ABSTRACT

Neutron activation analysis of U, Th and K has been made on some possible mantle materials, mainly peridotite nodules from continental and oceanic areas. The paper first describes the analytical procedures of U and Th determinations. Results of analysis showed that oceanic nodules have more uniform concentrations of U, Th and K and Th/U, K/U ratios than continental nodules. Average values for 7 oceanic lherzolites are: (1.87±0.27)×10^-8 gU/g, (5.48±0.87)×10^-8 gTh/g, and (54.2±7.5)×10^-6 gK/g, giving a rate of heat production, 26×10^-16 cal/cm³·sec, which is about 70% of that for the average chondrite. There is no indication that oceanic lherzolites are more enriched in these elements than the continental ones. These results suggest that, from the thermal point of view, peridotite nodules may not be the representative constituent of the upper mantle, unless the oceanic mantle is capable of transferring heat exceedingly well. The average values of the ratios are Th/U=2.93±0.61, and K/U=(0.29±0.06)×10⁴. The latter is significantly smaller than the crustal value (1×10⁴), which is already smaller than the chondritic value (5×10⁴).