Physics
Scientific paper
Mar 1983
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1983e%26psl..62..360h&link_type=abstract
Earth and Planetary Science Letters, Volume 62, Issue 3, p. 360-366.
Physics
8
Scientific paper
The content of K, Th and U in the continental crust is estimated based on the assumption that the concentration of these elements decreases with depth as Ax = A0e-x/D [11], with Ax and A0 the heat production rates at depth x and at the surface, respectively. Taking the weighted mean heat production rate of the intrusive rocks of the upper crust as A0 = 2.33 μW m-3, that of the granulites representing the lower crust as Ax = 0.72 μW m-3, and the mean scale height D = 9.5 km [1] the average vertical distance x = b between these intrusives and granulites is 11.2 km. With b known and the average concentrations of K, Th and U in granulites and intrusive rocks of the upper crust the scale heights of the vertical distribution of these elements are DK = 71 km, DTh = 9.5 km, DU = 5.8 km. The knowledge of these parameters permits to calculate the average concentrations of these elements in a 33.3 km thick crust: K = 2.19%, Th = 4.43 ppm, U = 0.66 ppm; Th/U = 6.7 and K/U = 3.3 × 104. The resulting heat flow is 23.0 mW m-2 which is practically identical with the value deduced from heat flow measurements. Assuming that the Th/U ratio of the entire crust-including the sediments-is 3.9, the high ratio of 6.7 in the crystalline crust indicates that about 7.2 × 1012 t U were extracted from it. All rocks with Th/U ratios <3.9 are possible sinks of this U. About half that amount is deposited in sedimentary rocks, mainly in black shales. The second important sink are the volcanic rocks of the continental margins.
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