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

• Main Authors: Castillo, Paterno R., Natland, James H., Niu, Yaoling, Lonsdale, Peter F.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 1998
• Subjects: East Pacific Rise; Isotope ratios; Mid-ocean ridge basalts; Nd-144/Nd-143; Pacific-Antarctic Ridge; Pb-208/Pb-204; Petrology; South Pacific; Sr-87/Sr-86; 1901 Earth and Planetary Sciences (miscellaneous); 1906 Geochemistry and Petrology; 1908 Geophysics; 1912 Space and Planetary Science; Antarctic; Pacific; Indian; Antarc*
• Online Access: https://espace.library.uq.edu.au/view/UQ:d37685e

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.