FOURIER TRANSFORM INFRARED SPECTROSCOPY (FTIR) STUDIES OF WATER IN VOLCANIC MATERIALS

High resolution Fourier transform infrared spectroscopy (FTIR) has been used to document the presence of water, and to empirically compare water concentrations in phenocrysts, in glass, and in spherulites of volcanic rocks. Analyses were done at the National Synchrotron Light Source at Brookhaven National Laboratory. Samples studied were 1) rhyolitic and dacitic pumice from the ~1 Ma Purico complex in the Altiplano-Puna Volcanic Complex of the central Andes (Schmitt, 1999), 2) recent (~3000 years; Reagan et al., 1992; Dunbar et al., 1994) anorthoclase phenocrysts from Mt. Erebus, Antarctica, and 3) middle Tertiary (~25 Ma) flow-banded spherulitic rhyodacite from the Atascosa Mountains of southern Arizona. In all cases, samples were 40-micron-thick, self-supporting slabs, analyzed in transmission mode. Broad peaks in the wavenumber range 3700-3200 cm-1 represent stretching bands of water species, resulting from radiation-induced vibration of O-H bonds. In rhyolitic and dacitic pumice from the Purico complex, large phenocrysts rarely contain measurable concentrations of water, but smaller phenocrysts show broad peaks in the 3700-3200 cm-1 range, suggesting that as crystallization progressed and water concentration in the magma increased, more water was incorporated into later growing crystals. Large (to ~9 cm long) anorthoclase crystals from Mt. Erebus, cut approximately parallel to crystallographic axes, contain melt inclusions that themselves host crystals. Melt inclusion water concentration decreases toward the edges of included anhydrous minerals. Consistent variations in anorthoclase water concentration in different crystallographic orientations have not been observed. In flow-banded spherulitic rocks from the Atascosa Mountains, larger spherules (~2 mm), that make up lighter colored bands, are more water-rich than smaller spherules (~ 0.3 mm), that make up darker colored bands, suggesting that larger spherule flow bands were originally more water-rich, and were possibly gas bubbles that were stretched to produce flow bands. These results suggest that further high resolution FTIR studies will be useful in providing data about variation in water concentration during crystallization, in the diffusive behavior of water in volcanic materials, and in understanding the origin of volcanic rock textures.