Priming for Supereruption: the hot pre-Peach Spring Tuff lavas and Peach Spring Tuff magmatic enclaves, Black Mountains, Arizona

Supereruptions are some of the most cataclysmic events on Earth, ejecting greater than 450 km3 of material during eruption. The 18.8 Ma Peach Spring Tuff (PST) erupted in what is now the southern Black Mountains, Arizona, with outflow covering an area greater than 35,000 km2. The volcanic deposits erupted prior to PST supereruption provide important insights on pre-supereruption magmatic conditions in the region. The pre-PST volcanic sequence consists of a ~1 km thick suite of trachyte lavas and a relatively thin sequence of more mafic lavas. We sampled pre-PST mafic lavas, one trachyte lava, and magmatic enclaves within the PST. Bulk analyses of samples were obtained with XRF, full elemental analyses determined through ICP-MS, and phenocryst compositions determined by SEM. Magmatic temperatures were estimated with Excel-MELTS and mineral-saturation thermometry. An atypically hot (~1025°C) aphyric lava, last of the trachyte sequence, contrasts with the rest of the sequence near 850°C (Rice et al., 2014), and is followed by the eruption of mafic lavas. Mafic lavas range from trachy-basalts to trachy-andesites (5-15% pheno.) and estimated temperatures range from 980-1095°C. Magmatic enclaves within the PST are basaltic trachy-andesite to trachy-andesite (5-20% pheno.), and are similar geochemically to the mafic lavas. Estimated temperatures of enclave magmas range from 1000-1070°C, similar to the mafic lavas and the only definitive enclave identified previously (Pamukcu et al., 2013). Full elemental analyses of three enclaves and two lavas further imply relation between the two sample types. The hot trachyte flow, followed by mafic lavas and related enclaves within the PST, indicate heat input into the Black Mountains magmatic system preceding PST supereruption and are possible evidence of the eruption trigger

Zircon U-Pb and geochemical signatures in high-pressure, low-temperature metamorphic rocks as recorders of subduction zone processes, Sikinos and Ios islands, Greece

Zircon U-Pb dating is a powerful and widely used geochronologic technique to constrain the timing and rates of magmatic and high and lower-grade metamorphic processes, as well as sediment provenance. Zircon trace element (TE) compositions also record magmatic and metamorphic processes during zircon growth. In this study, zircon laser ablation split-stream (LA-SS)-ICP-MS U-Pb and TE depth-profiling and novel two-dimensional zircon mapping techniques are used in combination with oxygen isotope analyses (secondary ion mass spectrometry, SIMS) to reconstruct the timing and metamorphic conditions recorded by recrystallization and growth of zircon rims, which provide valuable insight into the petro-tectonic evolution of high-pressure/low-temperature (HP/LT) metamorphic rocks formed in subduction zones. These techniques are applied to zircon grains from HP/LT metamorphic rocks of the Cycladic Blueschist Unit (CBU) and Cycladic Basement (CB) on Sikinos and Ios islands, Greece, which experienced metamorphism and deformation associated with subduction and subsequent back-arc exhumation. Zircon records multiple episodes of non-magmatic zircon rim growth at similar to 50 Ma and similar to 26 Ma. Eocene metamorphic rims are associated with HP/LT metamorphism and are observed in both units, suggesting likely juxtaposition prior to or during subduction and associated HP metamorphism. The similarity between TE concentrations and delta O-18 values of the Eocene rims and their corresponding cores is an indicator for recrystallization and precipitation as a mechanism of zircon growth. In contrast, Oligocene zircon rims appear to be restricted to a < 0.5 km thick zone along the CB-CBU contact, characterized by garnet break-down, and show HREE enrichment and higher delta O-18 values in the rims compared to the cores, consistent with a model suggesting metasomatic infiltration of fluids derived from dehydrating sedimentary rocks during progressive subduction and underplating prior to back-arc extension. This metamorphism appears to be static in nature and does not support major late Cenozoic reactivation of the contact as an extensional shear zone during back-arc extension