USING CRYSTAL AND GLASS FORENSICS TO DECIPHER THE MAGMA PROCESSES ASSOCIATED WITH THE TIETON ANDESITE, CASCADES OF WASHINGTON STATE

The Tieton andesite consists of a series of long lava flows erupted from the Bear Creek Mountain area located in the Southern Washington Cascades. Systematic sampling of these lavas was conducted to investigate changes in magma, crystal, and glass chemistry, and crystal populations to enable documented changes to be related to processes of transport, eruption, and emplacement. One thin section was selected for this initial study. The sample came from Pinegrass Ridge ~2.5 km NE of the vent area. The sample is a vesicular, porphyritic andesite with a glassy groundmass and 60.42 wt% SiO₂, and contains numerous rounded xenoliths of fine-grained diorite. We focus on the plagioclase chemistry and textures in addition to interstitial and groundmass glass. SEM-EDX data were collected using a Hitachi TN3030Plus tabletop microscope with a Bruker Quantax 70 EDS. The mineral and glass chemistry show the importance of mixing and assimilation between distinct mafic and felsic melts to generate the andesitic composition. Plagioclase textures provide evidence of melt conditions. Identified textures comprise oscillatory zoning, dissolved or patchy cores, resorbed or fine-sieve rims, and coarse-sieve. Oscillatory zoning is indicative of convection driven small scale physical-chemical perturbations at the crystal/melt interface. Intermediate rates of decompression in H₂O undersaturated conditions produce sieve textures. High rates of decompression at H₂O undersaturated conditions in a volatile-rich melt produce dissolved cores. While resorbed rims indicate a mafic recharge event of hotter and volatile-rich magma. Initial glass compositions range from silicic to mafic. More mafic compositions reside within or marginal to diorite xenoliths. More silicic compositions are in the groundmass associated with microlites. The range in textures provides evidence to indicate the crystals formed under varying conditions. Fine-sieve on the rim, with coarse-sieve, oscillatory zoning and glomerocrysts are evidence of mixing between different compositions. Initial glass compositions support this conclusion. However, there is evidence of xenoliths disintegrating and dissolving into the melt. The next step is to obtain EMPA for major and trace element data and collect crystal size distribution data on the crystal population.