Architecture of a Super-sized Magma Chamber and Remobilization of its Basal Cumulate (Peach Spring Tuff, USA)
Authors: Foley Michelle L , Miller Calvin F , Gualda Guilherme A R 
Published: February 2020

Using a combination of petrological and geochemical approaches, we investigate processes prior to and during eruption of the Miocene supereruption of the Peach Spring Tuff (PST; Arizona–California–Nevada), including those leading to assembly and destruction of its reservoir(s). We compare the dominant high-silica rhyolite outflow of the PST with the sparsely exposed but distinctive crystal-rich trachyte capping unit, which matches intracaldera trachyte in composition, texture, and phenocryst content. The details of the diverse glass chemistry in fiamme and pumice in the capping unit, coupled with glass compositions in the rhyolite outflow and phase chemistry in general, illuminate critical aspects of chamber geometry, conditions, and processes at the onset of the supereruption. Our results are consistent with a relatively simple single-chamber reservoir for the PST where the crystal-poor, high-silica rhyolite portion directly overlies a mushy, cumulate base. Rhyolite-MELTS phase-equilibria and amphibole geobarometers indicate that the high-silica rhyolite was extracted from its cumulate mush at a depth of ∼9·5–11 km (∼260–300 MPa) and subsequently stored and crystallized at ∼7·0–8·5 km (190–230 MPa). Three types of glass are distinguishable in PST pumice: trachyte (Trg; ∼68 wt% SiO2), low-silica rhyolite (LSRg; ∼72), and high-silica rhyolite (HSRg; ∼76·5). As many as three discrete, complexly mingled glasses are present in single trachyte fiamme. Trace element concentration profiles in sanidine and plagioclase phenocrysts from both the trachyte and HSR support growth from multiple distinct melts (Trg, LSRg, and HSRg). Glasses in trachyte fiamme have zircon saturation temperatures ≥100 °C higher than HSR glasses (850–920 vs ∼770 °C) and compositions indicating dissolution of cumulate phases: very high Zr and Zr/Hf (zircon), REE (chevkinite and titanite), Ba and Sr (feldspars), and P (apatite). Dominant processes of crystal accumulation in the formation of a mushy base, followed by efficient melt extraction, led to the formation of the voluminous high-silica rhyolite melt-rich body overlying a residual cumulate of trachytic composition. This was followed by heating, partial dissolution, and remobilization of the basal cumulate. This history is reflected in the contrasts that are evident in the PST (elemental compositions of pumice, phenocrysts, and glasses; crystal-fraction; temperatures). Reheating was presumably a result of injection of hot mafic magma, but isotopic uniformity of trachyte and rhyolite indicates minimal chemical interaction with this magma. Variability in dissolution textures in phenocrysts in the trachyte, revealed by resorbed and embayed shapes, and the large range of glass trace element concentrations, together with variable temperatures recorded in glasses by zircon and apatite saturation thermometry, suggest that heat transfer from the hotter rejuvenating magma was unevenly distributed. The late-stage heating event probably contributed to the onset of eruption, providing the thermal energy necessary to reduce the crystal fraction within the cumulate below the mechanical lock point. We estimate ∼50 % of the original cumulate phenocrysts dissolved before eruption, using Rhyolite-MELTS and trace element modeling. Sharp contacts with micron-scale compositional gradients between contrasting glass types in individual trachyte fiamme suggest that juxtaposition of contrasting magmas from different parts of the reservoir occurred during eruption.