Osmium isotopes in the Wiedemann Fjord mantle xenoliths: A unique record of cratonic mantle formation by melt depletion in the Archaean

The Wiedemann Fjord mantle xenoliths from Tertiary basanitic dykes in east Greenland contain extremely depleted spinel harzburgites. The harzburgites are unusual because of their highly forsteritic olivine and low modal orthopyroxene, consistent with an origin as residues of up to 40% melting. Also present are lherzolites with olivine contents as low as 50% and with up to 25% clinopyroxene, and there is a continuous range of compositions between the most depleted harzburgites and the lherzolites. Osmium isotopic data show that the depleted harzburgites were formed by melt depletion in the Archaean. In contrast to most other Archaean xenoliths (e.g., from the Kaapvaal and Siberian cratons) the composition of the Wiedemann Fjord harzburgites is consistent with depletion by polybaric melting. The lherzolites have less refractory mineral compositions than the harzburgites (lower bulk Mg # and Cr #, higher aluminum contents in orthopyroxene (opx)) and also have more radiogenic Os isotopic compositions. The data suggest that the Wiedemann Fjord xenoliths formed as restites from high degrees of melting in the Archaean and that some of the restites were partly refertilized by metasomatism involving addition of pyroxene and reequilibration of mineral and isotopic compositions.