Primitive Basalts Record Small-Scale Mantle Heterogeneities in the Lassen Region of the Southern Cascades

Abstract:

The Poison Lake chain (PLC), located the Lassen region of the Southern Cascades, encompasses six geochemical, lithological and geographically distinct groups of primitive basalts, defined as MgO >6%, Ni >100 ppm, and Cr >200 ppm. In total, 22 cinder cones and flows of the PLC erupted in a small area (<30 km²) over a very short time frame (100 ka +/- 10 ka). The diversity of primitive compositions in the small spatial and temporal scales provide an ideal area in which to explore variations in mantle compositions that produce primitive basalts in the Lassen Segment of the Cascade Arc. We present major, trace, and isotope data and spinel-olivine compositions that reveal the diversity of mantle domains present in this small area. Chromium compositions of spinel in primitive basalts of the PLC indicate the presence of three distinct mantle sources: (1) depleted with Cr# = 0.44-0.52; (2) enriched with Cr# = 0.2-0.3 and (3) an intermediate composition, with Cr# = 0.4. Major and trace element compositions of PLC primitive basalts confirm the relative differences in depletion and reveal distinctions in the depth of melt generation. REE patterns and trace element ratios indicate variability in the presence of garnet in the source and define the source regions with varying depth. These mantle domains are geographically distributed with depleted compositions (high Cr# spinel, lower incompatible elements) in the northwest part of the PLC, grading to more enriched compositions (low Cr# spinel and higher incompatible elements) in the southeast. Previous workers recognize variations in the Cascadian sub-arc mantle at large scales across and along the arc, and at the scale of individual volcanic centers. However, the small area (30km²) and short timescale (within 10 ka) represented by the primitive basalts of the PLC allow us to hold time and space relatively constant while examining geochemical variations. Using primitive mantle proxy basalts from the PLC, we suggest that mantle in the Lassen region, and potentially elsewhere in similar arcs, may be as small as tens of kilometers - a new measure for the scale at which mantle variations can occur.