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Precambrian Research 126 (2003) 331348 Graphite and carbonates in the 3.8 Ga old Isua
 

Summary: Precambrian Research 126 (2003) 331348
Graphite and carbonates in the 3.8 Ga old Isua
Supracrustal Belt, southern West Greenland
Mark A. van Zuilena,, Aivo Leplandb, Jane Teranesa, John Finarellic,
Martin Wahlena, Gustaf Arrheniusa
a Scripps Institution of Oceanography, University of California San Diego, La Jolla, San Diego, CA 92093-0236, USA
b Geological Survey of Norway, Leiv Eirikssons vei 39, Trondheim 7491, Norway
c Committee on Evolutionary Biology, University of Chicago, 1025 E. 57th Street Culver Hall 402, Chicago, IL 60637, USA
Accepted 5 February 2003
Abstract
We present a systematic study of abundance, isotopic composition and petrographic associations of graphite in rocks from the
ca. 3.8 Ga Isua Supracrustal Belt (ISB) in southern West Greenland. Most of the graphite in the ISB occurs in carbonate-rich
metasomatic rocks (metacarbonates) while sedimentary units, including banded iron formations (BIFs) and metacherts, have
exceedingly low graphite concentrations. Regardless of isotopic composition of graphite in metacarbonate rocks, their secondary
origin disqualifies them from providing evidence for traces of life stemming from 3.8 Ga. Recognition of the secondary origin of
Isua metacarbonates thus calls for reevaluation of earlier interpretations that suggested the occurrence of 3.8 Ga biogenic graphite
in these rocks. Thermal decomposition of siderite; 6FeCO3 = 2Fe3O4 + 5CO2 + C, is the process seemingly responsible for
the graphite formation. The cation composition (Fe, Mg, Mn, and Ca) of the carbonate minerals, carbon isotope ratios of car-
bonates and associated graphite and petrographic assemblages of a suite of metacarbonates support the conclusion that multiple
pulses of metasomatism affected the ISB, causing the deposition of Fe-bearing carbonates and subsequent partial disproportion-

  

Source: Arrhenius, Gustaf - Integrative Oceanography Division, Scripps Institution of Oceanography

 

Collections: Environmental Sciences and Ecology; Geosciences; Biology and Medicine