Isua supracrustals

Models for the formation of Precambrian sediments suggest that the chemical sediments (such as cherts) of the Isua supracrustal belt have formed as shallow water deposits. This is in agreement with structures locally preserved in the metacherts of the sequence. After deposition, the supracrustals were folded and metamorphosed. Finally, the metamorphism reached lower amphibolite facies and in consequence, most of the primary minerals became recrystallized. As a result all chert now appears as quartzite. But apparently metacherts, magnetite iron formation and quartz carbonate rocks have retained their major element chemistry largely unaltered during metamorphism (Nutman et al., 1984) 119). 

In summary, it can be stated that there is nothing markedly unusual about the Isua supracrustal facies and their organic contents. They can be compared closely with younger Precambrian, and in some cases Phanerozoic lithologies and successions. Depositional mechanisms and hydrosphere — atmosphere chemical equilibria appear to have been within the range of more recent times (Nutman et al., 1984)119). It seems from all indication that the advent of photoautrophy preceeded Isua times. It was probably the most crucial single event to make an impact on the evolution of the terrestrial atmosphere. 

There exists a continuous record of organic microstructures in ancient minerals and sediments from the 3.8 Ga old metasediments of the Isua Supracrustal Belt (Fig. 40). It has been pointed out that variations in the organic carbon content of... 

Blichert-Toft J., Albarede E., Rosing M., Erei R., and Bridgwater D. (1999) The Nd and Hf isotope evolution of the mantle through the Archean results from the Isua supracrustals. West Greenland, and from the Birimian terranes of West Africa. Geochim. Cosmochim. Acta 63, 3901-3914. 

Compston W., Kinny P. D., Williams I. S., and Foster J. J. (1986) The age and Pb loss behavior of zircons from the Isua supracrustal belt as determined by ion microprobe. Earth Planet. Sci. Lett. 80, 71—81. 

Dymek R. F. and Klein C. (1988) Chemistry, petrology and origin of banded iron-formation lithologies from the 3,800 Ma Isua supracrustal belt. West Greenland. Precamb. Res. 39, 247-302. 

Dymek, R. F., Brothers, S. C. Schiffries, C. M. 1988. Petrogenesis of ultramafic metamor phic rocks from the 3800 Ma Isua supracrustal belt. West Greenland. Journal of Petrology 29,... 

Frei, R. Rosing, M. T. 2001. The least terrestrial leads implications for the early Archaean crustal evolution and hydrothermal-metasomatic processes in the Isua supracrustal belt (west Greenland). Chemical Geology, 181, 47-66. 

Rose, N. M., Rosing, M. T. Bridgwater, D. 1996. The origin of metacarbonate rocks in the Archaean Isua supracrustal belt, West Greenland. American Journal of Science, 296, 1004-1044. 

Rosing, M. T. Frei, R. 1999. Late Archaean metasomatism and kyanite formation in the >3700 Ma Isua supracrustals, west Greenland. Journal of Conference Abstracts, 4, 144. 

Blichert-Toft, J. and Frei, R., 2001. Complex Sm-Nd and Lu-Flf isotope systematics in metamorphic garnets from Isua supracrustal belt, west Greenland. Geochim. Cosmochim. Acta, 65, 3177-87. 

Frei, R. and Jensen, B.T., 2003. Re-Os, Sm-Nd isotope and REE systematics on ultramafic rocks and pillow basalts from the Earth s oldest oceanic crustal fragments (Isua supracrustal belt and Ujaragssuit nunat area, W. Greenland). Chem. Geol., 196, 163-91. 

Frei, R., Polat, A., and Meibom, A., 2004. The Hadean upper mantle conundrum evidence for source depletion and enrichment from Sm-Nd, Re-Os and Pb-isotopic compositions in 3.71 Ga boninite-like metabasalts from the Isua Supracrustal Belt, Greenland. Geochim. 

Lepland, A., Arrhenius, G., and Cornell, D., 2002. Apatite in early Archaean Isua supracrustal rocks, southern west Greenland its origin, association with graphite and potential as a biomarker. Precambrian Res., 118, 221-41. 

Nutman, A.P., Allart, J.P., Bridgwater, D., Dimroth, E., and Rosing, M., 1984. Stratigraphic and geochemical evidence for the depositional environment of the early Archaean Isua supracrustal belt, southern west Greenland. Precambrian Res., 25, 365-96. 

Graphite and carbonates in the 3.8 Ga-old Isua supracrustal belt, southern west Greenland. Precambrian Res., 126, 331 18. 

Van Zuilen, M.A., Mathew, K., Wopenka, B., Lepland, A., Marti, K., and Arrhenius, G., 2005. Nitrogen and argon isotopic signatures in graphite from the 3.8 Ga-old Isua supracrustal belt, southern west Greenland. Geochim. Cosmochim. Acta, 69, 1241-52. 

The oldest existing rocks on Earth, from the Isua supracrustal belt of West Greenland, are dated at 3.8 X 10 years (Moorbath et al., 1973) and consist of metamorphosed sediments. It has been suggested that the carbon in the Isua banded iron formation may be photosynthetic in origin (Schidlowski et al., 1979). Whether this remarkable conclusion is substantiated by further work remains to be seen. It is in any case abundantly clear from the available evidence that the conditions of deposition of the oldest known sediments were remarkably like those prevailing during the rest of the Archaean (4.6 — 2.6 X 10 years). Furthermore, with the exception of the appearance... 

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