RADIOACTIVE DISEQUILIBRIA FROM 2D MODELS OF MELT GENERATION BY PLUMES AND RIDGES

We present a simple method of calculating radiogenic excesses caused by the melting process in 2D models of melting regions. U-238 series disequilibrium in basalt from recent Hawaiian eruptions can be explained using an axisymmetric plume model and a melt parameterisation based on laboratory experiments. The model is also applied to mid-ocean ridges and shows that the disequilibria are controlled by the mean melting rate at the base of the melting region and by the melt fraction present during melting. (Th-230/U-235) data constrains the mean melting rate at the onset of melting to less than 8 x 10(-8) yr(-1), which is consistent with that expected from the experimental parameterisation. Limited (Pa-231/U-235) data give a maximum initial melting rate of 0.4 x 10(-8) yr(-1). Melting begins in the garnet peridotite stability field and melt extraction is rapid (similar to 2000 yr). Extraction on this time scale is most easily explained if melt transport is by channel flow. The geochemical observations require the melt fraction in the region where U, Th and Ra are extracted from the source to be similar to 0.1%, in agreement with estimates from a simple fluid dynamical model of melt separation.