Time scales and mechanisms of uranium uptake in altered ocean crust; observations from the ∼15 million year-old site 1256 in the eastern equatorial Pacific

The alteration of ocean crust through hydrothermal seawater circulation facilitates chemical exchange between Earth's surface and interior. Hydrothermal alteration leads to uranium (U) removal from seawater and net U uptake by the ocean crust, particularly during low temperature alteration that occurs on the vast ocean ridge flanks away from the spreading axes. Determining the timescales of U uptake and its associated ²³⁸U/²³⁵U signature has important implications for understanding U exchange processes during subduction and recycling into the mantle. Here we study the U systematics of ∼15 million year-old ocean crust drilled at Site 1256 on the eastern flank of the East Pacific Rise. Analysis of cores from the upper ∼1300 m of intact ocean crust at this site, reveal large variability in U concentrations and ²³⁸U/²³⁵U ratios. Many of the samples from the upper ∼600 m of extrusive lavas have elevated U concentrations and ²³⁸U/²³⁵U ratios lower than seawater, consistent with mechanisms of U uptake under relatively oxidised conditions. Samples from the underlying sheeted dikes and gabbros show evidence for hydrothermal U mobilisation, but negligible net U uptake. In contrast, in the transition zone between the extrusive lavas and the sheeted dikes, samples revealed large U enrichments and high ²³⁸U/²³⁵U ratios above seawater. This is consistent with uptake of the reduced U⁺⁴ species under relatively reducing conditions from seawater-derived hydrothermal fluids. In addition, large secular disequilibrium in ²³⁴U/²³⁸U ratios from samples in the lava-dike transition and upper sheeted dikes give evidence for U mobility within the last ∼1.5 million years, likely driven by deep channelled flow of seawater-derived hydrothermal fluids combined with preferential leaching of ²³⁴U from the rock matrix. Both the total estimated U uptake and mean ²³⁸U/²³⁵U at Site 1256 is lower than similar estimates from significantly older (>100 million years) altered ocean crusts at drill Sites 801 and 417/418. This shows the variable total U uptake and ²³⁸U/²³⁵U ratio in altered ocean crust over time, which needs to be taken into consideration when estimating global U budgets.