Non-biogenic graphite in 3.8-Ga metamorphic rocks from the Isua district, Greenland

A 3.8-Ga-old metamorphic rock that contains up to ∼ 5.5 wt% reduced carbon as graphite was found in the Isua district, Greenland. This rock exhibits a strong schistosity, comprises mostly talc, contains high MgO (28.0 wt%), Ni (553 ppm) and Cr (267 ppm) and low Al₂O₃ (2.13 wt%), and bears a high Fe₂O₃/FeO ratio (0.94). The original rock was probably a submarine ultramafic tuff. The carbon content and the δ¹³C value of the rock vary on a microscopic scale (a few mm³ scale) from 0.7 to 5.5 wt% and from -15 to -12‰, respectively. The second rock, a quartzite, contains 0.13 wt% C as reduced carbon with a δ¹³C value of -11.2‰. These δ¹³C values fall within the range -27 to -10‰ reported by previous investigators for graphite from the Isua district. The δ¹³C values of the Isua graphite have been interpreted by previous investigators (e.g., Schidlowski and Aharon, 1992) as metamorphic modification of organic matter with initial δ¹³C values of ∼ -30‰. An important implication of this model has been that the ∼ 3.8-Ga ocean already supported some biological activity. However, an examination of the carbon content vs. δ¹³C relationship of the graphite-bearing rocks from Isua has revealed that the δ¹³C value generally increases with increasing carbon contents: the rocks with δ¹³C values less than ∼ -20‰ contain extremely small amounts of carbon (< 0.01 wt%), while the rocks with carbon contents greater than ∼ 1 wt% have δ¹³C values around -12‰. Such relationships are not consistent with the previous model for the origin of graphite in Isua, but are consistent with a two-component mixing model in which a major component had δ¹³C values of ∼ -12‰ and a minor component ∼ -25‰. The minor component may be biogenic, but it is not certain whether it represents a remnant of Archean biological products or more recent ones. The mineralogical and elemental characteristics of the rocks of this study - a thermodynamic consideration of the temperature and composition of metamorphic fluids - and carbon isotope mass calculations suggest that the major component of graphite with δ¹³C values around -12‰ was formed by an inorganic, rather than by a biological process. The graphite appeared to have formed at temperatures between 700° and 400°C by one or both of the following two processes: (1) reactions between CO₂ and CH₄ (CO₂ + CH₄ → 2C + 2H₂O) during cooling of fluids with a CO₂/CH₄ molar ratio of ∼ 1; and/or (2) reactions between the FeO component in the ultramafic rock with CO₂-rich fluids (4"FeO" + CO₂ → 2"Fe₂O₃" + C). The δ¹³C_ΣC values for the fluids for both (1) and (2) were probably between -12 and -5‰, suggesting that the carbon-bearing fluids could have been derived from the mantle.