CONSEQUENCES OF MELT TRANSPORT FOR URANIUM SERIES DISEQUILIBRIUM IN YOUNG LAVAS

Radioactive disequilibrium of U-238 nuclides is commonly observed in y oung lavas and has often been used to infer the rates of melting and m elt migration. However, previous calculations do not actually include melt transport. Here we explore the behaviour of short-lived radionucl ides in a new calculation that includes the fluid dynamics of melt seg regation. We emphasize that series disequilibrium results from the dif ferences in residence time of parent and daughter nuclides. Unlike pre vious models, contrasts in residence times are controlled by differenc es in transport velocities caused by melt separation and continued mel t-solid interaction throughout the melting column. This ''chromatograp hic'' effect can produce larger excesses of both Th-230 and Ra-226 wit hin the same physical regime compared to previous calculations which d o not include melt transport. Using this effect to account for U-serie s excesses leads to radically different inferences about the rates of melt migration. Where previous models require rapid melt extraction, o ur calculation can produce larger excesses with slow melt extraction. Nevertheless, reproducing the large (Ra-226/Th-230) activity ratios ob served in fresh mid-ocean ridge glasses is still problematic if the re sidence times are controlled solely by bulk equilibrium partitioning. While it Still remains to be shown conclusively that the large Ra-226 excesses are produced during melting, our calculation only requires di fferences in transport velocities to produce secular disequilibrium. T hus we speculate that other processes, such as crystal surface interac tion, may also contribute to the production of the observed excesses.