Water Speciation Around Vesicles within Obsidian Fragments from Mono Craters, CA : Testing the 'Bubble Geobarometer'

Abstract

Obsidian glasses preserve textural evidence that multiple cycles of fragmentation and annealing occurred within magmatic conduits prior to eruption. The nature and timescales of these processes are poorly understood. Dissolved volatile concentrations provide valuable insights into eruption dynamics, as solubility and speciation are sensitive indicators to changing pressures and temperatures. Recently, Watkins et al. (2012) postulated a 'bubble geobarometry' technique utilizing water diffusion profiles around vesicles in obsidian fragments collected from tephra deposits of Mono Craters, California. They suggested that the gradients in water content were the product of volatile resorption following pressure increases of 5-30 MPa prior to eruption. The purpose of this study was to test this hypothesis by presenting infrared spectroscopic measurements of dissolved water and carbon dioxide contents in Mono Craters obsidian fragments.