Thorium and uranium concentrations, and the isotopic composition of strontium in the differentiated Tasmanian dolentes

Abstract

The differentiation of the Tasmanian dolerites of tholeiitic affinities has been studied with respect to their Th, U, K, and Rb contents as well as to their initial Sr 87/Sr 86 ratios. A remarkable linear relationship between Th and K, and U and K is observed. The value of the Sr 87/Sr 86 ratio stays constant throughout the differentiated series, and only a slight increase in the K/Rb ratio with differentiation is noted. The observed Th/K, U/K, K/Rb and Sr 87/Sr 86 ratios are similar to those observed in “granitic” or “crustal” rocks, and are markedly different from those commonly observed in basalts, but more work on continental vs. oceanic basalts is needed. The implications of these observations on hypotheses for the “source” of the doleritic magmas are discussed. Contamination of basalt magma by crustal material, though a distinct possibility, is not favoured because of the homogeneity and uniform chemical composition of the large masses of doleritic intrusions. The low value of the K Rb ratio (~ 210) could not easily be derived from contamination of basalt magma with average crustal materials. It is concluded that if contamination has been active it must have been in the form of selective diffusion of certain elements into the magma. If the magma is generated from within the crust the temperature gradient in Tasmania must have been considerably steeper in the Jurassic than at present; if it is from the mantle an unusual composition of the mantle material under Tasmania is indicated.