Utilizing Trace Element Diffusion Models in Clinopyroxene and Olivine to Determine Ascent Rates of Cinder Cone Magmas in the Southern Cascades

While geophysical and U-series disequilibria methods have robustly characterized pre-eruptive histories for a range of composite magmatic systems, timescales of ascent for cinder cone magmas remain poorly constrained.... [ view full abstract ]

While geophysical and U-series disequilibria methods have robustly characterized pre-eruptive histories for a range of composite magmatic systems, timescales of ascent for cinder cone magmas remain poorly constrained. Alternatively, models of trace element diffusion within olivine and pyroxene using a finite difference approximation of Fick’s Second Law is a powerful technique for calculating timescales. We utilize this approach with two cinder cone magmas that erupted in the Pleistocene from the Lassen Volcanic Zone at the southern terminus of the Cascade Arc: the basaltic andesite of Box Creek (mbx) and the basaltic andesite of Bunchgrass Meadow (mbg). Mbx and mbg are composed of primitive (<54% SiO2), clinopyroxene (cpx) dominated material that is likely the result of a relatively simple and potentially deep storage history. We model major and trace element diffusion in one dimension along multiple crystallographic axes for 40 phenocrysts of cpx and olivine to determine preliminary timescales of ascent.