Processes of mantle enrichment and magmatic differentiation in the eastern Snake River Plain: Th isotope evidence

The ∼ 85 km long Great Rift of southern Idaho bisects the volcanic depression that forms the eastern Snake River Plain. Thorium isotope systematics of compositionally diverse lavas of the Great Rift, including those of Craters of the Moon, record a spectrum of crust and mantle processes. (²³⁰Th) (²³²Th) ratios range from 0.87 to 1.11 and are enriched in (²³⁰Th) with respect to (²³⁸U) by up to 13%. Covariations in Th isotope systematics in lavas from Craters of the Moon reflect crustal assimilation accompanied by fractional crstallization involving accessory phases. Temperatures based on apatite and zircon saturation confirm other estimates of magmatic temperatures and, considered together with volcanic histories, suggest cooling in upper crustal magma chambers at rates of > 10° C/ka. Thermal and chemical evolution of the Great Rift basalts support their differentiation at lower crustal conditions. Although basalts of the eastern Snake River Plain are generally interpreted as having originated in ancient enriched mantle lithosphere beneath southern Idaho, they share remarkable chemical similarities with oceanic basalts derived from enriched sources attributed to mantle plumes. Thorium isotope signatures of the Great Rift basalts are those of depleted mantle, resulting in the most extreme case of disparity between ²³²Th ²³⁸U ratios delimited by Th and Pb isotope systematics. It is difficult to account for this decoupling by invoking subduction-related U enrichment. A more likely explanation is metasomatism of hybrid lithospheric mantle in the wake of the Yellowstone plume.