Fractionation of U and Th during mantle melting: a reprise

238U-230Th disequilibrium in basalts has frequently been interpreted as an index of elemental U-Th fractionation during melting. However, time-dependent melting models can account for 238U-230Th disequilibrium as a result of 230Th ingrowth, that is not necessarily related to net elemental fractionations. Such ‘ingrowth’ models have been key in explaining 238U-230Th disequilibrium in mid-ocean ridge. basalts (MORB), but their importance in accounting for 230Th-excesses in ocean island basalts (OIB) is less certain. A new compilation of OIB data shows that 238U-230Th disequilibrium does not show consistent covariation with commonly used indices of degree of melting. Thus, 230Th ingrowth must also be important in the melting regimes beneath, at least some, ocean islands. The relative contribution of ingrown 230Th to total 238U-230Th disequilibrium should depend, most critically, on rates of melting relative to overall degree of melting. Fittingly, in the buoyant Hawaiian plume, there appears to be little time for 230Th ingrowth, and net U-Th fractionation appears to be most important in generating the largest degrees of 238U-230Th disequilibrium. Conversely, in the more sluggish Icelandic plume, 230Th ingrowth appears to be significant and results in striking disequilibrium even in the largest degree melts. It is apparent that each OIB dataset must be carefully assessed in turn as to whether 238U-230Th disequilibrium more dominantly reflects degree of melting or rates of melting/mantle upwelling. Certainly, it cannot be taken for granted that OIB (230Th/ 232Th) ratios are robust measures of the U/Th ratios of plume sources.