Alkali basalts from the Kerguelen islands have entrained many harzburgites and dunites in addition to ultramafic and mafic xenoliths (clinopyroxenites, wehrlites, websterites, hornblendites, basic granulites). The harzburgites and the dunites have been reequilibrated in the spinel peridotite stability field. They attest firstly to the existence of a refractory upper mantle beneath the southeastern provinces of the Kerguelen islands. To date no fertile mantle lherzolite has been observed.
The harzburgites can be divided into protogranular harzburgites which contain Cr-diopside and poikilitic harzburgites which are characterized by poikilitic textures and magmatic Mg-augite. They bear the imprint of two main processes, 1) high degree of partial melting probably related to the growth of the Kerguelen islands at the South East Indian Ridge and 2) interaction with mantle plume basaltic magmas. Partial melting event is well established by the high refractory indexes of both whole rocks and minerals (Fo up to 92, Cr-rich spinel, high MgO and low CaO, Al2O3, Na2O and Fe2O3 total bulk rock contents). The development of coarse granular to poikilitic textures, U-shaped REE patterns in both types of harzburgite and the presence of the
Cr-Na-rich Mg-augite sometimes associated with phlogopite in the poikilitic harzburgites are the evidence of the interaction with basaltic melts. Moreover, mg-numbers of Ol, Cpx and Opx in poikilitic harzburgites and dunites are systematically
lower than those predicted by partial melting models. These characteristics suggest reactions between previously depleted harzburgites and LREE-enriched basaltic melts. In the composite xenoliths, the dunites are wall rocks of magmatic dykes of tholeiitic (websterites) or alkaline affinity (biotite bearing clinopyroxenites, hornblendites). They show the same textural (coarse granular to poikilitic textures), chemical (low mg number of mafic phases) and trace element patterns than the poikilitic harzburgites. All these features are consistent with a model whereby the dunites were formed by reaction between formerly depleted harzburgites and basaltic melts propagating within open cracks.