Depleted spinel harzburgite xenoliths in Tertiary dykes from East Greenland: Restites from high degree melting

A new collection of mantle xenoliths in Tertiary dykes from the Wiedemann Fjord area in Southeast Greenland shows that this part of the central Greenland craton is underlain by highly depleted peridotites. The samples are mostly spinel harzburgites with highly forsteritic olivines (Fo_87-94, average Fo_92.7). This, together with unusually high modal olivine contents (70- > 95%), places the Wiedemann harzburgites in a unique compositional field. Relative to depleted Kaapvaal harzburgites with comparable Fo in olivine, the Wiedemann samples have considerably lower bulk SiO₂ (average 42.6 wt% versus 44-49 wt%). Spinel compositions are similar to those in other sub-cratonic harzburgites. Pyroxene equilibrium temperatures average 850°C, which is above an Archaean cratonic geotherm at an inferred pressure of 1-2 GPa, but low enough so that it is unlikely that the xenoliths represent residual peridotites created during Tertiary magmatism. Among mantle samples, the Wiedemann harzburgites are, in terms of their bulk composition, most similar to harzburgites from the ophiolites of Papua New Guinea (PNG) and New Caledonia (NC). One hypothesis is that the Wiedemann harzburgites, along with PNG and NC harzburgites, formed via dissolution of pyroxene from previously depleted peridotites, possibly beneath a volcanic arc. If so, higher spinel Cr/Al in Wiedemann samples may reflect a deeper origin compared to PNG and NC peridotites. Alternatively, using proposed primitive mantle compositions as a protolith, the Wiedemann harzburgites can be modeled as the residue after extraction of some 40% melt. The composition of this calculated hypothetical melt in terms of CaO, Al₂O₃, FeO, MgO and SiO₂ is similar to published experimental data on high degree melts of peridotite at 2-3 GPa. Munro-type komatiites lie close to these calculated and experimental melts but are slightly displaced towards low degree experimental melts at higher pressure (e.g., 6 GPa). We conclude that the Wiedemann harzburgites formed as a residue after about 40% melting, and that they may represent shallow, refractory residues after polybaric melting initiated at pressures > 7 GPa and continuing to relatively low pressures (2-3 GPa or less). Extraction and aggregation of polybaric melts would produce liquids similar to Munro-type komatiites.