Isotope geochemistry of Pacific Mid‐Ocean Ridge Basalt

Abstract

We report Sr, Nd, and Pb isotope ratios of 51 mid‐ocean ridge basalts (MORB) from the Juan de Fuca Ridge, the Gorda Ridge, the East Pacific Rise (EPR) and the Galapagos Spreading Center (GSC). The results confirm the general isotopic homogeneity of Pacific MORB, though high Pb isotope ratios (206 pb/204 pb up to 19.24) occur in basalts from the Easter microplate, reflecting the influence of the Easter hotspot. There is no evidence in our data of a “Dupal” geochemical anomaly in the mantle beneath the EPR in the southern hemisphere. Sr, Nd, and Pb isotope ratios are all correlated with one another, but correlations involving 87 Sr/86 Sr are poorer than other correlations, indicating that Rb‐Sr fractionation in the sub‐Pacific mantle has to some degree been decoupled from Sm‐Nd, U‐Pb, and Th‐Pb fractionation. Small but significant isotopic variations occur in MORB on regional scales of 1–100 km. These variations are, however, of lesser amplitude than those observed on larger scales. Small‐scale isotopic variations indicate either that large magma chambers do not exist beneath the EPR or that they are not efficient homogenizers of melts passing through them. On regional scales, Pb isotope ratios are more highly correlated than in Pacific MORB overall. We interpret these correlations as isochrons which record the age of events that created the regional mantle heterogeneities. These ages range from 1.4 to 2.4 Ga. Regional coherence of isotopic compositions indicates that the MORB source region consists of integral volumes which may be related to convection patterns. A comparison of the mean isotopic compositions of Pacific spreading ridges with means of Indian and Atlantic MORB shows that Pacific MORB have generally lower 87 Sr/86 Sr than MORB from the Mid‐Atlantic Ridge (MAR) and Indian Ocean ridges but have generally lower 143 Nd/144 Nd and higher 206 pb/204 pb than the MAR. Presently available data do not support the relationship of isotopic diversity and mean isotopic composition to spreading rate suggested by others. Differences between isotopic compositions and between isotopic diversity appear to be inherent features of the mantle beneath the Pacific, Indian, and Atlantic ocean basins.