Clifford Walters, Akira Shimoyama,l Cyril Ponnamperuma. Laboratory of .... Oehler,:O.Z. and Smith, J.W. Precambrian Res, 2:221 (1977). (20). Schid1owski, M.
ORGANIC GEOCHEMISTRY OF THE ISUA SUPRACRUSTALS
Clifford Walters, Akira Shimoyama,l Cyril Ponnamperuma Laboratory of Chemical Evolution, Dept. of Chemistry University of Maryland, College Park, MD. 20742
The Isua supracrustals of western Greenland are the oldest terrestrial rocks known, dated at > 3,750 m.y. Metamorphosed to lower amphibolite facies, they still can provide indications of their original environment of deposition. Graphite is widely dispersed throughout the meta-sediments, particularly within the ironstones. Most of the graphite is very well ordered. However, several samples show a marked disorder of crystallinity. These samples, when pyrolyzed at high temperatures, liberate organic fragments of low molecular weight (m/e < 200). The fragments suggest that at least some of the Isua graphite is derived from condensation of kerogen. Carbon isotope data has been interpreted as indication of photosynthetic fractionation. Whether the original organics, which are now seen as graphite, were biologically produced has not yet been unequivocally established. The pyrolyzed organics detected within the metasediments may well be the oldest molecular fossils yet found on the Earth. The Isua ~upracrusta1 formation is located ~150 km northeast of Godthab ('Nnk) ,West Greenland and borders the inland ice. (Figute l)A semicircular arc of meta-volcanics and metasediments bll~rounded by quartzo-fe1dspathic Amitsoq gneiss, the formation is recognized as the oldest known terrestrial rocks. Radiometric dating of the Isua meta-sediments (1,2) and the 1present address:
Mining College, Akita University Akita 010, Japan 473
Y. Wolman fed.), Origin of Life, 473-479. Copyright © 1981 by D. Reidel Publishing Company.
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THE ISUA SUPRACRUSTAL FORMATION
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ORGANIC GEOCHEMISTRY OF THE ISUA SUPRACRUSTALS
475
Amitsoq gneisses yield (3,4) essentially identical ages, the earliest age determination being 3.824 ±:~~; b.y. by U-Pb analysis of single zircons from acid boulders in a conglomerate unit (5). The geology and major geochemical features of the Isua supracrustals have been described elsewhere (6,7). The manner in which the formation was developed is not known. Windley (8) has suggested that batholiths of active continental margins has similar lithographies. Others, however, suggest the various lithological units at Isua are unrelated and the present assemblage is a result of tectonic activity (9).
An immediate problem to any use of the Isua meta-sediments for studies concerning prebiotic or early biological evolution is the degree of metamorphism. Up to four periods of postdepositional recrystallization have effected the supracrustal rocks (10). This amphibolite facies regional metamorphism is well established, and earlier descriptions of greenschist facies mineralogy is a result of retrogressive metamorphism. It is therefore highly unlikely, if not impossible, that any morphological features of abiotic structures or biological organisms could survive. Reports of microfossils termed "Isuasphaera" (11,12) have been shown to be mineral stained fluid inclusions (13). Since micropaleontological studies, which have provided the best evidence for living organisms in younger Archean relatively unmetamorphosed sedimentary rocks- cannot be applied, only organic geochemical investigations remain. While the degree of metamorphism severely limits the amount of information which can be discerned, it is hoped that such geochemical studies can prove the existance of syngenetic organic matter within the Isua metasediments and whether this organic matter is abiotic or biological in origin. Present studies are now centered on the graphite found within the Isua metasediments. A survey of over fifty ironstone outcrops has shown graphite to be most pervasive. Abundances range from
