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 U-238/U-235 signature has important implications for understanding U exchange processes during subduction and recycling into the mantle. Here we study the U systematics of similar to 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 similar to 1300 m of intact ocean crust at this site, reveal large variability in U concentrations and U-238/U-235 ratios. Many of the samples from the upper similar to 600 m of extrusive lavas have elevated U concentrations and U-238/U-235 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 U-238/U-235 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 U-234/U-238 ratios from samples in the lava-dike transition and upper sheeted dikes give evidence for U mobility within the last similar to 1.5 million years, likely driven by deep channelled flow of seawater-derived hydrothermal fluids combined with preferential leaching of U-234 from the rock matrix. Both the total estimated U uptake and mean U-238/U-235 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 U-238/U-235 ratio in altered ocean crust over time, which needs to be taken into consideration when estimating global U budgets.