post-caldera and the caldera-related rhyolites at Yellowstone (Bindeman, et al 2001;Vazquez and Reid 2002), and a number of Long Valley rhyolites (Reid and Coath 2000;Simon and Reid 2005;Simon, et al 2007). These differences amplify our need to distinguish between evidence for protracted fractional crystallization differentiation within long-lived shallow magma reservoirs and evidence for remobilization of crystals formed in relatively short-lived melt bodies (cf.…”

“…Red (dark-shaded) boxes for evolved magmas show range of uncertainty in volume and repose interval (*inverse eruption frequency). Lower panel shows pre-eruption crystallization intervals for select Pleistocene Coso and pre-caldera Long Valley rhyolites, DK Devils Kitchen (Miller and Wooden 2004), CP Cactus Peak (this study), and OGM older Glass Mountain, YGM younger Glass Mountain, BT Bishop Tuff (Reid and Coath 2000;Simon and Reid 2005;Simon, et al 2007). The general inverse relationship between size and pre-eruption crystallization interval displayed among different silicic fields (i.e., upper panel) exists among individual eruptions at both systems.…”