Elemental abundance patterns and Sr-, Nd- and Hf-isotope systematics for the Yellowstone hotspot and Columbia River flood basalts: Bearing on petrogenesis

The Yellowstone hotspot track in the northwestern US has a 17-myr history that began with eruption of the voluminous Columbia River basalts (CRB), and produced an eastward track with the bimodal rhyolite and basaltic eruptive centers of the Snake River Plain (SRP) as the North American Plate drifted westwards. We have generated the first Hf isotopic data on basalts in this large igneous province along with bulk-rock trace-element abundance patterns and Sr-Nd isotopic ratios to assess the hypothesized kinship with co-located rhyolites throughout the hotspot track. The isotopic compositions for all three elements span much of the compositional range observed in mantle-derived basalts worldwide. Varying between a high of +11.3 in parts of the CRB and a low of −8.3 in some of the late SRP basalts, the new ε _Hf values are geographically controlled, similar to the shift first documented with Sr and Nd isotopes across the “0.706 Sr line,” which marks the boundary between thin- accreted young ocean lithosphere and arc terranes in the west to thick, old cratonized crust in the east. This observation shows that there is significant lithospheric control on the isotopic compositions of both basalts and rhyolites associated with the Yellowstone hotspot track. Modeling of assimilation and fractional crystallization (AFC) processes in seven basaltic lava flows filling the 2-Ma Big Bend Ridge Caldera at Mesa Falls, Idaho, near the eastern end of the hotspot track suggests that the crustal contribution to the Nd and Hf concentration budgets increases systematically with time and was significant. A comparison of new and previously published bulk-rock major-oxide concentrations (e.g., the alkalis Na ₂ O and K ₂ O) with our previously published data for H ₂ O, Cl, F and S in olivine-hosted melt inclusions suggests that the volatiles are sub-crustal, possibly from the lithospheric mantle, and degas extensively during magma ascent and crystallization.