PEACH SPRING TUFF, AZ-CA-NV: SUPERERUPTION OF A STRATIFIED, MUSH-BASED MAGMA CHAMBER

The 18.78 Ma, >700 km³ Peach Spring Tuff (PST) is exposed through much of S Nev, SE Calif, and NW Ariz (e.g. Geology: Glazner et al 1986; Ferguson et al in press). Studies of pumice from its caldera (Silver Creek, Black Mtns, AZ) and outflow ignimbrite shed light on its magma chamber and processes therein prior to the supereruption (Pamukcu et al JPet in press; Frazier et al this meeting; McCracken et al AGU abst 2012; Carley MS thesis 2010). Phenocryst assemblages are uniformly san > plag + bio+ hbl > qtz + px, + sphn + zrc + chev, but intracaldera and some proximal outflow pumice is trachyte (TP), whereas distal outflow is rhyolite (RP). TP & RP are distinct in texture and composition (TP: ~30-40% phenocrysts, strongly resorbed; 65-70 wt% SiO₂, av Ba~1100 ppm, Sr 190, Zr 580, La 140; RP: 74-76 wt% SiO₂, av Ba ~40 ppm, Sr 30, Zr 240, La 64). Ti-enriched zircon rims in TP document late growth from hotter, less evolved melt (to 50 ppm Ti: T >~900 C), in contrast to low-Ti rims in RP. Resorption textures also point to late heating in TP but not RP. Glass in RP pumice is consistently high-Si rhyolite (76.8 wt%), whereas TP glass SiO₂is <70 wt%. Compositions of sparse glass shards and crystals in outflow tuff attest to entrainment of some trachyte throughout the eruption, but the distribution of RP & TP indicates initial tapping primarily of cooler rhyolite followed by hotter, crystal-rich trachyte.

Isotope data for all pumice reveal distinctive and uniform Sr, Nd, Hf, and Pb compositions, unusual for regional volcanic sequences and plutonic complexes of comparable scale, which suggests that RP & TP are comagmatic (Frazier et al this meeting). This supports a relatively simple view of the pre-eruption magma chamber consistent with textures, mineral zoning, pumice glass and whole-pumice chemistry, and Rhyolite-MELTS modelling: TP represents a lower cumulate zone that reached a high crystal fraction (‘rigid sponge’) before experiencing intense reheating (yielding ‘remobilized mush’). RP was melt-rich magma derived from the same parent as TP cumulate, present in a cooler upper zone that experienced little reheating; it may have felt the dynamic impact of the mobilization of the chamber base in the lead-up to eruption. The high-Si rhyolite glass reflects crystal-melt equilibria within the relatively shallow chamber (~10 km; cf Gualda & Ghiorso JGeol in rev).