Magma Generation at a Large, Hyperactive Silicic Volcano (Taupo, New Zealand) Revealed by U–Th and U–Pb Systematics in Zircons

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

“…Postcaldera Nd data and Ar/Ar data from Davies and Halliday (1998) plotted as ''x's,'' which shows secular shifts in Nd over time periods of 100's ka, but melt compositions likely changed more rapidly (10's ka) perhaps continuously as indicated by e Nd (e.g., Christensen and Depaolo 1993;Halliday et al 1984) and e Hf (this study, see Fig. 7 Sr-excesses may be antecrysts (e.g., Charlier et al 2005)-crystals related to the system but not necessarily to the magma in which they are erupted.…”

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

“…Classic inheritance (sensu Gulson and Krogh 1973) in several silicic extrusions has been identified by ion microprobe spot analyses of crystal cores, e.g., in crystal-rich units like the Tertiary Amalia Tuff of New Mexico (Johnson et al 1989), the Fish Canyon Tuff of Colorado (Lanphere and Baadsgaard 2001), and the *1.2 Ma Ongatiti ignimbrite from the Taupo volcanic zone, New Zealand (Brown and Smith 2004). Similar evidence for the incorporation of basement material also exists for a couple crystal-poor units at Taupo (the *20 ka unit X and 6-7 ka unit G; Charlier et al 2005). Unit X largely contains Pleistocene zircon and exhibits less than *15% basement aged zircon-much less abundant than the *50% or more found in the post-230 ka Coso rocksand as inherited zircon cores rather than as whole crystals.…”

“…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. Bachmann and Bergantz 2003;Bacon and Lowenstern 2005;Charlier et al 2005;Schmitt and Simon 2004) and ultimately why there exists such a range of behavior within and among long-lived silicic magma centers.…”

“…In other rhyolites (e.g., Bachmann et al 2007), widely distributed ages for zircons make distinctions between autocrysts, xenocrysts, and antecrysts ambiguous (e.g., Charlier et al 2005). Classic inheritance (sensu Gulson and Krogh 1973) in several silicic extrusions has been identified by ion microprobe spot analyses of crystal cores, e.g., in crystal-rich units like the Tertiary Amalia Tuff of New Mexico (Johnson et al 1989), the Fish Canyon Tuff of Colorado (Lanphere and Baadsgaard 2001), and the *1.2 Ma Ongatiti ignimbrite from the Taupo volcanic zone, New Zealand (Brown and Smith 2004).…”

“…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.…”