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 two Pleistocene high-Mg basaltic-andesite cinder cones, the Basaltic Andesite of Box Canyon (MBX) and Basalt of Bunchgrass Meadow (MBG). Lava samples from MBX and MBG 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 from MBX 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), while MBG samples have similar MgO and SiO₂. 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. Composition differences within MBX and between MBX and MBG can be attributed to fractionation of pyroxene and olivine. Pressure conditions were determined by comparing calculations from the Neave & Piturka (2017) clinopyroxene-liquid barometer and experimentally derived phase diagrams from Blatter et al. (2013). Pressure conditions for MBX were likely between 8-9kbar, which correlates to depths of 35km. MBG was likely formed under lower pressures (3kbar), placing their storage depths around 10km. Although further work is still required, our initial results provide new insights into the storage and evolution of basaltic magmas in the Cascade Arc.