Sr, Nd and Pb isotopic variation along the Pacific–Antarctic risecrest, 53–57°S: Implications for the composition and dynamics of the South Pacific upper mantle

Abstract

Sr, Nd and Pb isotope data for basalts from spreading axes and off-axis volcanoes near the Pacific–Antarctic risecrest, from Vacquier transform to just south of Udintsev transform, reveal an isotopically heterogeneous upper mantle. The isotopic composition of the mantle is represented by three end-members: (1) the `depleted' source of the bulk of Pacific normal-type mid-ocean ridge basalts (N-MORB); (2) an `enriched' source that produces basalts of the Hollister Ridge; and (3) a source, restricted to two adjacent sample locales, similar to that of Indian MORB. The distribution of these isotopic heterogeneities along the Pacific–Antarctic risecrest suggests two alternative hypotheses on the nature and dynamics of the south Pacific upper mantle. The whole area could be a single N-MORB mantle domain that shows a weak but continuous increase in 143Nd/ 144Nd from northeast to southwest across more than 2000 km of sea floor. The gradient is unrelated to the Louisville hotspot because Louisville basalts have low 143Nd/ 144Nd and the hotspot's influence along the ridge is spatially limited and near the high 143Nd/ 144Nd southwestern end of the gradient. The gradient appears consistent with a southwestward flow of the Pacific N-MORB-type mantle that has been proposed mainly on the basis of ridge morphology. That the N-MORB mantle domain is continuous across Heezen suggests that large-scale magmatic segmentation is not related to the largest structural offsets of the Pacific ridges. Alternatively, the higher 87Sr/ 86Sr, ΔNd and Δ8/4 of samples from southwest of the Heezen transform relative to those from the northeast could result from southwestward pumping of both plume and Indian Ocean-type mantle material by the Louisville hotspot. The Heezen transform forms a prominent tectonic and mantle domain boundary that prohibits the Louisville- and Indian Ocean-type mantle from flowing towards and contaminating the depleted Pacific-type source in the northeast.