FTIR STUDY OF THE EFFECTS OF HETEROGENEITY IN WATER CONCENTRATION ON THE ORIGIN OF FLOW BANDED RHYOLITES

This study focuses on the origin of flow-banded rhyolites that consist of darker and lighter flow bands that are not compositionally distinct magmas, but contrast in texture and color. Synchrotron-generated infrared radiation was used to obtain Fourier transform infrared (FTIR) spectra from which water concentrations were calculated, and to map variations in water concentrations across zones of spherules and glass from the 23 million year old Bartolo Mountain lava flow of southern Arizona. Lighter-colored, thicker flow bands consist of tan glass and large (2.5 to 5 mm) spherules. Darker-colored, thinner flow bands consist of brown glass and smaller (0.1 to 0.3 mm) spherules. Zones of orange glass separate lighter and darker flow bands. The center of the spherules is occupied by either 1) a quartz or sanidine crystal, 2) a granophyric intergrowth, or 3) (in some larger spherules) a void, suggesting that the spherule nucleated on a vapor bubble. In large spherules in the lighter bands, typically two zones of quartz/feldspar intergrowth radiate from the center of the spherule. The two zones are separated by a concentric zone of glass. Sanidine crystals that make up the cores of large spherules contain up to 250 ppm water. Inner radiating quartz/feldspar zones typically contain less (~2500 ppm) water than outer radiating quartz/feldspar zones (~3800 ppm). The transitional glass zones that separate the inner and outer zones contain to 7500 ppm water. Small spherules in darker bands have only one generation of radiating crystal growth. Water concentrations in small spherules range from a few hundred to approximately 1800 ppm. The orange glass that separates darker and lighter bands typically contains 1000-2000 ppm water. Overall the coarser-spherule, lighter-colored bands are much more water-rich than the finer-spherule, darker-colored bands, consistent with abundant water facilitating the growth of large spherules, and supporting the work of Hausback (1987), who suggested that flow banding may consist of alternating layers of stretched lithophysae, within which vapor phase crystallization occurred, and original less water-rich magma. Differences in original water concentration in the alternating layers resulted in differences in undercooling textures in spherules in the two types of flow bands.