33 Petrology (Posters)

Sunday, 31 October 2010
8:00 AM - 6:00 PM
Hall D

EFFECTS OF LOW-TEMPERATURE HYDROUS ALTERATION on THE CHEMISTRY OF NATURAL OBSIDIAN FROM THE NEAR EAST

Paper number: 33-26
Program: Technical Sessions
Time: 8:00 AM – 6:00 PM

Authors

Abstract

The diffusion of water into glass is of great importance to industry, archaeology, and geology. Meteoric water interacts with the silicate glassy structure to form non-bridging oxygens that radically change the character of glass. The hydration degrades the properties of glass and allows for exchange of elements into and out of the original glass. Understanding the fundamental chemical reactions at the diffusion front is currently of great interest to both the glass industry (to minimize the rate of glass decomposition) and the study of ancient glasses (to better preserve glass artifacts and to gain insights into their ages). Hydration of glass is characterized by the equilibrium of two H-bearing species (molecular H2O and OH hydroxyl groups). For natural obsidians, the relative abundances of the two hydrous species provide insights into their magmatic and post-eruptive histories. In particular, the species concentrations allow geologists to distinguish glasses that have retained their initial magmatic water content from those that have experienced low-temperature, post-eruption hydration.

In this study, we present analyses of the geochemistry (using electron microprobe and XRF techniques) and hydrous species concentrations (using micro-IR spectroscopy) of obsidians and perlites on over 80 individual glasses sampled from over 30 Tertiary felsic lava flows in Turkey, Azerbaijan, Armenia, and Georgia. As observed in previous geochemical studies of recent magmatism in the Near East, our analyses distinguish two distinct magmatic trends, despite being limited to specimens with >70% SiO2. We also distinguish a third geochemical trend that shows enrichment in K2O and depletion in Na2O with increasing water content, in agreement with previous studies of North American obsidians. SiO2, Al2O3, CaO, FeO, and other HFSE’s also decrease with hydration. We hope to perform additional analyses to better resolve the respective degrees of elemental depletion.