High Field Strength Element Fractionation in the Upper Mantle: Evidence from Amphibole-Rich Composite Mantle Xenoliths from the Kerguelen Islands (Indian Ocean)

Abstract

A basanite dyke in the Kerguelen Archipelago contains abundant INTRODUCTION composite mantle xenoliths consisting of spinel-bearing dunites Composite ultramafic xenoliths in volcanic rocks provide cross-cut by amphibole-rich veins. Two types of veins (thick and unique information about the extraction, interaction and thin) have been distinguished: the thick veins represent almost migration processes of melts in the upper mantle (e.g. complete crystallization products of highly alkaline melts similar Irving, 1980; Nielson & Wilshire, 1993; Witt-Eickschen to the host basanites, whereas thin veins are precipitates from et al., 1998). Studies of such xenoliths have improved our fractionates of the parental melts to the thick veins. These understanding of trace-element fractionation processes fractionated fluids are enriched in H2O relative to the parental involved in the geochemical evolution of the upper melts. The amphiboles in the thin veins are lower in Ti and mantle. In addition, amphibole, ilmenite and phlogopite higher in Nb, Ta, Zr and Hf than amphiboles in the thick are metasomatic phases that play a crucial role in the veins. This fractionation of high field strength elements (HFSE) dispersion and residence of trace elements in the upper is consistent with a combination of the changing composition of mantle because they incorporate (sometimes exclusively) the fractionated fluids and the change in intrinsic amphibole–fluid some of the important trace elements such as high field partition coefficients for HFSE in fluids with higher aH2O and strength elements (HFSE; Ti, Zr, Hf, Nb, Ta) or large ion lower aTiO2. The trace element content of amphiboles disseminated lithophile elements (LILE; Ba, Sr) that are incompatible in in dunitic wall-rocks is closely related to the composition of olivine–orthopyroxene–clinopyroxene–spinel peridotites adjacent veins and thus these amphiboles are precipitates from (e.g. Griffin et al., 1988; O’Reilly & Griffin, 1988; O’Reilly fluids percolating into the dunite from the veins. Disseminated et al., 1991; Ionov et al., 1997). amphibole reflects the composition of the percolating melt, which We report in this paper results of a majorand traceis similar to that of the associated veins. element study of constituent minerals in composite mantle xenoliths of spinel-bearing dunites cross-cut by amphibole-rich (± ilmenite ± phlogopite) veins. Our data