Altered Lavas from the Floor of the Eastern North Atlantic

Abstract

RITTMANN and others have long argued that basic lavas erupted on the ocean floor under great depths of water may suffer immediate alteration and solidify as rocks of the serpentine–spilite association, and recent experimental work by Clark and Fyfe1 has directed attention to this possibility. The altered basalts, the physical properties and bulk chemistry of which were outlined in a previous communication2, were almost certainly extruded under deep water and may provide an example of mild alteration under these conditions. The specimens were dredged from a small abyssal hill, named Swallow Bank, standing 4–5 km. above the Mohorovicic discontinuity under 4,960 m. of water at 41° 21′ N., 14° 28′ W. These specimens, and a second collection from a nearby small seamount (the Western Seamount, summit depth 2,600 m.), have been examined petrographically. In thin section the altered lavas are 50 per cent opaque; none retains unaltered groundmass material, and only three specimens out of four hundred preserve unaltered phenocrysts or glass. The present communication outlines the mineralogy of the alteration process which is believed to have affected them.