THE SNAKE RIVER PLAIN LARGE IGNEOUS PROVINCE (SRP-LIP) THROUGH TIME

The Snake River Plain Large Igneous Province (SRP-LIP) is the world’s best modern example of a time-transgressive hotspot track beneath continental crust. The SR-LIP preserves a record of volcanic activity that spans over 17 Ma and is still active today, with basalts as young as 200 ka in the west and 2 ka in the east. The SRP began with massive eruptions of the Columbia River basalts mostly west of the Sr 0.706 line and rhyolites largely east of the 0.706 line, circa 17-16 Ma. Bi-modal volcanism lasted to 12 Ma, when it became focused on the Bruneau-Jarbidge eruptive center and along the western SRP rift valley. After about 10 Ma, volcanism progressed NE towards Yellowstone, the current locus of activity. Basalt geochemistry evolves through fractional crystallization in mid-crustal magma chambers and episodic magma recharge, with scant evidence of crustal assimilation. Trace elements imply 7-12% partial melting of EMORB-like spinel-facies peridotite. Basalt elemental abundances and He isotope systematics are consistent with a deep sublithospheric mantle source similar to Hawaii. Surface and subsurface basalt flows show systematic variations in Sr-Nd-Pb isotopes with distance from Yellowstone interpreted to reflect changes in the proportion of plume source and the underlying heterogeneous cratonic lithosphere, which varies in age, composition, and thickness from west to east. Sr-Nd-Pb isotopes suggest <5% lithospheric input into a system dominated by OIB-like plume-derived basalts.

Tomographic imaging of upper mantle velocities beneath the SRP suggest that melt buoyancy drove upper mantle convection as the plume rose and interacted with the SCLM as North America migrated westward over the Yellowstone swell. SCLM is displaced until residuum and melt buoyancy equate. The youngest SRP basalts erupted on the western Yellowstone swell have higher K and Mg, trace element ratios similar to the plume component of Hawaiian basalts, lower 87Sr/86Sr isotopic ratios, and Pb isotope compositions that lie within the Pacific MORB array. These observations suggest that the ancient SCLM was replaced by plume residuum after the North American Craton passed over the hotspot.