SPINEL COMPOSITIONS IN BASALTS OF THE POISON LAKE CHAIN, NORTHERN CALIFORNIA

The Poison Lake chain (PLC) is a series of spatially (<30 km²) and chronologically (100 ka +/- 10 ka) related cinder cones and lava flows found approximately 30 km east of Lassen Peak in northern California. PLC cinder cones and flows are organized into geochemical, lithological, and geographically distinct groups. Six groups include primitive basalts, defined as MgO >6%, Ni >100 ppm, and Cr >200 ppm, all of which contain olivine hosted spinel, which collectively can be used to constrain mantle sources. Olivine- Cr spinel pairs occur in five of the primitive basalt groups and are useful for determining the early composition of the primitive mantle liquids because Cr spinels are one of the first phases to crystallize and are preserved as inclusions in olivine crystals, which protects the spinel from further interaction with liquid. These olivine- Cr spinel pairs can be used to determine the most primitive composition of the magma. New and previous major, trace, isotope, and olivine-spinel compositional data suggests that mantle sources for five of the six PLC groups with primitive basalts are the result of varying degrees of partial melting of a primitive mantle source. Using the Cr# and Mg# in Cr- spinel compositions, and the Fo content of the coexisting olivine, Lenz et al., 2014 identified that three mantle sources are required to produce these five groups, and defined them as: depleted (Cr#= 0.44-0.52), intermediate (Cr^#= 0.4), and enriched (Cr^#= 0.2-0.3). An additional PLC group, the basalts of Pittville Road have spinels that are Ti and Fe³⁺ -rich. Further analysis of the Fe-Ti spinels and the whole rock compositions of their host basalts, distinguish the Pittville Road basalts from other PLC basalt groups. Mineral and whole rock analyses will be presented to determine the degree of post-mantle processes and/or source heterogeneities that contributed to the petrogenesis of the basalts of Pittville Road.