When did continental crust form in the Archean?
Some komatiites in older Archean belts have relatively high positive epsilon Nd values indicating that they were derived from Sm relative to Nd enriched mantle material, that is, from depleted mantle. These komatiites are also characterised by low Al and Yb values, but high Ca values, and some rocks of this type are also enriched in Fe and Ti. The low Al and Yb could be accounted for by removal of garnet (HREE depletion) from the mantle source, but enigmatically would not account for the high epsilon Nd values reflecting high Sm/Nd (LREE depletion) of the mantle source. On the other hand Archean gneisses exhibit epNd values ranging from juvenile values of +3.5 at 4 Ga and +7 at 2.7 Ga to crustal values of -9 at 2.7 Ga. Whereas the negative epNd values likely reflect crustal recycling and/or crustal contamination, the variation in positive values of komatiitic rocks may represent primary variations in mantle chemistry or crustal recycling. The existence of values as high as +3.5 in the
