Syn-eruptive Processes as Recorded by Crystals and Vesicles in the Bishop Tuff, California: A Combination of Experimental Petrology and 3D Imaging

Abstract

The Bishop Tuff, a 0.76 million-year-old rhyolitic deposit in California, contains two crystal phases, described by Pamukcu et al. (2012). The first contains long-lived crystal grown over 5000+ years; the second consists of microlites <50μm large, grown in <2 years as a result of eruptive decompression. In this study I combine vesicle size distribution (VSD) investigation with experimentally produced data on decompression crystallization to better understand the syn-eruptive evolution of a supereruption. Isothermal experiments were run by Jim Gardner using late-erupted Bishop Tuff material, which was water-saturated and melted, then decompressed at various rates and initial temperatures. Seven experiments have been analyzed; each began at 130MPa, and ended near 10MPa. All seven experiments had initial temperatures of 710⁰C or 785⁰C and the decompression rates were 20MPa/hr, 5.5MPa/hr, 1.7MPa/hr, or 0.75MPa/hr. I compared textures and mineral assemblages of Bishop Tuff and experimental products using Scanning Electron Microscope imaging. We were able to replicate growth of feldspar microlites and quartz rims in 6-3 day experiments. I used tomographic analysis of progressively larger pumice chips to obtain VSDs and calculated chip volumes and bulk density using image analysis. Plotting bulk porosity against volume generated a cumulative-like size distribution inconsistent with a true cumulative distribution. We demonstrate that this may result from censorship of large bubbles during sampling. We conclude that decompression crystallization took days to weeks and decompression rate fluctuated during eruption.