Xenoliths evidence for a refractory oceanic mantle percolated by basaltic melts beneath the Kerguelen archipelago

Alkali basalts from the Kerguelen islands have entrained many mantle peridotites (harzburgites and dunites) in addition to various other ultramafic and mafic xenoliths. The harzburgites and the dunites were equilibrated in the spinel peridotite stability field (T = 850-1150°C). They attest to the existence of a metasomatized refractory upper mantle beneath the southeastern province of the Kerguelen islands. To date no fertile mantle Iherzolite has been found in this area. The harzburgites can be divided into protogranular Cr-diopside-bearing harzburgites and poikilitic harzburgites which contain an interstitial magnesian-augite. They bear the imprint of two main processes: (1) high degree of partial melting, and (2) interactions with basaltic magmas. The partial melting event is well established by the high refractory indices of both whole rocks and minerals (Fo up to 92, Cr-rich spinel, high MgO and low CaO, Al₂O₃, Na₂O and total Fe₂O₃ contents in bulk-rock analyses). Evidence of the interaction with basaltic melts is apparent in the development of coarse granular to poikilitic textures, U-shaped REE patterns in both types of harzburgites and in the presence of Cr-Na-rich magnesian-augite sometimes associated with phlogopite in the poikilitic harzburgites. Moreover, the mg* numbers of minerals and bulk rocks in the poikilitic harzburgites and dunites are systematically lower than those predicted by partial-melting models. These characteristics suggest an interaction between previously depleted harzburgites and LREE-enriched alkali basaltic melts. The dunites are the wall-rocks of magmatic dykes of websterites, clinopyroxenites and hornblendites. They show the same textural (coarse granular to poikilitic textures), chemical (low mg* number of mafic phases) and REE patterns as the harzburgites. All these features are consistent with a model whereby the dunites were formed by interactions between formerly depleted harzburgites and basaltic melts of transitional to alkaline affinities propagating within open cracks.