SrRbK and Sr isotopic relationships in ultramafic rocks, southeastern Alaska

Geologic evidence suggests that a series of ultramafic complexes of the ‘Duke Island type’ located along a 560 km-long belt in southeastern Alaska crystallized from magmas of ultramafic composition. Some geologists have proposed that these magmas were derived by fractional fusion of ultramafic material in the upper mantle. The⁸⁷Sr/⁸⁶Sr ratios, corrected for growth of radiogenic⁸⁷Sr, in total rock samples and in minerals from four ultramafic complexes range from 0.7026 to 0.7068, and the K/Rb ratios in total rock samples range from 300 to 1200. Other workers have reported that the isotopic composition of strontium in alpine peridotites is significantly different from that of oceanic volcanic rocks, but the⁸⁷Sr/⁸⁶Sr ratios of the Alaskan ultramafites and of oceanic volcanic rocks are similar.