Strontium isotopic composition of basaltic lavas from the snake river plain, Southern Idaho

Abstract

The isotopic composition of strontium has been measured in 54 basalts and associated igneous rocks from the snake River Plain, southern Idaho. Thirty Pliocene-Holocene olivine tholeiites have 87Sr/ 86Sr ratios ranging from 0.7060 to 0.7080 and averaging 0.7070 ± 0.0003 (2σ); 17 basalts and associated latites from Craters of the Moon, King Hill, and the Blackfoot River Canyon have ratios ranging from 0.7080 to 0.7180. The 87Sr/ 86Sr ratios in the tholeiites are similar to these in other continental tholeiites, but are greater than those in oceanic basalts and in alkali-olivine basalts from the Basin-Range province. The constancy in isotopic composition suggests that the high ratios are characteristic of the parent magma(s) and are not due to crustal contamination. Major and trace element data suggest that the Craters of the Moon lavas are differentiates of the olivine tholeiites. The high 87Sr/ 86Sr ratios of these lavas may be explained by crustal contamination of small volumes of tholeiitic magma which differentiated at shallow depth in a Precambrian metamorphic terrain near the margins of the Snake River Plain. Pliocene silicic volcanic rocks which may underlie the Snake River basalts are not a likely source of contamination, because five such rocks have present-day 87Sr/ 86Sr ratios ranging from 0.7110 to 0.7134.