Search

Neutron activation and spectrographic data on 58 ultramafic rocks, representative of diverse geographic and geologic settings, indicate considerable differences in abundances of Al, Si, Ti, Co, Cr, Fe, Sc, Mn, Cu, V, and Ni. These wide differences in composition vitiate any calculation of mean elemental abundances for ultramafic rocks taken as a single homogeneous group and indicate gross chemical differentiations either within the mantle or between the mantle and the ultramafic rocks.

The results of 399 new analyses and 144 analyses from the literature indicate that the carbonate-free fraction of sediments from active oceanic ridges is characterized by very low aluminum and titanium content and high iron and manganese content compared with sediments from other volcanic regions and from inactive ridges. These active ridge sediments appear to originate by volcanic processes associated with ocean-floor spreading and high heat flow. The total mass of these sediments is probably only a few per cent of all deep-sea sediments.