Graphite and carbonates in the 3.8 Ga old Isua Supracrustal Belt, southern West Greenland

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 carbonates 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 disproportionation to graphite and magnetite. Equilibrium isotope fractionation between carbonate and graphite in the rocks indicates peak metamorphic temperatures between 500 and 600 ◦ C, in agreement with other estimates of metamorphic temperature for the ISB.