TRACE ELEMENT ANALYSIS OF PLAGIOCLASE IN PRIMITIVE BASALTS AT POISON LAKE CHAIN, LASSEN REGION

The Poison Lake chain (PLC) is a volcanic field of mafic lava flows and calc-alkaline cinder cones located 30 km ENE of Lassen Peak in the southernmost Cascade Arc. PLC lavas erupted between 100-110 ka in a 30 km² area that contains nine compositionally distinct groups (Muffler et al., 2011). Six groups include primitive basalts with high MgO content (>6%; Muffler et al., 2001). Within the six groups, three groups (Basalt of Cone Lake road (bc), Basalts of Robbers Spring (br), and Basalt of old railroad grade (bg)) are interpreted to have compositions that did not undergo any processes (e.g. mixing, recharge, assimilation), so are classified as primary basalts. Whole rock trace element analysis further constrains the primary basalts as having their own unique mantle source. A fourth group (Basalt of Pittville Road (bt)) is the only composition with whole rock trace elements that plot in the garnet stability field (Righter et al., 2002). Basalts of Pittville Road are not primary mantle compositions, but their affinity to the garnet stability field establishes that bt samples represent an additional mantle source, distinct from other PLC groups. Plagioclase compositions are used to test if primary samples underwent magmatic processes (open system crystallization) or ascended from the mantle without interaction with other compositions (closed system crystallization). Open or closed system crystallization is determined by analyzing the chemical changes along a transect (core to rim) of a plagioclase phenocryst where the change in trace element content is correlated with fractionation, as recorded by changes in anorthite (An) content. Incompatible trace elements (e.g. Ba) will increase in closed system crystallization and compatible trace elements (e.g. Sr) will decrease in closed system crystallization. Plagioclase in groups br, bc, and bg have increasing Ba with fractionation (decreasing %An), indicating closed system crystallization, which is consistent with their previous classification as primary basalts. The Sr in plagioclase in bt basalts increase and decrease between the core and rim, suggesting open system crystallization. Further trace element analysis of plagioclase in bt basalts will help constrain what type, and to what extent, open system processes occurred.