MAGMATIC EVOLUTION AND ERUPTION PROCESSES IN A TUFF CONE DEPOSIT, CON SHEA VOLCANO, WESTERN SNAKE RIVER PLAIN, IDAHO
Matrix glass fragments from five units were analyzed using an electron microprobe. The samples were all low in silica (46.9 wt% to 47.8%), with the slightly lower values occurring in later deposits. The most notable compositional changes occurred in the FeO (11.7 wt% to 16.8%) and TiO2 (2.5 wt% to 4.6%) values, both increasing with time. Slight increases were also noted in the P2O5 and MnO contents, in addition to decreases Al2O3 and Na2O. Fourier transform infrared spectroscopy (FTIR) was used to analyze the volatile (H2O and CO2) content of melt inclusions found within olivine crystals and fragments of the matrix glass. Melt inclusions from two units of different stratigraphic height were analyzed. A notable decrease in water content (0.39 wt% to 0.11%) in the inclusions occurred as the eruption progressed; whereas, only a minor decrease in carbon dioxide (527 ppm to 360 ppm) was observed. Pristine matrix glass from six units was analyzed, revealing a decreasing amount of dissolved water (0.43 wt% to 0.04wt%), corresponding to an increase in stratigraphic height. This decrease in water content correlates with petrographic observations of increasing vesicularity and microcrystalinity of the matrix glass with increasing stratigraphic height. Through petrographic, electron microprobe, and FTIR analysis, a tholeiitic compositional trend and drying out of the system have been identified and correlated with the increased presence of vesicles and olivine and plagioclase microlites found in the matrix glass.