Fission-track mapping of uranium concentrations in basalts recovered from Leg 37 of the Deep Sea Drilling Project indicates that secondary enrichment or redistribution of the element is confined to specific zones of altered basalt. Primary magmatic variation in uranium concentration corresponding to major and trace-element variation within the basalts can also be discerned. Uranium, strontium, and other trace element values for the basalts fall into two distinct groupings, providing clear indication that two separate magmas were involved in the formation of the upper oceanic crust at site 332. Different degrees of partial melting of upper mantle material is the most probable mode of formation of the two distinct magma sources, which were tapped repeatedly and alternately to form the observed sequence of basalts encountered during the drilling of hole 332B.

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