Extreme isotopic homogeneity among basalts from the southern East Pacific Rise: mantle or mixing effect?

Abstract

Basaltic lavas which have erupted along the great oceanic ridge system are characterized by their overall chemical and isotopic homogeneity. Nevertheless, they show evidence for variability in the chemical and isotopic composition of the ocean ridge basalt (ORB) source whose scale and distribution remain uncertain. Isotopic compositions of Sr, Nd, Pb and Hf and ratios of some highly incompatible elements have been used as tracers of ORB source compositions, usually permitting source characteristics to be distinguished from petrogenetic effects. The Mid-Atlantic Ridge (MAR), in particular, shows correlations in the isotopic compositions of several elements, between isotopic compositions and topography and between these parameters and incompatible element concentrations and ratios^1–3. We report here isotopic measurements for basalts, dredged between approximately 30 and 34° S on the East Pacific Rise and adjacent Maddingly Rise, which show highly uniform Sr and Nd isotopic compositions, possibly resulting, in part, from mixing associated with a high rate of magma generation and rapid spreading (∼170 mm yr⁻¹). However, a ridge segment of comparable length between 18 and 22° S and with only a slightly lower spreading rate shows much greater isotopic variability. Hence we are apparently sampling small but detectable regional differences in mantle composition. The isotopic homogeneity among the basalts from 30 to 34° S implies that any heterogeneity in the underlying upper mantle is uniformly distributed on a scale much smaller than the volume sampled in individual magma batches, and is thus averaged out in the resulting lavas.