SMALL SCALE MANTLE HETEROGENEITIES OF THE CASCADE ARC RECORDED IN PRIMITIVE BASALTS OF THE POISON LAKE CHAIN IN THE LASSEN REGION OF THE SOUTHERN CASCADES

Small-volume primitive basalts throughout the Cascades have been used to interpret large-scale heterogeneities and processes in the sub-arc mantle. Major and trace element and isotope data from a set of six small-volume contemporaneous basalt units in the Poison Lake chain (PLC) of the Lassen Segment of the Cascade Arc are used to identify mantle-proxy compositions to evaluate the scale of mantle heterogeneities in this small area (<30km²). PLC samples that represent mantle compositions are referred to as “primitive mantle-proxy basalts,” and are restricted to primitive basalts (1) with MgO >6%, Ni>100 ppm; Cr>200 ppm and (2) have no evidence of petrogenetic processing such as fractionation, mixing and assimilation. The second criterion limits samples that can be used to represent mantle compositions to very few in the PLC, but also helps distinguish magmas that are the best representations of the mantle. Detailed major and trace element and isotope analyses and modeling of intracrustal processes of PLC basalts reveal that some magma batches were modified by fractionation and cannot be used as mantle proxies. Four primitive magmas erupted in the PLC are mantle-proxy basalts and represent unique mantle compositions, distinguished by varying depths of melting and slab (fluid) contribution. The four mantle source regions are: 1) deep, garnet peridotite mantle moderately modified by slab fluids, 2) intermediate depth mantle moderately modified by slab fluids, 3) shallow mantle, moderately modified by slab fluids and 4) shallow, depleted mantle strongly modified by fluids. Although variations of the sub-arc mantle in the Cascades are recognized at large scales across and along the arc, and at the scale of individual volcanic centers including the Lassen Volcanic Center, the small area (30km²) and range of ages (within 10 ka) represented by the primitive basalts of the PLC provided a view of the mantle at a single point in time and space. Primitive mantle-proxy basalts from the PLC indicate that the scale of mantle heterogeneity in the Lassen region, and potentially elsewhere in the Cascade Arc is as small as 10’s of kilometers- a new measure for the small scale at which mantle variations can occur.