The Foundation seamount chain is 1400km long and is built on the Pacific plate. The chain begins at 32°S and 127°W and crosses the Farallon fossil spreading axis and a fossil microplate. It ends at the Pacific-Antarctic spreading axis at 38°S. The seamounts mapped west of 116ºW form isolated structures whereas those towards the easterly part of the spreading axis form elongated East-West ridges as seen in figure below (Devey et al., 1995a). Preliminary studies of the height of the volcanoes versus the thickness (Binard and Maia, 1995) and/or the age of the lithosphere (Maia et al., 1995) leave very little doubt as to the mantle-plume related origin of this chain. Ar-Ar dating in progress will provide a definitive answer to this question.
Trace element geochemistry shows the enriched character of the well shaped volcanoes of the western part of the chain ((Nb/Zr)N > 2) whereas East-West ridges exhibit more variable values (0.3 < (Nb/Zr)N < 2). The spreading axis itself shows moderate trace element depletion ((Nb/Zr)N × 0.8) and a high proportion of evolved products (Devey et al., 1995b). Initial radiogenic isotope data (Sr, Nd and Pb) favour the hotspot model because the volcanoes of the chain itself exhibit an isotopic signature distinct from that of South Pacific MORB. Nevertheless, rather unradiogenic 87Sr/86Sr (O.7028) and radiogenic 143Nd/144Nd (O.51293) and Pb (206Pb/204Pb ×19.85 and 208Pb/204Pb × 39.4) favour a mixed mantle source beneath this hotspot.
Interactions with fossil Farallon or active Pacific-Antarctic spreading axes have produced contrasting geochemical signatures likely linked to specific structural environments. On one hand, seamount 5 named "Becquerel" and built on a former microplate has a less radiogenic Sr and Pb and more radiogenic Nd isotope signature which probably traces some past interaction of the plume with the former Farallon spreading system; On the other hand, the two
elongated ridges out of three that we sampled and which link the actual active spreading axis and the part of the chain made of well individualised seamounts, have intermediate isotopic characteristics between MORB and plume values. This implies that the plume material which has built this hotspot track has interacted twice with spreading axes in two distinct structural settings, namely a microplate and a superfast spreading axis. This provides an opportunity using isotope and element geochemistry to understand the plume-depleted mantle interactions under these various structural constraints.
Binard, N. & Maia, M., Meeting of the Geological Society of France on Marine Geosciences, Brest, Abstract volume, 15. (1995).
Devey, C.W. and the scientific party of the cruise Sonne 100, Berichte/Report no. 75, Geological Institute, University of Kiel, 123pp. (1995a).
Devey, C. W. and the scientific party of the cruise Sonne 100. Submitted to Nature (1995b).
Maia, M., Binard, N., Devey, C.W., Hékinian, R. & Stoffers P., Meeting of the Geological Society of France on Marine Geosciences, Brest, Abstract volume 48 (1995)
Mammerickx J., Earth Planet.Sci. Lett. 113, 293-306 (1992).
Fig. 1: Map of the Foundation seamount chain deduced from gravimetric data (Mammerickx, 1992) with the position of the volcano summits (black crosses).