Amitsoq Gneiss

Gancarz A. J. and Wasserburg G. J. (1977) Initial Pb of the Amitsoq Gneiss, West Greenland, and implications for the age of the Earth. Geochim. Cosmochim. Acta 41, 1283-1301. 

Baadsgaard H., Nutman A. P., and Bridgwater D. (1986) Geochronology and isotopic variation of the early Archaean Amitsoq Gneisses of the Isukasia area, southern West Greenland. Geochim. Cosmochim. Acta 50, 2173 - 2183. 

McGregor V. M. (2000) Initial Pb of the Amitsoq gneiss revisited implications for the timing of early Archaean crustal evolution in West Greenland. Chem. Geol. 166, 301-308. 

Nutman A. P., Bridgwater D., and Fryer B. (1984) The iron rich suite from the Amitsoq gneisses of southern West Greenland early Archaean plutonic rocks of mixed crustal and mantle origin. Contrib. Mineral. Petrol. 87, 24-34. 

Baadsgaard, H. 1973. U-Th-Pb dates on zircons from the Early Precambrian Amitsoq gneisses. Earth and Planetary Science Letters, 33, 261-267. 

Moorbath, S., Allaart, J. H., Bridgwater, D. McGregor, V. R. 1977. Rb-Sr ages of early Archaean supracrustal rocks and Amitsoq gneisses at Isua. Nature, 270, 43-45. 

Alternative views of early Archaean mantle evolution require that mantle depletion started as early as ca. 4.5 Ga (see compilation in Rollinson, 1993). These models imply significant mantle Sm-Nd fractionation in the very early Archaean and have major implications for the differentiation of the early Earth. One such study is that of Bennett et al. (1993) who measured very high eNd values (+3.5 to +4.5) in 3.81 Ga Amitsoq gneiss samples. Collerson et al. (1991) also calculated an isochron eNd value of +3.0 for 3.8 Ga-old peridotites from northern Labrador. The extreme deviation from CHUR early in Earth history (Fig. 3.27) was interpreted by Bennett et al. (1993) as evidence for an extreme and very early fractionation of the Earth s mantle relative to CHUR. Such an event implies the formation of extensive continental crust prior to 3.8 Ga, for which there is no independent geological evidence. This apparent paradox and the claim for very early extensive mantle differentiation led to a detailed reexamination of the Bennett... 

The results of the fast decay scheme (Fig. 3.29b) are similar to the original curve by Vervoort and Blichert-Toft (1999) and imply a very early (pre-3.5 Ga) mantle differentiation event, followed by less extreme mantle depletion. The early depletion event was most likely caused by the production of a very early mafic crust. Thus the ultimate source of the parental magmas of the Jack-Hills zircons, the Amitsoq gneisses, and some Barberton komatiites, was mantle that had already been depleted in the early Archaean. Bizzarro et. al. (2003) proposed that this early differentiation of the mantle took place prior to 4.33 Ga. This result is consistent with evidence from 142Nd for a very early mantle melting event, as outlined in the previous section of this chapter. 

A, zircons from Amitsoq Gneisses, west Greenland B [black squares], komatiites and basalts from Barberton Greenstone Belt, South Africa JH, zircons from the Jack Hills,... 

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