Ultra-depleted, shallow cratonic mantle beneath West Greenland: dunitic xenoliths from Ubekendt Ejland

Abstract

Dunitic xenoliths from late Palaeogene, alkaline basalt flows on Ubekendt Ejland, West Greenland contain olivine with 100 · Mg/(Mg + Fe), or Mg#, between 92.0 and 93.7. Orthopyroxene has very low Al2O3 and CaO contents (0.024-1.639 and 0.062-0.275 wt%, respectively). Spinel has 100 · Cr/ (Cr + Al), or Cr#, between 46.98 and 95.67. Clinopy- roxene is absent. The osmium isotopic composition of olivine and spinel mineral separates shows a consid- erable span of 187 Os/ 188 Os values. The most unradio- genic 187 Os/ 188 Os value of 0.1046 corresponds to a Re-depletion age of ca. 3.3 Gy, while the most radio- genic value of 0.1336 is higher than present-day chon- drite. The Os isotopic composition of the xenoliths is consistent with their origin as restites from a melt extraction event in the Archaean, followed by one or more subsequent metasomatic event(s). The high Cr# in spinel and low modal pyroxene of the Ubekendt Ejland xenoliths are similar to values of some highly depleted mantle peridotites from arc settings. How- ever, highly depleted, arc-related peridotites have higher Cr# in spinel for a given proportion of modal olivine, compared to cratonic xenolith suites from Greenland, which instead form coherent trends with abyssal peridotites, dredged from modern mid-ocean ridges. This suggests that depleted cratonic harzburg- ites and dunites from shallow lithospheric mantle rep- resent the residue from dry melting in the Archaean.