Equilibrated Nd–unequilibrated Sr isotopes in mantle xenoliths

Abstract

Isotopic analyses of mantle xenoliths transported to the surface by alkali basalt eruptions complement data obtained from studying basic magmas. These xenoliths also enable mantle characteristics to be examined before they have been affected by the fractionation processes which occur during magma genesis. The data on ultramafic nodules demonstrate Sr isotopic disequilibrium between mineral phases within a single xenolith^1–5. This suggests that isotopic disequilibrium on a grain-size scale may be important in determining the isotopic composition of melts derived from the mantle. However, recent work on carefully cleaned mineral separates from alpine peridotites⁶ and ultramafic nodules² has shown that, in many cases, the highly radiogenic ⁸⁷Sr/⁸⁶Sr found in minerals with low Sr concentrations (olivine, orthopyroxene) can be considerably reduced by leaching the sample in dilute acids before analysis. To investigate the nature of isotopic disequilibria between minerals of lherzolitic nodules, we report here a combined study of the Nd and Sr isotopic composition of two spinel Iherzolites from New Mexico and Arizona. These samples were selected from a large number of ultramafic xenoliths for which major and trace element compositions had been determined⁷. The two nodules measured represent two end members of a distinct group of nodules which have major element compositions that would make them suitable parent materials for basaltic melts. These two nodules show no obvious signs of metasomatic alteration or re-equilibration under more than one pressure and temperature regime. Interstitial glass does not occur in either nodule investigated. Hence, the Nd and Sr isotopic composition of these nodules and constituent minerals were expected to provide important constraints on the nature of basalt source regions in the mantle in general, and, in particular, that underlying the American South-West.