33 Volcanology (Posters)

Sunday, 9 October 2011
9:00 AM - 6:00 PM
Hall C

CHEMICAL EXCHANGE ACCOMPANYING OBSIDIAN-PERLITE TRANSITION IN THE NATURAL ENVIRONMENT

Paper number: 33-7
Program: Technical Sessions
Time: 9:00 AM – 6:00 PM

Authors

Abstract

The interaction of water and silicate glass is of great interest to geology, industry, and archaeology. Hydration radically changes the character of both natural and manufactured glass and allows for rapid exchange of elements between glass and its surrounding environment. Following eruption, rhyolite glass commonly experiences extreme hydration (>5 wt% H2O) through reaction with hydrothermal fluids to form perlite. This natural process represents a potential analog for glass hydration at low temperatures. Detailed geochemical studies of the natural transition of obsidian into perlite may offer valuable insights into the degradation of industrial glass, as well as the decomposition of ancient glass artifacts. Understanding the fundamental chemical reactions that occur during this transition is essential to minimize the rate of industrial glass degradation and effectively preserve glass artifacts, in addition to gaining insights into their ages.

We present geochemical analyses (using XRF and electron microscopy) and detailed measurements of the water content and hydrous species concentrations (using micro-IR spectroscopy) of natural obsidian-perlite pairs collected from Ruby Mountain, Colorado. As observed in previous studies of naturally hydrated obsidian, the perlite exhibits dramatic depletion in Na2O (-20 wt%) with accompanying enrichment in K2O (+8 wt%) compared to the adjacent glass. These results agree with our previous studies of hydrated obsidian from the Near East. In addition, our geochemical data indicate enriched CaO concentrations (+25 wt%) in the Ruby Mountain perlite. As shown in studies of the alteration of basaltic glass to palagonite (e.g., Staudigel and Hart, 1982), hydration is associated with nearly isomolar exchange of cations with the hydrothermal solution. We also discuss the behavior of trace constituents during the transition from obsidian to perlite, including mobility of the large ion lithophile, high field strength, and rare earth elements.