Ion microprobe analysis of graphite from ca. 3.8 Ga metasediments, Isua supracrustal belt, West Greenland: Relationship between metamorphism and carbon isotopic composition

摘要

In-situ ion microprobe measurements of carbon isotopic compositions of graphite were made in seven metasediments and two carbonate rocks from the ca. 3.8 Ga Isua supracrustal belt, West Greenland. The delta(13)C values of micron-scale graphite globules in the metasediments and the carbonate rocks vary from -18 to +2% and from -7 to -3%., respectively. The maximum delta(13)C value of graphite globules in the metasediment rises from -14 to -5parts per thousand, as the metamorphic grade increases from epidote-amphibolite to upper amphibolite facies. In a single hand specimen, the delta(13)C values of graphite inclusions in garnet are similar to7parts per thousand lower on average than those outside garnet. Similarly, graphite armored by quartz apparently shows a few permil lower delta(13)C values than those on grain boundaries between noncarbonate minerals. The fact that early crystallized minerals include relatively C-13-depleted graphite indicates that the regional metamorphism increased the delta(13)C values of the Isua graphite. This is consistent with the regional trend of C-13-enrichment accompanied by the increase of metamorphic grade. The minimum fractionation between graphite and carbonate is consistent with the equilibrium fractionation at about 400 to 550 degreesC. These observations indicate that isotopic exchange with isotopically heavy carbonate caused C-13-enrichment of Isua graphite. The delta(13)C values of graphite reported here (delta(13)C > -18parts per thousand) were produced either as a metamorphic modification of organic carbon with initially much lower delta(13)C Values, or as an abiological reaction such as decomposition of carbonate. If the isotopic exchange between carbonate and graphite during regional metamorphism controlled the 13 C-enrichment of Isua graphite, previously reported large C-13-depletion of graphite, especially armored by apatite (Mojzsis et al., 1996) was probably premetamorphic in origin. This supports the existence of life at Isua time (ca. 3.8 Ga). Copyright (C) 2002 Elsevier Science Ltd. [References: 30]