VOLATILES IN NOMINALLY ANHYDROUS MINERALS AND MELT INCLUSIONS IN ICELANDIC BASALTS: VARIATIONS OVER SPACE AND TIME

Concentrations of volatiles, particularly water and carbon dioxide, are integral to understanding the petrogenesis and eruptive history of magmas. Icelandic basalts were previously thought to be uniformly low in water content (e.g. Gunnarsson et al., 1998). However, more recent studies suggest that hotspot locales, like the Iceland mantle plume currently centered under the Vatnajokull glacier, may be a source of hydrous basaltic melts (Nichols et al., 2002). To better explore a possible link between hotspot or rift location and volatile concentration, samples were collected from eleven volcanic centers throughout Iceland. Samples represent a variety of eruption styles, ranging from effusive basalts to explosive rhyolites, and span a range of ages and tectonic settings. Volatile concentrations are measured on approximately 100 micron thick rock sections using Fourier transform infrared (FTIR) spectroscopy. In this study, we focus on water in melt inclusions hosted in phenocrysts, such as olivine, plagioclase, and clinopyroxene, or in preserved glass if inclusions are not present. Preliminary results for a subset of basalt samples indicate significant differences between water concentrations in melt inclusions ranging from 88 ppm to over 3000 ppm. Within this subset, the rift-related West Volcanic Zone has substantially higher water, averaging approximately 1450 ppm for two volcanoes, compared to the intraplate, Snaefellsnes Volcanic belt, which averages approximately 500 ppm. Carbon dioxide, though present in small amounts, shows similar patterns with samples from the West Volcanic Zone having higher concentrations the Snaefellsnes Volcanic Belt. Measurements will also be made on the nominally anhydrous phenocrysts to assess whether their water concentrations vary directly with water concentrations in their melt inclusions. The collection of additional FTIR data, as well as geochemical data, will produce a comprehensive dataset of a complex volcanologic area to identify correlations between melt composition and volatile abundance and speciation, as well as to identify trends in volatile concentration in Icelandic basaltic melts with respect to the location of the center of the Iceland plume and over the course of geologic history.