Document Type

Thesis

Abstract

Supereruptions are some of the most cataclysmic events on Earth, ejecting greater than 450 km? 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 km’. 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 trachybasalts to trachy-andesites (5-15% pheno.) and estimated temperatures range from 980- 1095°C. Magmatic enclaves within the PST are basaltic trachy-andesite to trachyandesite (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.

Date Awarded

2015

Department

Geology

Advisor 1

Christopher M. Bailey

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