Radioactive disequilibrium in altered mid-oceanic basalts

A young (<1 m.y.) tholeiitic basalt dredged from the Mid-Atlantic Ridge displays a²³⁴U excess and a²³⁰Th deficiency that have resulted from the addition of seawater uranium during weathering at seafloor temperatures. Two older samples, though they acquired substantial amounts of uranium from seawater, are depleted in²³⁴U, indicating preferential leaching of this isotope. Hydrothermally altered samples suggest that some uranium loss may have occurred. Possible isotopic effects of preferential²³⁴U leaching, however, are obscured by secondary addition of seawater uranium at lower temperatures.