GEOLOGIC APPLICATIONS OF WATER CONCENTRATION MAPPING USING FTIR SPECTROSCOPY WITH A FOCAL PLANE ARRAY DETECTOR

Two applications of mapping of water concentration variations across ~100 micron-thick rock slabs of thin section proportions are described. In both projects, a Bruker Vertex 70 spectrometer was used with a Hyperion 3000 microscope. The system has both a single element detector for collection of single spectra, and a 64 x 64 focal plane array detector for mapping chosen areas of samples. The imaging process involves the collection of data across the entire spectral range, in 64 x 64-pixel blocks. Each pixel represents approximately a 2.6 x 2.6-micron square of the sample. In both projects described here, the area (for minerals) or the height (for glasses) of the broad OH and H₂O band that extends from approximately 2900-2700 cm^-1 was mapped. The first project is an examination of water concentration variations in middle Tertiary (~25 Ma) rhyolitic flows from the Atascosa Mountains of southern Arizona. Spherulites composed of radiating sanidine crystals increase in water concentration from the core to the rim of the spherulites. The crystals incorporate more water as they grow because magma at the crystal/magma interface becomes enriched in water that is incompatible in the sanidine crystals. Further, flow bands that host large spherulites (400-600 microns vs ~200 microns diameter) are enriched in water (5000-7000 ppm) relative to flow bands that host smaller spherulites (2500-3000 ppm), suggesting that 1) flow bands were zones of contrasting water concentration in the magma before spherulites grew and 2) larger spherulites grow in water-richer magma with higher diffusion coefficients. The second project is an examination of mobilization of water in structural sites and in fluid inclusions in quartz and feldspar crystals as a result of deformation in the 2.6 Ga Stevenson granite of northern Saskatchewan. With increasing strain, water moves from the interiors of nominally anhydrous minerals to boundaries of small matrix grains, where it lowers melting temperature and promotes the generation of melt films that ultimately coat matrix grains, possibly leading to the inception of microshear zones in lower crustal granites. Mapping of concentrations of compounds of interest allows the investigator to evaluate mobility of compounds during geologic processes and to associate specific compounds with textural features.