TEXTURAL CONSTRAINTS OF PLAGIOCLASE PHENOCRYSTS IN THE TIETON ANDESITE FLOW: CLUES TO THE MAGMATIC SYSTEM

Tieton Andesite and associated basaltic andesite flows are located in the Southern Washington Cascades. This study investigates textures in plagioclase crystals in order to further understand the plumbing of the magma system. Thin sections were prepared from four different localities: two samples from the Pinegrass Ridge, near the vent (PG1), further down flow (PG2), Naches Heights and a basaltic andesite. Thin sections were photographed using the Olympus BX51 microscope. Photographs were spliced together in Adobe Photoshop to make a map of each section. maps were used to obtain SEM data, because the groundmass and the plagioclase have similar mean atomic numbers and therefore similar shades of gray in SEM-EDX mode. EDX analysis was performed using the Hitachi TN3030Plus Tabletop Microscope.

Plagioclase textures are indicators of melt conditions. Textures identified include: coarse sieve, fine sieve, oscillatory zoning, dissolved cores, and resorbed rims. Sieve texture indicates intermediate rates of decompression in H2O undersaturated conditions. Oscillatory zoning is indicative of movement in the magma chamber. A dissolved core indicates a high rate of decompression at H2O undersaturated conditions in volatile-rich magma and resorbed rims indicate a recharge of more primitive, hotter and volatile-rich magma. Each of the crystal populations and textures imply they experienced differing conditions. The Pinegrass Ridge samples have different crystal textures. The PG1 has both fine and coarse sieve textures and resorption rims on some crystals. However, in the PG2 sample, coarse sieve, oscillatory zoning, dissolved cores and resorption rims are visible. These textures indicate the conditions the crystals formed in were different. The Naches Heights crystals have fine sieve texture around the rim, some coarse sieve, instances of oscillatory zoning, and large glomerocrysts, which are evidence of mixing between different compositions. Based on these observations each sample underwent different conditions. The next step of this project is to look further into the textures and compositions of each slide by preforming EDX analysis of the plagioclase crystals with the SEM, a point count of the different textures in each slide, and determining crystal size distribution of each sample.