GREENLAND'S LITHOSPHERIC MANTLE
Trace-element systematics of ultra-depleted mantle xenoliths from Greenland. Constraints on mantle melting in the Archaean and the structure of Greenland lithospheric mantle.

Stefan Bernstein in collaboration with Kent Brooks (Univ. Copenhagen), Peter B. Kelemen and Karen Hanghøj (Columbia Univ., NY, USA).

Continental lithospheric mantle, and particularly the Archaean mantle, is the source of diamonds and is the platform for the growth of continental crust. The research at GEUS aims at understanding the evolution and structure of the cratonic mantle in general, and more specifically the evolution of the lithospheric mantle in Greenland which in some regions of Archaean mantle has a diamond potential. Mantle xenoliths in alkaline rocks are the primary source of knowledge of the composition of the lithospheric mantle. We have analysed a large collection of mantle xenoliths from Ubekendt Ejland (West Greenland) and Wiedemann Fjord (East Greenland). Major element and modal analyses together with Os isotopic measurements support the hypothesis that these rocks formed by high-degree mantle melting in the Archaean. Further, our modelling shows that the xenoliths are restites from komatiite extraction and that, more controversially, there is evidence that komatiites formed from normal decompressional melting at pressures in the range 7–2 GPa. Using trace-element data on orthopyroxene, obtained by ion-microprobe, we are presently engaged in the modelling of melting processes in the early Archaean, which in turn will provide the basis for understanding how the thicker portions of cratonic lithospheric mantle was formed, including the diamond-bearing peridotites.

Fig.SB4a Dunitic nodules brought from the lithospheric mantle atabout 40km depth to the Earths surface by lamprophyre dykes inWiedeman Fjord, East Greenland. Fig.SB4b The dunitic and harzburgitic nodules have considerablehigher contents of olivine and higher magnesium in the olivines(as measured by the percentage of forsterite) than average peridotite nodule from other Archaean cratons, such as Kaapvaal in South Africa.

Publications: Bernstein, S., Kelemen, P.B., and Brooks, C.K. 1998: Depleted spinel harzburgite xenoliths in Tertiary dykes from East Greenland: Restites from high degree melting. Earth and Planetary Science Letters 154, 221–235. Hanghøj, K., Kelemen, P.B., Bernstein, S., Blusztajn, J., and Frei, R. 2000: Osmium isotopes in the Wiedemann Fjord mantle xenoliths, a unique record of cratonic mantle formation by melt depletion in the Archaean. Geochemistry, Geophysics and Geosystems 2, 2000GC000085.