Sedimentary rocks in the supracrustal belt, including chert, BIF, carbonate rock and terrigenous turbidite, contain much amount of graphite grains. However, their origins, namely biological or abiological, are still highly controversial. In order to obtain evidence for the biological origin, in-situ ion microprobe measurements of carbon isotopic compositions of graphite (Fig. 23) were made in seven metasediments and two carbonate rocks from the ca. 3.8 Ga Isua supracrustal belt, West Greenland (Fig. 24; Ueno et al., 2002). The δ¹³C 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 δ¹³C value of graphite globules in the metasediments rises from -14 to -5‰, as the metamorphic grade increases from epidote-amphibolite to upper amphibolite facies. In a single hand specimen, the δ¹³C values of graphite inclusions in garnet are 7‰ lower on average than those outside garnet. Similarly, graphite armored by quartz apparently shows a few permil lower δ¹³C values than those on grain boundaries between noncarbonate minerals. The fact that early crystallized minerals include relatively δ¹³C-depleted graphite indicates that the regional metamorphism increased the δ¹³C values of the Isua graphite. This is consistent with the regional trend of δ¹³C-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 °C. These observations indicate that isotopic exchange with isotopically heavy carbonate caused δ¹³C-enrichment of Isua graphite. The δ¹³C values of graphite reported here (δ¹³C = -18‰) were produced either as a metamorphic modification of organic carbon with initially much lower δ¹³C values, or as an abiological reaction such as decomposition of carbonate. If the isotopic exchange between carbonate and graphite during regional metamorphism controlled the δ¹³C-enrichment of Isua graphite, previously reported large δ¹³C-depletion of graphite, especially armored by apatite (Mojzsis et al., 1996) was probably pre-metamorphic in origin. This supports the existence of life at Isua time (ca. 3.8 Ga). Furthermore, recent experiment has revealed that magnetite in the quartz-magnetite BIF of uncontested sedimentary origin from northeastern ISB includes highly ¹³C-depleted kerogenous material (Nishizawa et al., 2005). Carbon and nitrogen isotopes (δ¹³C = -30‰, δ¹⁵N = -3‰) and C/N elemental ratio (86) are within the range of those of kerogen in Archean metasediments. Since the magnetite is concordant with bedding surface, it is plausible that the kerogenous material would have been incorporated into the magnetite during diagenesis. This is also consistent with the existence of life at Isua time.

Figure 23:Graphite grains within a chert in the Isua supracrustal belt, southern West Greenland.

Figure 24:Isotopic fractionations between carbonate and minute graphite globule (Δ_{carb-graphite}=δ¹³C_carb-δ₁₃C^graphite). Horizontal lines indicate the equilibrium fractionations at 300, 400, 500, and 600 ℃. “A” to “D” show the metamorphic grade (Hayashi et al., 2000). Arrows indicate hypothetical trend of carbon isotopic modification during prograde metamorphism