| Summary: | 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 Nd/Nd 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 Nd/Nd and the hotspot's influence along the ridge is spatially limited and near the high Nd/ Nd 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 Sr/Sr, 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.
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