The O, Sr, Nd and Pb isotope geochemistry of MORB

Abstract

We report analyses of O, Sr, Nd and Pb isotope ratios in 52 fresh glasses of mid-ocean ridge basalt (MORB). δ 18O-values in basalts range from +5.35 to +6.05‰ and do not correlate with indices of fractional crystallization or with 87 Sr 86 Sr . Values up to +6.47‰ occur in andesites and dacites from the propagating rift at 95°W on the Galápagos Spreading Center. Neither fractional crystallization nor assimilation of altered material appear to be dominant controls on the δ 18O of basalts. A weak, but statistically significant correlation was found between δ 18O and Nd and Pb isotope ratios in Pacific MORB which warrants further investigation. Otherwise, no statistically significant correlations between δ 18O and radiogenic isotope ratios were found, indicating that any O isotope variations in the depleted upper mantle are largely unrelated to incompatible-element enrichments and depletions. There are important and significant differences in variances of radiogenic isotopes in MORB from different ocean basins, but these differences do not appear to be related to spreading rate. We conclude that they reflect real differences in the homogeneity of the upper mantle beneath ocean ridges and that efficacy of mixing during magmatic processes is not a dominant control of the degree of heterogeneity observed in MORB. Pacific MORB has more uniform isotope ratios than MORB from other ocean basins. Radiogenic isotope ratios are most strongly correlated among Pacific MORB and least correlated among Indian Ocean MORB. Means of radiogenic isotope ratios of MORB from various ridges also vary considerably. However, there is no consistent relationship between means of various isotope ratios: means of both 87 Sr 86 Sr and 143 Nd 144 Nd are lower in Pacific than in Atlantic MORB; Pb isotope ratios in MORB from the two oceans are similar. Indian Ocean MORB has higher 87 Sr 86 Sr and lower 143 Nd 144 Nd than MORB from the Atlantic and Pacific but lower 206 Pb 204 Pb . Our data strongly reinforce the view that the depleted upper mantle consists of a number of distinct subreservoirs produced by a variety of processes. Although the Pb presently in the upper mantle may not have long resided there, it could not have been derived from a primitive reservoir. A positive correlation between 206 Pb 204 Pb and 208 Pb ★ 206 Pb ★ suggests the Pb in the mantle beneath the Pacific and Atlantic is derived from an enriched reservoir. The Pb isotope ratios in Indian Ocean MORB are suggestive of the presence of a recycled component.