The vertical distribution of uranium, thorium, and potassium in the Continental Crust

Details The vertical distribution of uranium, thorium, and potassium in the Continental Crust The vertical distribution of uranium, thorium, and potassium in the Continental Crust

Mar 1967

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1967gecoa..31..377l&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 31, Issue 3, pp.377-390

Computer Science

25

Scientific paper

Granulite faeies rocks exposed at the earth's surface may not be representative of the lowermost parts of the continental crust, but they do provide samples of the deepest levels made available in any areal extent at the surface. A concentration-depth relation for elements can thus be defined by statistically comparing their abundances in granulite faeies rocks with their abundance in the common surface rocks. U, Th and K have been determined in approximately 400 rocks from the Australian shield. Felsic amphibolite facies gneisses have U and Th contents indistinguishable from those of hornblende granulite subfacies rocks and intrusive shield granites. Pyroxene granulite subfacies rocks of similar bulk chemical composition have significantly lower abundances of U and Th. In the mafic rooks studied the U and Th contents and Th/K and U/K ratios show no variation with metamorphic grade. The lower U and Th concentrations in felsic pyroxene granulite subfacies rocks is related to processes of metamorphism, including partial melting and upward movement of granitic material and vapour. The overall average contents of Th and K in the surface rocks of the West Australian basement shield appear to be similar to the overall average contents of these elements in the Palaeozoic rocks outcropping in East Australia. However the shield seems to be depleted in U relative to the younger orogenic region. It has been calculated that the surface heat production of the shield is only about 25 per cent less than that of the Palaeozoic rocks. This is compared with the difference of about 60 per cent in the average heat flows measured in the two regions.

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