Petrography and geochemistry of apatites in banded iron formation, Akilia, W. Greenland: Consquences for oldest life evidence
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, during which, open system behaviour can corrupt any biogenic carbon-isotopic signatures. A 13C-depleted signature from apatites in the Akilia BIF (quartz + magnetite + pyroxene ± garnet), was reported as strong evidence for oldest life. Specifically, this signature was attributed to reduced C inclusions (now graphite), regarded as protected in the 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 for life on Earth before 3,800 million years ago. Nature, 384, 55-59). Neither Lepland et al. (Lepland, A., van Zuilen, M.A., Arrhenius, G., Whitehouse, M.J., Fedo, C.M., 2005. Questioning the evidence for Earth's earliest life-Akilia revisited. Geology, 33, 77-79.) nor us have found these graphite inclusions. Instead, infrared absorbance spectroscopy shows that carbon occurs in Akilia apatites as carbonate substituting at phosphate and to a lesser degree hydroxyl lattice sites. Furthermore, although Isua and Akilia BIF have similar whole rock REE patterns, compared with Isua BIF apatites the REE patterns for Akilia apatites show depletion of the heavy versus light REE. This can be explained by the Akilia apatites having equilibrated with garnet, strongly suggesting that the Akilia apatites are metamorphic in origin. More detailed evidence is required to support the previous proposal that Akilia metasediments host the evidence for Earth's oldest life via 13C-depleted graphite inclusions protected within apatite since the pre-metamorphic history of the rock. © 2006 Elsevier B.V. All rights reserved.
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