Abstract

Assisted by detailed bathymetric and bottom photographic coverage, a series of closely spaced samples of rock were taken by dredging along a traverse from the center of the Median Valley to the adjacent crest mountains on the Mid-Atlantic Ridge. The specimens show a gradation from tholeiitic to alkali basalts. Chemical variations, and the alkali content in particular, can be correlated with the depth of extrusion and with the topographic relationship of the volcanoes to the axis of the Median Valley.Although the basalts show considerable evidence of gravity-controlled crystal fractionation, the trends so established are evidently not responsible for alkali enrichment, but appear to be superimposed onto the more fundamental, continuous trend from tholeiitic to alkali basalts.

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