FRACTIONATION OF U AND TH DURING MANTLE MELTING - A REPRISE

U-238-Th-230 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 U-238-Th-230 disequilibr ium as a result of Th-230 ingrowth, that is not necessarily related to net elemental fractionations. Such 'ingrowth' models have been key in explaining U-238-Th-230 disequilibrium in mid-ocean ridge basalts (MO RB), but their importance in accounting for Th-230-excesses in ocean i sland basalts (OIB) is less certain. A new compilation of OIB data sho ws that U-238-Th-230 disequilibrium does not show consistent covariati on with commonly used indices of degree of melting. Thus, Th-230 ingro wth must also be important in the melting regimes beneath, at least so me, ocean islands. The relative contribution of ingrown Th-230 to tota l U-238-Th-230 disequilibrium should depend, most critically, on rates of melting relative to overall degree of melting. Fittingly, in the b uoyant Hawaiian plume, there appears to be little time for Th-230 ingr owth, and net U-Th fractionation appears to be most important in gener ating the largest degrees of U-238-Th-230 disequilibrium. Conversely, in the more sluggish Icelandic plume, Th-230 ingrowth appears to be si gnificant and results in striking disequilibrium even in the largest d egree melts. It is apparent that each OIB dataset must be carefully as sessed in turn as to whether U-238-Th-230 disequilibrium more dominant ly reflects degree of melting or rates of melting/mantle upwelling. Ce rtainly, it cannot be taken for granted that OIB (Th-230/Th-232) ratio s are robust measures of the U/Th ratios of plume sources. (C) 1997 El sevier Science B.V.