SR, ND AND PB ISOTOPIC VARIATION ALONG THE PACIFIC-ANTARCTIC RISECREST, 53-57-DEGREES-S - IMPLICATIONS FOR THE COMPOSITION AND DYNAMICS OF THE SOUTH-PACIFIC UPPER-MANTLE

Sr, Nd and Pb isotope data for basalts from spreading axes and off-axi s volcanoes near the Pacific-Antarctic risecrest, from Vacquier transf orm to just south of Udintsev transform, reveal an isotopically hetero geneous upper mantle. The isotopic composition of the mantle is repres ented by three end-members: (1) the 'depleted' source of the bulk of P acific 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 MORE. The distribution of these isotopic heterogeneities along the Pa cific-Antarctic risecrest suggests two alternative hypotheses on the n ature and dynamics of the south Pacific upper mantle. The whole area c ould be a single N-MORB mantle domain that shows a weak but continuous increase in Nd-143/Nd-144 from northeast to southwest across more tha n 2000 km of sea floor. The gradient is unrelated to the Louisville ho tspot because Louisville basalts have low Nd-143/Nd-144 and the hotspo t's influence along the ridge is spatially limited and near the high N d-143/Nd-144 southwestern end of the gradient. The gradient appears co nsistent with a southwestward flow of the Pacific N-MORB-type mantle t hat has been proposed mainly on the basis of ridge morphology. That th e N-MORB mantle domain is continuous across Heezen suggests that large -scale magmatic segmentation is not Sr-87/Sr-86, Delta Nd and Delta 8/ 4 of samples related to the largest structural offsets of the Pacific ridges. Alternatively, the higher from southwest of the Heezen transfo rm relative to those from the northeast could result from southwestwar d pumping of both plume and Indian Ocean-type mantle material by the L ouisville 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 Pacif ic-type source in the northeast. (C) 1998 Elsevier Science B.V.