Carbon isotopes and petrography of kerogens in ∼3.5-Ga hydrothermal silica dikes in the North Pole area, Western Australia

More than 600 specimens of similar to3.5 Ga-old hydrothermal silica dikes from the North Pole area, Pilbara craton, Western Australia, have been studied petrographically. The kerogens in 44 samples have been analyzed isotopically (C and N) and chemically (C, N, and H). The silica dikes are composed mainly of fine-grained silica (modal abundance: >97%) and are classified into two types by minor mineral assemblages: B(black)-type and G(gray)-type. The B-type silica dikes contain kerogen (0.37 to 6.72 mgC/g; average 2.44 mgC/g, n = 21) and disseminated sulfides, dominantly pyrite and Fe-poor sphalerite. In some cases, carbonate and apatite are also present. Their silica-dominated and sulfide-poor mineral assemblages suggest precipitation from low-temperature reducing hydrothermal fluid (likely 100-200degreesC). On the other hand, the G-type silica dikes are sulfide-free and concentrations of kerogen are relatively low (0.05 to 0.41 mgC/g; average 0.17 mgC/g, n = 13). They typically contain Fe-oxide (mainly hematite) which commonly replaces cubic pyrite and rhombic carbonate. Some G-types occur along secondary quartz veins. These textures indicate that the G-type silica dikes were formed by postdepositional metasomatism (oxidation) of the B-types, and that the B-types probably possess premetasomatic signatures. The delta(13)C values of kerogen in the B-types are -38.1 to -33.1parts per thousand (average -35.9parts per thousand, n = 21), which are -4parts per thousand lower than those of the G-types (-34.5 to -30.0parts per thousand; average -32.2parts per thousand, n = 19), and -6parts per thousand lower than bedded chert (-31.2 to -29.4parts per thousand; average -30.5parts per thousand, n = 4). This indicates the preferential loss Of 12 C during the metasomatism (estimated fractionation factor: 0.9985). Considering the metasomatic effect on carbon isotopes with probably minor diagenetic and metamorphic overprints, we conclude that the original delta(13)C Values of the kerogen in the silica dikes would have been heterogeneous (similar to5parts per thousand) and at least some material had initial delta(13)C values of < -38parts per thousand. The inferred 13 C-depletions of organic carbon could have been produced by anaerobic chemoautotrophs such as methanogen, but not by aerobic photoautotrophs. This is consistent with the estimated physical and chemical condition of the hydrothermal fluid, which was probably habitable for anaerobic and thermophilic/hyperthermophilic chemoautotrophs. Alternatively, the organic matter may have been possibly produced by abiological reaction such as Fischer-Tropsch Type (FTT) synthesis under the hydrothermal condition. However, the estimated condition is inconsistent with the presence of the effective catalysts for the FTT reaction (i.e., Fe-Ni alloy, magnetite, and hematite). These lines of evidence suggest the possible existence of biosphere in the similar to3.5 Ga sub-seafloor hydrothermal system. Copyright (C) 2004 Elsevier Ltd.