Potassium, rubidium, strontium, thorium, uranium, and the ratio of strontium-87 to strontium-86 in oceanic tholeiitic basalt

The average concentrations of potassium, rubidium, strontium, thorium, and uranium in oceanic tholeiitic basalt are (in parts per million) K, 1400; Rb, 1.2; Sr, 120; Th, 0.2; and U, 0.1. The ratio Sr⁸⁷ to Sr⁸⁶ is about 0.702, that of K to U is 1.4 x 10⁴, and of Th to U is 1.8. These amounts of K, Th, U, and radiogenic Sr⁸⁷ are less than in other common igneous rocks. The ratios of Th to U and Sr⁸⁷ to Sr⁸⁶ suggest that the source region of the oceanic tholeiites was differentiated from the original mantle material some time in the geologic past.