NB-TA-RICH MANTLE AMPHIBOLES AND MICAS - IMPLICATIONS FOR SUBDUCTION-RELATED METASOMATIC TRACE-ELEMENT FRACTIONATIONS

We report the trace element compositions of amphibole and mica separat ed from mantle xenoliths in alkali basalts and analyzed by ICP-MS. Nb and Ta are highly (50-200-fold) enriched in vein amphibole and mica re lative to primitive mantle compositions, whereas Th and U are depleted . Some disseminated amphiboles do not have such extreme Nb-Ta enrichme nts, but Nb-Ta partition coefficients between amphibole and clinopyrox ene are remarkably high, ranging from 10 to 85. In apparent contrast w ith the results on natural mantle amphiboles, recently reported Nb and Ta partition coefficients between amphibole and melts are very low [1 ,2]. The reason for the apparent contradiction may lie in either the c omposition of the amphibole or the fluid phase (silica-rich aqueous fl uid rather than silicate or carbonate melt). In either case, our obser vations show that amphibole and mica can be important hosts for Nb and Ta and cannot be ignored in identifying the underlying cause of the n early universal relative Nb-Ta depletion of subduction-related volcani c rocks. We propose a metasomatic model for creating source regions th at are depleted in Nb and Ta relative to Th, U and the LREE. Fluids ge nerated by dehydration of the subducted slab ascend through the overly ing mantle wedge and precipitate amphiboles. Highly incompatible eleme nts including Nb and Ta are transferred with the fluid into the wedge where the 'open-system' precipitation of amphibole fractionates the tr ace elements and thus generates low (Nb,Ta)/(Th,U,LREE) ratios in the residual fluid. As this fluid travels further it either directly induc es partial melting in hot regions of the wedge or is consumed through 'closed-system' crystallization of disseminated amphibole in host peri dotite that can later undergo partial melting. In either case the resu lting source regions of subduction-related magmas are enriched in high ly incompatible trace elements but not in Nb and Ta. This model may be considered either as a complement or as an alternative to published m odels explaining the chemistry of are magmas.