Paper No. 17-4
Presentation Time: 2:35 PM
HETEROGENEITIES IN PUMICE AND FIAMME PROVIDES INSIGHTS INTO THERMAL AND MECHANICAL HISTORY OF PARTIALLY REMELTED AND REMOBILIZED CRYSTAL MUSH IN THE MAGMA CHAMBER THAT FED THE PEACH SPRING SUPERERUPTION (AZ, USA)
The ~1000 km3 Peach Spring Tuff (PST) is the product of an 18.8 Ma supereruption from Silver Creek caldera, southern Black Mountains, AZ (Ferguson et al 2013). Four zones are distinguishable in thick outflow sections of the ignimbrite near Kingman, AZ on the basis of welding characteristics and upward increasing phenocryst abundances (Ferguson & Cook 2015), but are uniformly high-silica rhyolite composition (74-76 wt% SiO2; Pamukcu et al 2013; Barry et al 2015). An upper, sporadically exposed zone that caps the sequence (Warm Springs Zone [WSZ]) is a phenocryst rich (~35%) trachyte (65-69 wt% SiO2); WSZ pumice is similar to intracaldera fiamme in texture and composition (Pamukcu et al 2013; Foley et al 2014). Phenocrysts in intracaldera and WSZ pumice are large (sanidine to 6 mm) and exhibit variable evidence of resorption and reaction; many are anhedral and heavily embayed. The WSZ and intracaldera pumice have far higher Ba, Sr, Zr, and REE (La, Gd) concentrations in bulk pumice and fiamme (avg 1000, 230, 570, 165, 11 ppm respectively), than in the highly evolved outflow (80, 30, 215, 70, 6 ppm). This contrast, along with isotopic similarity (McDowell et al 2016), supports a closed-system model of a single, relatively simple, vertically stratified PST reservoir with a crystal-rich base (feldspar + accessory mineral-rich cumulate) and complementary crystal-poor upper zone (Foley et al 2016). Zircon saturation thermometry (Boehnke et al. 2013) using glass compositions shows a thermal contrast between trachyte (~860-915°C) and the high-silica rhyolite outflow (~732°C). Higher temperatures, together with crystal resorption and rim compositions, indicate the cumulate was reheated and remobilized by a hotter magma. Trace elements in WSZ bulk pumice/fiamme indicate wide variability in proportions and compositions of crystals in source cumulate. Diversity of textures and wide ranges of glass compositions for trachyte fiamme/pumice (66-71 wt% SiO2) suggest that heat supplied by magma injection was not evenly distributed. Compositional glass heterogeneity is also apparent in individual fiamme/pumice; contacts between contrasting compositional domains are in some cases sharp on a scale of microns. This scale reveals that re-equilibration of melt was not achieved and eruption occurred quickly upon remobilization.