Upper Cenozoic basalts from southwestern Nevada and east-central California are unusually rich in both strontium (~ 1,200 ppm) and Sr87 (initial Sr87/Sr86 ~ 0.707). The average Rb/Sr ratio of these basalts is too low to have generated the observed Sr87/Sr86 ratio during the 4.6 b.y. of the Earth's existence, and the high strontium contents and low Rb/Sr ratios effectively rule out introduction to the basalts of the high Sr87/Sr86 values through contamination by more radiogenic material during ascent through the crust. Instead, the basalts must have been derived from unusual mantle material in which an originally high Rb/Sr ratio was markedly lowered during an earlier phase of magmatic activity.

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