PRIMITIVE BASALTS OF THE POISON LAKE CHAIN REVEAL MANTLE HETEROGENEITIES IN THE LASSEN REGION OF THE SOUTHERN CASCADES

The Poison Lake chain (PLC) includes lavas and tephra from calc-alkaline cinder cones that erupted over a short period of time (~100 ± 10ka) and cover a small area (<30 km²; 1). Geochemical analyses define six compositional groups that are dominated by primitive compositions (Mg# = 59-76). Investigation of mantle heterogeneities identified three basalt samples as mantle-proxy compositions, and an additional basalt suggests a fourth, compositionally distinctive mantle composition for the PLC area. Major- and trace element compositions and isotope ratios indicate groups are not related to one another by crustal processes but are consistent with derivation from heterogeneous mantle source components. Eruption of PLC basalts from monogenetic cones that are aligned with extensional faults of the Basin and Range likely facilitated small batches of magma reaching the surface with minimal modification (1). Geothermobarometry calculations (2) suggest mantle pressures range from 0.76-1.6 GPa and 1258-1359⁰C (3). Flow lengths (0.8-4.0 km) and cone heights (20-190 m) were used to calculate effusion rates ranging from 0.6-5.0 m³/s and eruption durations ranged from 27-174 hours (4). One-atmosphere cooling experiments suggest lavas cooled at approximately 5°C/hr, and calculations estimate that flow velocities approximately 0.8 m/s (4, 5). The diversity of unique primitive basalt compositions recorded in the PLC offers a unique example of small-scale mantle heterogeneities in the sub- Cascades arc mantle. Detailed sampling used to identify the compositional diversity of magmas represented in the very small geographic area and short timescale of PLC eruptions may be a useful strategy to determine if similar mantle diversity occurs in other regions of the Cascades. (1) Muffler et al., 2011; (2) Lee et al., 2009 (3) Davis et al., 2018; (4) Dodge et al., 2017; (5) Keszthelyi & Self, 1998.