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. However, magmatic storage and evolution of cinder cone systems is not well understood,... [ view full abstract ]

The Lassen Region in Northern California is dominated by cinder cones, small volcanic landforms that erupt once for a period of 1-10 years. 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). 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). 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 6.0 kbar, translating to a depth of ~20km. This indicates that the magma was stored and crystallized in the mid to lower crust. Trace and major element data indicate magmatic evolution by fractionation rather than contamination.