ERUPTIONS TRIGGERED BY MAGMA MIXING AT MOUNT JEFFERSON, CASCADE RANGE, OREGON

The Cascade Range represents the only active volcanic arc in the contiguous USA. It is important to understand the dynamics involved in triggering eruptions along the Cascades. Magma mixing has been demonstrated as an eruption trigger at Mt. Hood (Kent et al; 2010). This research measures chemical zonation of plagioclase to test for possible magma mixing as an eruption trigger at Mt. Jefferson. Mt. Jefferson has erupted heterogeneous compositions of magma ranging from basalt to rhyodacite (Conrey, 1991); Mt. Hood in comparison has erupted homogeneous compositions ranging from andesites to dacites (Kent et al; 2010). The more complicated internal structure of magmatic evolution under Mt. Jefferson is evident in the range of magma compositions. Four samples were analyzed for chemical zonation in feldspar crystals at the USGS by the JEOL 8900 Superprobe. SEM images were also obtained at USGS. Compositions of the rims ≤ 50µm were compared against compositions of the cores of same crystal to ascertain their compositional history. Two populations of feldspars are present in all the samples. Population 1 are small crystals that show minor zoning and no disequilibrium textures and are interpreted as forming after magma mixing. Population 2 are large heavily zoned crystals that exhibit disequilibrium textures and are interpreted with cores formed in a felsic magma chamber and rims forming after magma mixing. Two of the four samples analyzed showed evidence of mixing by an increase in Fe in plagioclase at the rim. There is a correlation between thickness of dehydration reaction rims in amphiboles and Fe content in plagioclase rims. The width of amphibole reaction rims increases towards the interior of the flow. Amphibole reaction rim thickness increases with slower ascension rates (Rutherford and Hill, 1993). Our interpretation is that magma mixing triggered an eruption that quickly erupted samples with plagioclase rims high in iron and amphiboles with no measurable rims. This eruption also triggered the extrusion of samples from deeper in the magma chamber that ascended more slowly, giving the amphiboles time to create reaction rims and the Fe in plagioclase to re-equilibrate. Magma mixing is likely a mechanism for eruption at Mt. Jefferson, but the evidence for magma is partially or completely erased during a slow eruption.