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 δ13CΣC 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.
All Science Journal Classification (ASJC) codes
- Geochemistry and Petrology