Populations of Major Phases in Glacier Peak Lavas Populations of Major Phases in Glacier Peak Lavas

Volcanoes can be studied through minerals that are present in their eruption products. These minerals can be studied through optical microscopy and geochemistry to better understand their origins and provide insight into which processes took place prior to eruption (i.e., fractionation of crystals, assimilation of the surrounding crust, magma mixing). In this study I examine mineral populations in dacite (lava of intermediate composition) from Dahkobed/Glacier Peak volcano in the north Cascade Arc. The samples consist of a felsic (high SiO 2 ) host component and a mafic (low SiO 2 ) inclusion component. To establish base mineral populations, minerals from both the host and inclusion were examined with a petrographic microscope. Then, representative crystals from each possible plagioclase, pyroxene, and olivine population were analyzed using a scanning electron microscope – electron diffusive X-ray spectroscopy (SEM-EDS) for their major element compositions. Results indicate that there are 3 sets of crystal populations, one from the felsic host (consisting of plagioclase, pyroxenes, amphibole, and quartz), one from the mafic inclusion (consisting of olivine, plagioclase, and pyroxenes), and one from an intermediate source (consisting of clusters of plagioclase and pyroxenes). Minerals in the host and inclusion all show evidence for disequilibrium re-crystallization and new crystallization following interaction. Specifically, host amphibole and quartz became unstable as evidenced by their pronounced clinopyroxene reaction rims, whereas host plagioclase and orthopyroxene continued crystallizing and nucleating as evidenced by the presence of both normally and reversely zoned crystals with overlapping compositions. In contrast, host clinopyroxene crystalized after the two magmas mingled as evidenced by normal zoning and lack of dissolution textures. Inclusion olivine became unstable after magma mingling, as evidenced by disequilibrium textures in the crystals. Inclusion plagioclase, clinopyroxene and orthopyroxene, on the other hand, crystallized after the magmas interacted and exhibit wide compositional ranges due to localized differences in the degree of magma mixing. A third set of mineral populations is present as clusters of sodic plagioclase and pyroxenes with moderate Mg contents. The origin of these clusters is unknown, but the crystal compositions did not resemble any of the established host and inclusion populations. These results suggest the magmatic system that produced the dacite samples is more complex than a two-magma system. Additionally, the mingled magmas did not erupt immediately after interacting as indicated by the continued crystallization of crystals. The exact reason for eruption is not clear; however, the host matrix is glass-rich and microcrystalline, suggesting mobilization by a new melt prior to eruption. smaller crystals ( ≤ 50 µm) with lower An (6–23) and is found in clusters with orthopyroxene (Mg# 58–75 ) and slightly lower Mg# clinopyroxene (73–76). These data suggest that two populations of plagioclase are found in the mineral clusters, one with higher An plagioclase and higher Mg# clinopyroxene (population Plg-C1) and another with lower An plagioclase, lower Mg# clinopyroxene, and orthopyroxene (population Plg-C2).