Scales of Geochemical Segmentation of the Pacific Ridge

A 1300 km long sampling gap (53° to 41° South) of the Pacific Antarctic Ridge was filled during the PACANTARCTIC 2 cruise in 2004-05. The data obtained from this new collection together with published analyses give a picture of the variation of geochemical parameters along the Pacific ridge axis. This study reveals three different scales of ridge segmentation that are related to different scales of mantle domains under the ridge axis. (1) Small scale: The regular sampling of the different segments limited by overlapping spreading centers along the 53-41°S ridge section shows regular variations in geochemical parameters. Most of the 100-260 km long segments are depleted with (Nb/Zr)N < 1. The centers of the segments have higher (Nb/Zr)N and more radiogenic Sr isotope ratios than the segment ends where the ridge becomes less robust. This small-scale geochemical segmentation together with observed variations of geophysical parameters suggests small-scale mantle flow below the ridge. (2) Medium scale: A major transform fault at 50°S, Menard TF, marks the boundary between two geochemical domains. To the south, trace element and isotope ratios are depleted and more variable than to the north where the variations are more subdued and the geochemical parameters indicate a more enriched source. (3) Large scale: Data compilation of published Sr, Nd, Pb, Hf and He isotopic analyses from along the entire Pacific ridge shows geochemical breaks related to the major tectonic discontinuities, such as the Eltanin fracture zone and the Easter microplate boundary, previously recognized as a large-scale chemical and thermal division of the Pacific mantle.