GEOTHERMOBAROMETRY OF SOURCE MANTLE MELTS OF PRIMITIVE MAGMAS AT 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 30km² area and erupted in a relatively short ~100 ±10 ka time period. The PLC is located 30 km ENE of Lassen Peak in northeastern California. These groups represent primitive basalts, some of which that have been interpreted to have unique mantle source domains. The pressure and temperature of the sources of these flows have not been previously constrained. By identifying the pressure and temperature of each domain we can better constrain the physical distribution of the heterogeneity of the mantle sources. Rare earth element concentrations of samples from individual PLC groups have been used to constrain multiple mantle source components in the Lassen region (OIB, MORB, fluid, and sediment; Borg et al., 1997). Given the source heterogeneity, new pressure and temperature calculations are calculated for two PLC groups with >6% MgO, as required by the geothermobarometer (Lee et al., 2009 and Leeman personal communication). The temperature of the mantle source of the old railroad grade (bg) group is calculated to have been 1258-1289 °C, with pressures of 0.80-0.97 GPa. Calculations for the mantle source of basalts of Cone Lake Road (bc) indicate that source was likely hotter (1299 °C-1337 °C) and deeper (1.08 GPa - 1.35 GPa). These initial results are comparable with high temperatures estimated for calc-alkaline basalts (≥1200-1300 °C; Clynne and Borg 1997). Calculated pressures are also consistent with high Al/Cr in bc spinels (high pressure) and lower Al/Cr in bg spinels (lower pressure; Clynne and Borg, 1997). Distinct P-T for bg and bc are also consistent with compositional heterogeneity of their trace element compositions that suggest they come from unique mantle sources. Future work will calculate P-T for all remaining groups once compositions are adjusted for fractionation effects.