Isotopic evidence for uranium exchange during low-temperature alteration of oceanic basalt

Abstract

Measurements of uranium concentration and the ²³⁴U/²³⁸U activity ratio in oceanic basalts which have undergone low-temperature seafloor alteration indicate that uranium uptake is a pervasive occurrence but that the various phases involved behave differently with respect to this process. Palagonite exhibits uranium contents 8-20 times higher than unaltered glass coupled with low ²³⁴U/²³⁸U, suggesting ongoing preferential leaching of ²³⁴U. Altered crystalline interiors of several old basalts have ²³⁴U/²³⁸U > 1, indicative of recent uranium exchange with seawater. The data also provide evidence for uranium sources with ²³⁴U/²³⁸U higher than the seawater value of 1.14. Manganese crusts on basalts of a variety of ages have isotopic ratios indicating that they either are recent deposits or also have experienced continuing uranium exchange with seawater.