THE FOUNDATION SEAMOUNT CHAIN - A FIRST SURVEY AND SAMPLING

The Foundation Seamounts form a 1400 km-long chain on the Pacific plat e from 32 degrees S, 127 degrees W to the Pacific-Antarctic spreading axis at 38 degrees S, 111 degrees W. Previously only known from sparse single-beam echosoundings and satellite altimetry, we present here th e first multibeam bathymetric survey and geological sampling results. We confirm that the submarine topography correlates with the altimetry , and that the chain is volcanic rather than tectonic or microcontinen tal in origin. The chain can be divided up morphologically and geochem ically into three section: (1) west of 125 degrees W large flat-topped volcanoes composed of incompatible-element depleted lavas (Nb/Zr-N ap proximate to 1) of a near-ridge origin with little or no plume influen ce, (2) between 125 and 115 degrees W true intraplate volcanoes with i ncompatible element enrichment (Nb/Zr-N>1.9) generated over the Founda tion plume, (3) east of 115 degrees W E-W-trending volcanic ridges wit h compositions (Nb/Zr-N 2.0-0.3) suggestive of interaction between the plume and the Pacific-Antarctic spreading axis. On the spreading axis moderate incompatible element enrichments (Nb/Zr-N approximate to 0.8 , cf. approximate to 0.3 outside the Foundation area) also suggest plu me influence. It appears that the activity of the Foundation plume in the last few million years has (1) significantly waned and (2) become wholly channeled towards the spreading axis. The Foundation plume may be in the process of ''dying''.