Cruise report SO-100: the Foundation seamount chain Suva (Fiji) - Valparaiso (Chile) 19950129-19950322

The SO-100 cruise from Suva to Valparaiso made the first-ever direct study of the Foundation seamount chain. In total some 40 volcanoes were mapped in whole or in part (all at least with summit position and diameter determined), of which the majority were sampled with dredge and TV-grab. The maps and samples will be used to characterise the evolution of the Foundation chain, and to help constrain the satellite gravimetry models for Pacific seamount chains. The volcanoes show a morphological evolution along the chain, from being large isolated edifices in the west to being associated on elongated ridges at the eastern end of the chain near the Pacific-Antarctic spreading axis. Immediately adjacent to the spreading axis, the Foundation chain is characterised by three east-west trending, flat topped ridges. On board chemical studies of the lavas dredged show a progressive decrease in the trance element enrichment of the magmas towards the spreading axis (as characterised by falling Nb/Zr ratio). This suggests that the evolution of the Foundation chain is towards less trace element enriched, MORB-like compositions with time. A geochemical gradient is also see along the spreading axis itself, the most enriched lavas in terms of Nb/Zr being found on the topographic bulge where the Foundation chain projects onto the spreading axis. Comparison of the satellite-derived gravity data with the measured seafloor bathymetry shows good agreement. Many non-circular gravity anomalies turned out to be more than one closely spaced volcanic edifice in the bathymetry. Preliminary modelling of the effective plate thicknesses suggest that the lithosphere beneath some of the older volcanoes was older at the time of magmatism than that beneath somewhat younger volvanoes. Many of the Foundation volcanoes yielded hydrothermal minerals during the samples. These samples will be studied in Kiel to determine the extent and type of hydrothermalism associated with the chain. (orig.)SIGLEAvailable from TIB Hannover: RA 6759(75)+a / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekBundesministerium fuer Bildung, Wissenschaft, Forschung und Technologie, Bonn (Germany)DEGerman