GROUNDMASS CRYSTALLINITIES OF PROXIMAL AND DISTAL LAVAS OF THE POISON LAKE CHAIN IN THE LASSEN REGION OF THE SOUTHERN CASCADES

The Poison Lake Chain (PLC) is a group of 39 calc-alkaline basaltic cinder cones located east of the crest of the Cascades arc in the Lassen Volcanic Center, approximately 30 km east of Lassen Peak. Primitive magmas erupted 100 ka (±10 ka) from monogenetic cones that are aligned with extensional faults of the Basin and Range (Muffler et al., 2011). The goal of this work is to characterize PLC lava flow emplacement. Effusion rates for 28 PLC flows range from 0.6-40 m³/s, and eruption durations from 20-205 hours, which are based on calculations using flow lengths and cone heights respectively (Walker et al., 1973). These results suggest eruptions of PLC lavas were relatively fast and were likely influenced by Basin and Range extension. We can constrain the emplacement and cooling history of PLC lavas by comparing vent and distal sample groundmass crystallinities that are quantified from SEM BSE images. Vent samples have smaller crystal lengths and widths of groundmass plagioclase (e.g., flow unit br2 has an average groundmass crystal area of 381μm² and average width of 10μm) and distal samples have larger crystal lengths and widths of groundmass plagioclase (e.g., flow unit br2 has an average groundmass crystal area of 1100μm² and average width of 18μm). Crystal sizes and population densities indicate that crystallization of groundmass plagioclase in vent samples are nucleation-dominated while crystallization of groundmass plagioclase in distal samples are growth-dominated. One atmosphere cooling experiments starting at from 1175°C (liquidus temperature) are used to recreate groundmass plagioclase textures observed in natural samples to quantify PLC lava cooling rates. CSD analyses of experiments with a slow cooling rate (5°C/hr) best replicate natural vent samples. Experimental results can be used with FLOWGO and Q-LAHVA models (Harris and Rowland, 2000; Mossoux et al., 2016) to further characterize the thermo-rheological evolution of channelized PLC lavas.