A Discussion on volcanism and the structure of the Earth - Silica saturation in cenozoic basalt

Abstract

Cenozoic basalts are conspicuously either over- or undersaturated with respect to silica. Undersaturated basalts are strongly dominant in the mediterranean areas and in oceanic islands other than Hawaii. Oversaturated basalts are strongly dominant in Hawaii and throughout the circumoceanic environment. Both over- and undersaturated basalts are abundant on the continents. Silica-saturated basalts are dominant only in the submarine ridges, where fresh representatives of either of the other saturation types are still unknown. Experimental work suggests that initial or early melting fractions of basalts or their plutonic equivalents or precursors would usually be either over- or undersaturated regardless of the saturation state of the starting material. The bimodal distribution of silica saturation in Cenozoic basalts is thus compatible with, and provides little basis for preference among, a variety of Earth models in which basaltic lavas are presumed to be early melting fractions of pre-existing solid materials.