Recognising early Archaean mantle: a reappraisal

Abstract

This paper examines 3.8 Ga peridotites from Greenland and Labrador to test claims that these samples are unmodified early Archaean mantle. Geochemical cri- teria were applied in which samples were compared to the mantle array in Mg/Si versus Al/Si (wt%) space, their REE patterns were compared to those of different mantle types and their chromite compositions were compared to mantle chromite compositions as expressed by their cr# and fe#. Geochemical data were used from the previously published works of Friend et al. (2002) and Bennett et al. (2002). Only two samples, from the region south of Isua satisfied all criteria, indicating that the area south of the Isua Greenstone Belt in west Greenland is a suitable place to search for early Archaean mantle. This study also confirms the observation by Friend et al. (2002) that early Archaean mantle from south of Isua is of a different character from Archaean mantle from the subcontinental lithosphere. Calculations presented here show that some mantle frag- ments from south of Isua experienced a lower degree of melt extraction and were probably more oxidising than early Archaean mantle preserved in the subcontinental lithosphere. Elemental concentrations of Os in early Ar- chaean mantle are lower than the new estimate for the primitive upper mantle of Becker et al. (2006). Peridotites from the Isua greenstone belt are not mantle, but have an affinity with the layered intrusions found south of Isua.