Constraints on melt generation and transport from U-series activity ratios

Disequilibrium activity ratios of (230Th/238U), (226Ra/230Th), (231Pa/235U) and (210Pb/226Ra) have been reported from MORB and plume-derived basalts, and equilibrium ratios of (228Ra/232Th) from Etna and the Vestmannaeyjar volcanic system south of Iceland. The constraints these observations impose on melt generation are explored using simple models, and suggest that the rate of melt generation and separation is too fast to allow radioactive equilibrium to be established for (230Th/238U) and (231Pa/235U) in the melting zone beneath Hawaii, and perhaps also for (230Th/238U) beneath ridges. The observed disequilibrium activity ratios require melt to separate from its source region when the melt fraction exceeds a few tenths of a %. The few measurements that are available for (210Pb/226Ra) and (228Ra/232Th) suggest that melt moves from being in chemical equilibrium with its residue to the surface in a time of between 6 and 20 years. However, measurements from ridges and from large intraplate volcanoes are needed to establish this result for the main sites of melt generation on Earth.