OLIVINE TRACE ELEMENT INDICATORS OF THERMAL HISTORY AND MANTLE SOURCE COMPOSITIONS FOR PRIMITIVE MAGMAS AT THE POISON LAKE CHAIN, LASSEN REGION

The Poison Lake Chain (PLC) is a group of calc-alkaline cinder cones and mafic lava flows that are separated into nine distinct compositional groups (Muffler et al., 2011). All groups erupted within a 30 km² area in a relatively short ~100 ±10 ka time. The PLC is located 30 km ENE of Lassen Peak in northeastern California. Three of the compositional groups include primary basalts, which have been interpreted to have unique mantle source domains. In the current study, pressures and temperatures of mantle source melting were calculated using whole rock compositions and a geothermobarometer of Lee et al. (2009). Results identify that the three unique primary basalt compositions are derived from three separate depths, ranging from 25 to 45 km depth. To further constrain the petrogenetic history, new work uses the trace element content of olivine to determine the mantle source lithology, primary source composition, olivine crystallization temperatures, and the degree of partial melting. Initial results suggest that all PLC groups are derived from garnet peridotite mantle lithologies, based on the Al and Mn contents of olivine (De Hoog et al., 2009). However, whole rock trace element compositions indicate only two PLC groups are related to the garnet stability field of Righter et al. (2002). Additional trace element analyses of olivine will be used to investigate general source lithologies (De Hoog et al., 2009) and specific mantle compositions (Putirka et al., 2018). Once mantle lithologies are resolved, crystallization temperatures of olivine can be calculated (e.g. De Hoog et al., 2009). The degree of partial melting for each PLC group can be calculated using olivine Cr# (De Hoog et al., 2009) and calculations from Putirka (2016), to add to our thermal reconstruction of PLC primary magmas.