High field strength element fractionation in the upper mantle: Evidence from amphibole-rich composite mantle xenoliths from the Kerguelen Islands (Indian Ocean)

A basanite dyke in the Kerguelen Archipelago contains abundant composite ma ntle xenoliths consisting of spinel-bearing dunites cross-cut by amphibole- rich veins. Two types of veins (thick and thin) have been distinguished: th e thick veins represent almost complete crystallization products of highly alkaline melts similar to the host basanites, whereas thin veins are precip itates from fractionates of the parental melts to the thick veins. These fr actionated fluids are enriched in H2O relative to the parental melts. The a mphiboles in the thin veins are lower in Ti and higher in Nb, Ta, Zr and Hf than amphiboles in the thick veins. This fractionation of high field stren gth elements (HFSE) is consistent with a combination of the changing compos ition of the fractionated fluids and the change in intrinsic amphibole-flui d partition coefficients for HFSE in fluids with higher a(H2O) and lower a( TiO2). The trace element content of amphiboles disseminated in dunitic wall -rocks is closely related to the composition of adjacent veins and thus the se amphiboles are precipitates from fluids percolating into the dunite from the veins. Disseminated amphibole reflects the composition of the percolat ing melt, which is similar to that of the associated veins.