Alkali/Silica Distinction between Hawaiian Tholeiites and Alkali Basalts

Abstract

In 1963, H. S. Yoder, Jr. suggested that the empirical dividing line between Hawaiian tholeiites and alkali basalts on an alkali/silica plot is the trace of the critical plane Di-Fo-Ab in the simple basalt tetrahedron Di-Fo-Ne-Qz. We show that the normative coordinates of the dividing line are generally similar to Hawaiian rock chemistry, but that it is not simply the projection of the whole critical plane onto such a plot. It is plausible, however, that over at least part of its length, the line is the projection of the intersection of a thermal divide (on or near the join Cpx-Ol-Pl) with the boundary surface separating the Cpx and Pl primary phase volumes in the generalized normative tetrahedron Cpx-Ol-Ne-Qz. This interpretation is compatible with experimentally determined crystallization sequences of Hawaiian volcanic rocks and provides an explanation for the actual existence of the dividing line in terms of the fact that low-pressure fractionation moves liquids away from the thermal divide. This interpretation can also account for the observation that some rocks with tholeiitic mineralogy fall in the alkali basalt field of an alkali/silica plot (and conversely) as an artifact of the projection implicit in making the plot.