In this study we use microsampling techniques to explore diagenetic processes in carbonates. These processes are important as they can affect the accuracy of U series chronometry. Fission track maps of deep-sea scleractinian corals show a threefold difference between the minimum and maximum [U] in modern corals, which is reduced to a factor of 2 in fossil corals. We use micromilling and MC-ICP-MS to make detailed analyses of the [U] and delta U-234(initial) distributions in corals from 218 ka to modern. Within each fossil coral we observe a large range of delta U-234(initial) values, with high delta U-234(initial) values typically associated with low [U]. A simple model shows that this observation is best explained by preferential movement of alpha-decay produced U-234 atoms (alpha-recoil diffusion). Open-system addition of U-234 may occur when alpha-recoil diffusion is coupled with a high [U] surface layer, such as organic material. This process can result in large, whole-coral delta U-234(initial) elevations with little effect on the final age. The diagenetic pathways that we model are relevant to both shallow-water and deep-sea scleractinian corals since both exhibit primary [U] heterogeneity and may be subject to U addition.