MAGMA CHAMBER DYNAMICS AS DOCUMENTED IN A SINGLE ERUPTIVE UNIT FROM MT. JEFFERSON, CENTRAL OREGON CASCADES

Mt. Jefferson, the second highest peak in Oregon, is a large composite volcano characterized by compositional diversity at all scales. On the large scale (m-km), the compositional suite of lavas ranges from basalt to rhyodacite (50-72 wt. % SiO₂). On the small scale (mm-cm), individual eruptive units contain multiple populations of plagioclase, and a variety of mafic enclave types.

To understand the small scale compositional diversity and how that is tied to the large scale diversity of erupted lavas, we measured major, trace element and volatile (H₂O, Cl, S, F) concentrations within phenocrysts and melt inclusions from a single eruptive unit, the Whitewater Creek andesite. This lava is one of the more heterogeneous units erupted at Mt. Jefferson and is characterized by the presence of four distinct textural/mineralogical components (olivine-orthopyroxene-plagioclase enclave, two pyroxene-plagioclase-magnetite enclaves, amphibole gabbro enclaves) all within a dacitic crystal-rich matrix (An_50-60 plagioclase, orthopyroxene, clinopyroxene, amphibole, quartz and magnetite).

Geothermometry/geobarometry calculations suggest that early-erupted components of the unit had undergone storage degassing at pressures of <2.5 kbar. Equilibration pressures and temperatures based on amphibole and amphibole/plagioclase pairs range from 2.8 to 5.5 kbar and 770-900^oC. Mixing and diffusive re-equilibration of the mixed mafic and silicic components preceded eruption, and continued during transport to the surface. Calculated liquid compositions in equilibrium with each of the observed components cover much of the entire range represented by the lavas at Mt. Jefferson. It is difficult to directly link them to specific magma types observed at Mt. Jefferson with the fact that components have undergone significant post mixing re-equilibration during storage and transport.