Pb, Nd and Sr isotopic investigations of kaersutite and clinopyroxene from ultramafic nodules, and their host basalts: The nature of the subcontinental mantle

Abstract

Pb, Sr, and Nd isotopic data are presented for amphibole and clinopyroxene megacrysts from alkaline volcanic rocks at eleven localities around the Pacific rim from Nunivak Island, Alaska, to New Zealand. Isotopic data are known for the host rocks at most localities. The relatively high-Pb concentrations in the kaersutites (0.107–0.967 ppm) and the lack of evidence for crustal contamination allows their use as a probe of the isotopic composition of Pb in a component of the upper mantle. The majority of 143Nd 144Nd and 87Sr 86Sr ratios in host basalt and kaersutite samples are between 0.51293 and 0.51308 ( ϵ( Nd) = +5.7 to +8.6) and 0.7028 and 0.7031, respectively. Pb, Sr, and Nd isotope ratios in localities as widespread as Dish Hill, Siberia Crater, Cima Dome (California), Geronimo (Arizona) and Potrillo Maar (New Mexico) are mostly similar to those observed in ocean island basalts. This homogeneity suggests that the amphiboles are derived predominantly from the asthenosphere and that sources with an affinity to enriched MORB or to oceanic island basalt exist beneath Proterozoic mobile belts in the western United States. Pb, Sr, and Nd isotope ratios in nodule clinopyroxenes differ substantially from those in co-existing amphiboles and host basalts in a manner indicating predominantly lithospheric isotopic signatures for clinopyroxenes. In agreement with other authors, the present data fit a two-component model source, in which, on the average, a fluid phase with asthenospheric affinity controls the abundances and isotopic composition of Pb, Sr, and Nd in kaersutites and host basalts, and a phase with lithospheric affinity controls the clinopyroxene data. The occasional overlap between isotope data from kaersutite or basalt and clinopyroxene suggests that, if such a model is true, either source may on occasion contribute to either group of rocks such that a continuum of compositions may exist at some scale.