Cuney and Kyser reported in 2010(?) in Chapter 5 of a text without a reference to the book [the editor's blunder] that magmatic processes entirely controlled uranium and other incompatible element fractionation during the early history of the Earth. Uranium in silicate magmas exhibits a strongly
incompatible behavior because of its large ionic radius and high valence, which prevents its incorporation into the structure of the main rock-forming silicates. As a result, during partial melting and crystal fractionation, U is preferentially fractionated into silicate melts. Such a behavior has several major geochemical, geophysical and metallogenic consequences:
- through geological time, U and other incompatible elements such as Th and K have been continuously transferred from the mantle to the Earth crust, and within the continental crust towards its upper part,
- radiogenic heat production is maximized in the upper crust, and thus the distribution of radiogenic heat flux production may be used to delineate radioelement enriched crustal blocks (see Chapter 4 [of the unrecognized book]),
- felsic melts tend to be the most enriched in U, and thus granites and rhyolites represent primary sources of U for the formation of deposits.
However, despite the strongly incompatible behavior of U, deposits dominantly resulting from magmatic processes are rare. In an average granitoid, with a U enrichment of 3 to 4 ppm, U is dominantly held within the crystal structure of accessory minerals such as zircon, apatite, monazite, titanite and xenotime from which U cannot be leached by most geological fluids.
The first mechanism of U enrichment were partial melting of the primitive mantle (U = 20.5 ppb, Table 5-1) followed by crystal fractionation during ascent of melts to the surface. Fractionation of basic melts did take place during the Hadean and Paleoarchean times, but to a limited extent because the Earth was relatively hot.
Before the Mesoarchean (<3.2−3.1 Ga), these processes led to the formation of a relatively thin and dominantly mafic crust, composed essentially
of komatiitic and tholeiitic basalt. Felsic rocks in the early Earth belonged to the classic tonalite-trondhjemite- granodiorite (TTG) suite with generally less than a few ppm U. At such low concentrations, U essentially enters into accessory minerals, and uraninite saturation of the melts was never attained.
Uraninite has never been described in pre-Mesoarchean plutonic rocks. The first uraninite-bearing granites appear during the Mesoarchean (Cuney 2010)
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