Petrography and geochemistry of apatites in banded iron formation, Akilia, W. Greenland: Consequences for oldest life evidence

摘要: Abstract The world's oldest sediments occur in high-grade terrains that were recrystallised during polyphase amphibolite to granulite metamorphism (>500 °C) accompanied by intense ductile deformation, which, open system behaviour can corrupt any biogenic carbon-isotopic signatures. A 13C-depleted signature from apatites the Akilia BIF (quartz + magnetite + pyroxene ± garnet), was reported as strong evidence for life. Specifically, this attributed reduced C inclusions (now graphite), regarded protected apatite crystals since deposition/diagenesis (Mojzsis, S.J., Arrhenius, G., McKeegan, K.D., Harrison, T.M., Nutman, A.P., Friend, C.R.L., 1996. Evidence life on Earth before 3,800 million years ago. Nature, 384, 55–59). Neither Lepland et al. (Lepland, A., van Zuilen, M.A., Whitehouse, M.J., Fedo, C.M., 2005. Questioning Earth's earliest life—Akilia revisited. Geology, 33, 77–79.) nor us have found these graphite inclusions. Instead, infrared absorbance spectroscopy shows carbon occurs carbonate substituting at phosphate and a lesser degree hydroxyl lattice sites. Furthermore, although Isua similar whole rock REE patterns, compared with patterns show depletion of heavy versus light REE. This be explained having equilibrated garnet, strongly suggesting are metamorphic origin. More detailed is required support previous proposal metasediments host via within pre-metamorphic history rock.