CONSTRAINTS ON PLUME–LITHOSPHERE INTERACTION BENEATH THE SNAKE RIVER PLAIN: RADIOGENIC ISOTOPE CONSTRAINTS

The Snake River Plain (SRP) represents ~16 Myr of volcanic activity that took place as North America migrated over a mantle hotspot. Seismic identification of a low-velocity body in the mantle beneath Yellowstone is compelling evidence that the Miocene to recent volcanism associated with the Columbia Plateau, Oregon High Lava Plains, the SRP, Northern Nevada Rift and the Yellowstone Plateau (YP) represents a single magmatic system related to a mantle plume. A remaining enigma is, why do radiogenic isotope signatures from basalts erupted over the Mesozoic-Paleozoic accreted terrains suggest an oceanic mantle source while basalts erupted over the Proterozoic-Archean craton margin suggest an ancient subcontinental lithospheric mantle (SCLM) source?

The Wyoming craton was enriched during ancient subduction events, and stabilized in the late Archean to Paleoproterozoic. Concentrations of radiogenic incompatible elements in OIB-plume sources are 10s to 100s times lower than found in the Wyoming craton. Assimilation of small melt fractions of the craton into large volume, larger degree partial melts derived from the plume source should result in hybrid magmas whose isotopic compositions are controlled by the isotopic composition of the cratonal component.

To test this prediction 50 SRP basalts were analyzed for Pb, Sr, and Nd isotopes. The results are consistent with mixing between an OIB-like component with 1-4% melt derived from the enriched SCML and show that the relative amount of plume component increases from 95-98% in the YP, to 98-99% in the central and western SRP. Main phase CRBG, central and eastern SRP, and YP basalts show a decrease in ²⁰⁶Pb/²⁰⁴Pb from west to east with distance from the YP, with OIB-like values in Oregon and Washington toward values typical of the lower crust and SCML of the Wyoming craton along the SRP and YP. Spatial heterogeneity in the craton is reflected by lack of covariation between ²⁰⁸Pb/²⁰⁴Pb, ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios along the SRP, indicating differences in Th/Pb, U/Pb. Rb/Sr, and Sm/Nd and the time-integrated effect of radioactive decay. These results suggest a progressive decrease in craton thickness from east to west approaching its margin, a concomitant decrease in the age, and compositional heterogeneity in the lower crust and SCML beneath the SRP.