Chemical systematics of an intermediate spreading ridge: The Pacific-Antarctic Ridge between 56 degrees S and 66 degrees S

Axial bathymetry, major/trace elements, and isotopes suggest:that the Pacif ic-Antarctic Ridge (PAR) between 56 degrees S and 66 degrees S is devoid of any hotspot influence. PAR (56-66 degrees 5) samples have in-average lower Sr-87/Sr-86 and Nd-143/Nd-144 and higher Pb-206/(204)pb than northern Paci fic midocean ridge basalts (MORB), and also than MORE from the other oceans . The high variability of Pb isotopic ratios (compared to Sr and Nd) can be due to either a;general high mu (HIMU) (high U/Pb) affinity of the souther n Pacific upper mantle or to a mantle event first recorded in time by Pb is otopes. Compiling the results of this study with those from the PAR between 53 degrees S and 57 degrees S gives a continuous view of mantle characteri stics from south Pitman Fracture Zone (FZ) to Vacquier FZ, representing abo ut 3000 km of spreading axis. The latitude of Udintsev FZ (56 degrees S) is a limit between, to the north, a domain with large geochemical Variations and, to the south, one with small Variations. The spreading rate has interm ediate values (54 mm/yr at 66 degrees S to 74 mm/yr at 56 degrees S) which increase along the PAR, while the axial morphology: changes from valley to dome. The morphological transition is not recorded by the chemical properti es of the ridge basalts nor by the inferred mantle temperature which displa ys few variations (30-40 degrees C) along the PAR. Contrary to what is obse rved along the South-East Indian Ridge, PAR morphology appears to be contro lled more by spreading rate rather than by mantle temperature. Much of the major and trace element variability results from segmentation control on th e shallowest thermal structure of the mantle. The cold edge of a fracture z one seems to be more efficient when occurring in an axial dome context.:It is expressed as an increase of the magnitude of the Transform Fault Effect along the valley-dome transition, resulting in a clear increase of trace el ement ratio variability (such as Nb/Zr). There is no strong evidence for th e previously proposed southwestward asthenospheric flow in the area. Howeve r, this flow model could explain the intrasegment asymmetric patterns.