THORIUM URANIUM AND POTASSIUM ABUNDANCES IN PERIDOTITE INCLUSIONS AND THEIR HOST BASALTS

The abundances of the radioactive elements uranium, thorium and potassium in six lherzolite inclusions and their host basanites have been determined by neutron activation analysis and γ-ray spectrometry. The lherzolite inclusions have marked differences in K/U, K/Th and smaller differences in Th/U from the basanites. The contents of those elements in lherzolite inclusions are not determined simply by varying degrees of contamination by the host magma. The lherzolites are inferred to be accidental xenoliths in the host magma and their geochemical characteristics are representative of at least some parts of the upper mantle. They are considered to represent residual material left after extraction from mantle pyrolite of an undersaturated basaltic magma or after selective extraction of minor elements by wall-rock reaction processes. Processes of magma extraction from the mantle may lead to strong depletion of K in residual mantle peridotite with very little accompanying depletion in U and Th. The abundances of U, Th and K in the lherzolite inclusions are also compared with various estimates of upper mantle composition based on heat flow data. © 1957.