Uranium in the oceanic crust

Thermal-neutron induced fission of uranium-235 recorded in appropriate nuclear-track detectors and visualized by chemical etching offers a unique means for measuring the concentration of uranium and mapping its spatial distribution within a rock. The accuracy in measuring uranium concentrations as low as 10-10 g/g and the high local resolution of the uranium atom sites within only a few microns makes this method extremely sensitive. Uranium mapping was applied to tholeiitic basalts coming from various parts of the world-wide mid-oceanic rift system. The results led to the following findings: - on a micro-scale, the analysis of the main constituent phases of the basalts (glass, minerals, matrix) makes it possible to follow the mode of uranium enrichment in function of the degree of sea-water alteration and to determine the pristine uranium content of a tholeiite. - On a regional scale, the pristine uranium contents of the samples provide a means to discriminate between different generations of basalts flows. - On a global scale, differences in bulk uranium contents among tholeiites from the various parts of the mid-oceanic rifts might be related to the interaction of three processes; the degree of partial melting of a chemically homogeneous upper mantle, the subsequent modification of the melts by fractional crystallization, and their local contamination by ''hot spot'' type material through lateral flow along the ridges. 7 ref.; 13 tabl.; 20 refs