EVIDENCE FOR MIXED ASTHENOSPHERE-LITHOSPHERE SOURCES FOR BASALTS OF THE EASTERN SNAKE RIVER PLAIN, IDAHO

Basalts of the eastern Snake River Plain have major and trace elements that suggest derivation from an asthenospheric source, but their isotopic compositions imply an enriched lithospheric source. Basalts from deep bore holes in the eastern SRP have low mg#s of 58-40 and define smooth trends on MgO-variation plots. La/Lu increases with fractionation, suggesting crustal or lithospheric assimilation. All of the basalts are LREE-enriched, with LaN ranging from 18 to 142 and (La/Lu)N=2.4 to 9.9. There are three distinct depth-correlated trends in composition: (1) below 2500 feet wide fluctuations in major and trace elements are unrelated to fractionation; (2) there are five mega-cycles of upward fractionation, defined by increases in Ti, P, Zr, La, and La/Lu up section, and by decreases in mg#s; and (3) there are two super-cycles in which Cr and Ni concentrations increase-up section. The first trend is apparently the result of variable assimilation of crust and/or lithospheric mantle material. This is supported by the Pb, Sr, and Nd isotope data. The second trend may result from the fractionation of large batches of primitive magma that were stored in the crust, while the third trend represents increased melt fractions or repeated melting of a refractory source (possibly due to continued melting of a single source region). K/P ratios vary widely below 2500 feet, but show a persistent upward increase above that depth; we infer assimilation of previously intruded basalt. Forward modeling shows that these trends cannot form by low pressure crystal fractionation; high pressure pyroxene fractionation is needed, but assimilation is also required. Trace element partial melting models require 7-12% partial melting of an E-MORB source at 12-18 kb. Mixing of 5% highly enriched lithospheric melt with 95% asthenospheric melt will generate basalts with the chemical and isotopic characteristics observed. We suggest this mixing occurred in the lithosphere as asthenospheric melts percolated upward, creating a hybrid magma with plume-like major and trace element characteristics, but with lithospheric isotopic compositions. Further assimilation occurred during storage and fractionation in the middle crust.