PRIMING FOR SUPERERUPTION: THE HOT PRE-PEACH SPRING TUFF LAVA FLOWS 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 volcanic material. The 18.8 Ma Peach Spring Tuff (PST) erupted in what is now the southern Black Mountains, Arizona, with outflow deposits covering an area greater than 35,000 km². The volcanic deposits erupted just prior to PST supereruption provide important insights on pre-supereruption magmatic conditions in the Black Mountains volcanic center. 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 PST itself. Bulk analyses of samples were obtained with XRF, full elemental analyses determined with an ICPMS, and phenocryst compositions determined by SEM. Magmatic temperatures were estimated through use of the Excel MELTS program and mineral-saturation thermometry. An atypically hot (~1015°C) aphyric trachyte lava, last of the trachyte sequence, contrasts with temperatures in the rest of the sequence near 850°C, and is followed by the eruption of relatively mafic magmas. This high temperature and closeness in time to mafic lavas suggests significant heat input. Mafic lavas range from trachybasalts to basaltic trachyandesites (5-15% phenocrysts) and estimated temperatures range from 1015-1085°C (assuming 2 wt. % water and constant pressure of 200 MPa). Magmatic enclaves within the PST itself range from basaltic trachy-andesite to trachyandesite (5-20% phenocrysts), and are similar geochemically and in phenocryst assemblage to the mafic lavas. Estimated temperatures of enclave magmas range from 1010-1085°C, similar to those of the mafic lavas. The only definitive magmatic enclave identified previously within the PST is similar in composition and petrographic characteristics to our enclaves. Full elemental analyses of two sampled enclaves and one lava further imply relation. The presence of mafic enclaves is an indicator of hotter mafic magma injected in the (PST) chamber and our data suggests that pre-PST lavas are related to these enclaves. The hot trachyte flow, followed by mafic lavas and related enclaves within PST, indicate heat input into the Black Mountains magmatic system preceding PST supereruption and are possible evidence of the eruption trigger.