Sr, Nd and Pb isotopic variation along the Pacific–Antarctic risecrest, 53–578S: Implications for the composition and dynamics of the South Pacific upper mantle

• Main Authors: Paterno R. Castillo A, James H. Natl, Yaoling Niu C, Peter F. Lonsdale A
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Published: 1997
• Subjects: Antarctic; Indian; Pacific; Antarc*
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.331.8707; http://www.dur.ac.uk/yaoling.niu/MyReprints-pdf/1998CastilloEtAl.pdf

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: Ž. 1 the ‘depleted ’ source of the bulk of Pacific normal-type mid-ocean ridge basalts Ž N-MORB.; Ž 2. an ‘enriched ’ source that produces basalts of the Hollister Ridge; and Ž. 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 143 Ndr 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 143 Ndr 144Nd and the hotspot’s influence along the ridge is spatially limited and near the high 143 Ndr 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 87 Srr 86Sr, DNd and D8r4 of samples