INVESTIGATING MAGMATIC PROCESSES WITH FOURIER TRANSFORM INFRARED SPECTROSCOPY AND IMAGING

Fourier transform infrared (FTIR) spectroscopy and mapping is a powerful means of interpreting aspects of both crystallization of magmas and partial melting of deep crustal granites. Further, the sensitivity of FTIR band positions in spectra of hornblende to M- and T-site occupancies makes FTIR spectra of hornblende a useful tool for evaluating processes that control hornblende compositional changes during crystallization. Two examples follow. First, the Silurian Cadillac Mountain granite hosts enclaves that typically consist of a single alkali feldspar solid solution and hornblende with minor apatite needles. Maps of water concentration from the interior of enclaves out into the host granite were collected using a Bruker Vertex 70 FTIR spectrometer and a Hyperion 3000 microscope equipped with a 64 x 64 focal plane array detector. FTIR maps show that water concentration is smoothly zoned, and, counterintuitively, from higher concentrations (to 500 +/- 75 ppm) in quartz in enclaves to lower concentrations (~100 ppm +/- 15 ppm) in quartz in the host granite, suggesting that mingling and hybridization of enclaves in the granitic magma delivered water to the granitic magma. FTIR spectra of hornblende crystals in the enclaves have a sharper pair of bands in the OH-stretching region at 3556 and 3415/cm than those of hornblende spectra in the host granite, for which the 3410/cm band is broader and more prominent. A second example of the use of FTIR mapping is that of the behavior of water in feldspar in the 2.6 Ga Stevenson granite of northern Saskatchewan. Mapping demonstrates that with increasing strain, water moved from the interiors of feldspar crystals to boundaries of small matrix grains, where it lowered melting temperature and promoted the generation of melt films that ultimately coated matrix grains, possibly leading to the inception of microshear zones in lower crustal granites.