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The three most readily apparent media for the transport of mantle-derived thorium and uranium to the crust are ultramafic and mafic magmas, alkaline undersaturated magmas, and volatiles. Each type constitutes a distinctive geologic and geochemical environment for the radioelements.

Ocean Basins

Mantle-derived magma has penetrated ocean basins along spreading ridges (Figs. 3, 5) and in plumes. Although irregularly distributed outwardly, the plumes appear to be controlled by regional tectonic features including lineaments and intersections. Along ocean ridges, mantle material intrudes more or less passively in a rising medium. Ocean-ridge magmas range widely in composition from olivine tholeiite to alkali basalt (Aumento, 1967), depending on the degree of partial melting of peridotitic or eclogitic mantle material rising beneath the ridge (Wyllie, 1971, p. 357-359). Most commonly, however, these magmas are tholeiites with very low uranium and thorium contents.

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