OLIVINE-HOSTED SPINELS REVEAL SMALL-SCALE MANTLE HETEROGENEITIES BENEATH THE LASSEN SEGMENT OF THE SOUTHERN CASCADES

Chromian spinel compositions from a geochemically diverse suite of calc-alkaline primitive basalts, erupted in the Poison Lake chain (PLC), reveal the presence of small-scale heterogeneities in the mantle beneath the Lassen region of the Southern Cascades. Primitive basalts of the PLC are contemporaneous (100ka +/- 10ka) and cover a small area (<30 km²); however, major and trace element data for compositions erupted in the PLC suggest that there are six geochemically distinct primitive basalt groups that are not genetically related. Instead, they are interpreted to result from several small, compositionally distinct batches of mantle melts. The small spatial scale, similar ages and wide range of geochemical compositions make the PLC an ideal laboratory to describe geochemical variations in the sub-arc mantle. We present electron microprobe analyses of olivine-hosted spinel to characterize the source of these primitive basalts. Focused on spinel inclusions in olivines that have Fo contents that indicate equilibrium with their host rocks’ Mg#s, we examine spinel compositions to determine early crystallizing phases (spinels with TiO₂ <1.5 and Mg# > 0.5) and use the most primitive compositions to differentiate mantle source compositions. Mg# and Cr# in spinel and Fo content in olivine for PLC basalts define geochemical trends that describe phases that crystallized from mantle melts. Spinel chromium compositions of inferred melts are consistent with three distinct mantle sources: (1) depleted/higher partial melt with Cr# = 0.44-0.52; (2) intermediate with Cr# = 0.4; and (3) enriched/lower partial melt with Cr# = 0.2-0.3. In addition, correlation between location and Cr# of spinel indicate that the mantle may be spatially variable, showing more depleted compositions in the northwest, grading to more enriched compositions in the southeastern part of the PLC. Mineral chemistry of olivine-spinel pairs and the small area of the PLC suggest that the mantle beneath the southern Cascades is heterogeneous on a much smaller scale than previously recognized.