Carbonaceous Matter in the Black Cherts from the Dixon Island Formation, Western Pilbara, Australia (3.2 Ga)

The Dixon Island Formation is one of the best locations that preserve an Archean sea-floor hydrothermal system. The black chert vein is a characteristic for the Archean sea-floor hydrothermal system, and it contains three types of carbonaceous particles that may be derived from Archean microbes (Kiyokawa et al., 2001). In this investigation, we tried to characterize the carbonaceous matter in the black cherts. After washing the samples with acetone, they were powdered and extracted sonifically with chloroform : methanol = 1 : 2. The extracts were analyzed by GC-MS, as TMS derivatives prepared with BSTFA. The residues were treated with HF/HCl (10M-1M) to remove silicates. After centrifuging and freeze-drying, black powders were obtained. One sample of them was saponified with 0.5 M KOH-MeOH and extracted with ether (basic-neutral fraction). The water layer was acidified (pH2.0) and extracted with ether in the same manner (acidic fraction). A portion of the black powders were subjected to elemental analyses. Gas chromatograms of the chloroform : methanol -extracts show various organic compounds including straight-chain hydrocarbons, however it remains uncertain whether they are indigenous or not. Compounds were rarely found in the ether fractions from saponified sample, suggesting that the carbonaceous macromolecular matter in the black chert has already lost ester bonds and that now it is on the way to graphite. This is consistent with the following results of the elemental analyses. H/C ratios of the residues after HF/HCl treatment range from 0.40 to 0.52, indicating the early metagenesis stage of which temperature is around 175 - 200 ^oC for oil source rocks. However, because the black cherts are much older than typical oil source rocks, the same maturity may be reached at much lower temperature.