PRE-ERUPTIVE STORAGE AND EVOLUTION OF HIGH-MG BASALTS IN THE SOUTHERN CASCADE ARC

The Lassen Region in Northern California is dominated by cinder cones, small volcanic landforms that erupt once for a period of 1-10 years. Though cinder cones are small, they are capable of producing violent Strombolian eruptions, which could have potentially disruptive affects on air travel and infrastructure. However, magmatic storage and evolution of cinder cone systems is not well understood, especially when compared to larger volcanic systems. Thus, we seek to better constrain magmatic storage depths of cinder cone magmas in the southern Cascades.

Here, we present the whole rock and mineral (olivine and pyroxene) geochemistry from one tephra and nine lava samples of a Pleistocene high-Mg basaltic-andesite cinder cone, the Basaltic Andesite of Box Canyon (mbx) Lava samples from mbx are dark gray, aphanitic with phenocrysts of pyroxene and olivine and groundmass plagioclase. Large glomerocrysts of intergrown olivine and pyroxene are common. Whole rock major element data shows that the lavas sampled have higher MgO (~10 wt%) and SiO2 (~ 52 wt%) than previously analyzed primitive basaltic products in the Lassen region compiled by Borg et al. (1997). Clinopyroxene phenocrysts are Ca-rich augites and are not zoned with respect to major elements. Additionally, there is little to no compositional variability between clinopyroxene derived from tephra and lava samples, which may indicate a single storage depth prior to eruption. Pressure conditions were calculated using the Neave & Piturka (2017) clinopyroxene-liquid barometer. Preliminary results return a storage pressure of 11.8 kbar, translating to a depth of ~44km. This indicates that the magma was stored and crystallized below the Moho (~38 km beneath the Lassen Region; Das & Nolet, 1998), which is significantly deeper than cinder cone magma storage pressures estimated utilizing olivine-hosted melt inclusions (~15 km; Walowski et al., 2015). Although further work is still required, our initial results provide new insights into the storage and evolution of basaltic magmas in the Cascade Arc.