Origin of the Columbia River Flood Basalts: Geochemical Evidence

Date: 4/26/2019

Time: 03:45 PM

Room: Puget Sound

The Columbia River Basalt Group (CRBG) consists of 210,000 km³ of basalt and basaltic andesite lava. Recent age estimates constrain 99% of this volume to have erupted between ~16.7 and 15.9 Ma for an overall magma flux of 0.26 km³/y; this value may have been exceeded by x5 during the peak eruptive periods. All CRBG lavas were processed through and evolved in crustal magma system(s); Grande Ronde magmas, in particular, experienced crustal contamination in these plumbing systems. Restoration to primitive compositions approximately doubles the total volume to ~0.5M km³ of mantle-derived magma, requiring 5M km³ of mantle source at nominal 10% melting. Hypotheses for the origin of the province fall into two broad groups: dominantly “bottom-up”, that is, driven by a deep-seated mantle plume; or “top-down”, fundamentally plate-driven. Geochemical support for a bottom-up origin is supplied by the following observations: 1. The basalts are derived from both enriched and depleted mantle, as expected for a rising plume entraining ambient depleted upper mantle. 2. The enriched component has high ³He/⁴He and bears isotopic affinities with the sources for some Pacific Ocean island groups. The following observations militate in favor of top-down mechanisms: 3. Highly magnesian melt compositions, which would provide clear evidence for derivation from deep-seated upwelling mantle with anomalously high potential temperature, are absent. 4. All CRB lavas, including those with the least isotopic evidence for crustal contamination, have elevated LILE/HFSE ratios expected from subduction-related processes. 5. In addition to overall LILE enrichment, all CRBG formations exhibit the strong additional Ba enrichment that characterises many mid-late Cenozoic magmatic provinces in the inland western U.S., arguing for a source component that is shared throughout the Cordillera. The geochemical features of the CRBG can best be reconciled by a compromise model, where lithospheric instabilities affected regional upper mantle which was simultaneously being invaded by a rising plume.

Presenting Author: John A. Wolff

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