Potassium:Rubidium Ratio in Ultramafic Rocks: Differentiation History of the Upper Mantle

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Science  12 Aug 1966:
Vol. 153, Issue 3737, pp. 740-741
DOI: 10.1126/science.153.3737.740


The increase in K:Rb ratio with decrease in potassium content found in basaltic rocks does not seem to apply to ultramafic rocks. The ratios in a series of alpine ultramafic rocks and ultramafic inclusions in basals and kimberlite pipes are about 200 to 500-significantly lower than those in oceanic tholeiites. This characteristic of ultramafic rocks appears to be consistent with a simplified model in which early differentiation of the primitive mantle led to formation of an upper mantle region enriched in alkali elements and having a low K:Rb ratio. Alpine ultramafic rocks may be residuals from such an upper mantle region.