Interpretation of the provenance of small-scale heterogeneity as documented in a single eruptive unit from Mt. Jefferson, Central Oregon Cascades

Abstract

Mt. Jefferson is a large composite volcano located in the central Oregon Cascades that has erupted a diverse compositional suite of lavas from basalt to rhyodacite (50–72 wt. % SiO2). Individual eruptive units contain multiple populations of plagioclase, and a variety of mafic textural/mineralogical components often preserved as large centimeter to millimeter-sized enclaves. Understanding the processes active in any volcanic center requires that we document the products of those processes. In this contribution, we documented the small-scale compositional diversity within a single eruptive unit at Mt. Jefferson, the Whitewater Creek andesite, in order to answer three questions: (1) what are the characteristics and scale of diversity in a single eruptive unit, (2) what is the provenance of the observed components, and (3) how does that observed small-scale diversity relate to the larger-scale diversity observed between other flows at Mt. Jefferson. Our analyses are based on major, trace element concentrations for phenocrysts, and melt inclusions from a single eruptive unit, the Whitewater Creek andesite which is one of the most heterogeneous units erupted at Mt. Jefferson. We have identified at least four distinct components present at the centimeter scale. These components, identified on the basis of their mineralogy and composition, include three mafic (two pyroxene + plagioclase; plagioclase-hornblende, and olivine-orthopyroxene) and one silicic component (dacitic groundmass). To understand the relationship between the observed textural components and the magma types erupted at Mt Jefferson, equilibrium liquid compositions were calculated from the phenocryst compositions. The range exhibited by those calculated liquids covers much of the entire range represented by the lavas at Mt. Jefferson. However, it is difficult to directly connect them to specific magma types observed at Mt. Jefferson. We attribute our inability to directly link the textural components to the known magma types to post mixing diffusive reequilibration during storage and transport. These results suggest that great care should be taken in interpretation of whole rock data.