Consequences of melt transport for uranium series disequilibrium in young lavas

Abstract

Radioactive disequilibrium of 238U nuclides is commonly observed in young lavas and has often been used to infer the rates of melting and melt migration. However, previous calculations do not actually include melt transport. Here we explore the behaviour of short-lived radionuclides in a new calculation that includes the fluid dynamics of melt segregation. We emphasize that series disequilibrium results from the differences in residence time of parent and daughter nuclides. Unlike previous models, contrasts in residence times are controlled by differences in transport velocities caused by melt separation and continued melt-solid interaction throughout the melting column. This “chromatographic” effect can produce larger excesses of both 230Th and 226Ra within the same physical regime compared to previous calculations which do not include melt transport. Using this effect to account for U-series excesses leads to radically different inferences about the rates of melt migration. Where previous models require rapid melt extraction, our calculation can produce larger excesses with slow melt extraction. Nevertheless, reproducing the large ( 226Ra/ 230Th) activity ratios observed in fresh mid-ocean ridge glasses is still problematic if the residence times are controlled solely by bulk equilibrium partitioning. While it still remains to be shown conclusively that the large 226Ra excesses are produced during melting, our calculation only requires differences in transport velocities to produce secular disequilibrium. Thus we speculate that other processes, such as crystal surface interaction, may also contribute to the production of the observed excesses.