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Widespread occurrences of variably crystalline 13C-depleted graphitic carbon in banded iron formations

Dodd, MS; Papineau, D; She, ZB; Manikyamba, C; Wan, YS; O'Neil, J; Karhu, JA; ... Pirajno, F; + view all (2019) Widespread occurrences of variably crystalline 13C-depleted graphitic carbon in banded iron formations. Earth and Planetary Science Letters , 512 pp. 163-174. 10.1016/j.epsl.2019.01.054. Green open access

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Abstract

Almost all evidence for the oldest traces of life on Earth rely on particles of graphitic carbon preserved in rocks of sedimentary protolith. Yet, the source of carbon in such ancient graphite is debated, as it could possibly be non-biological and/or non-indigenous in origin. Here we describe the co-occurrence of poorly crystalline and crystalline varieties of graphitic carbon with apatite in ten different and variably metamorphosed banded iron formations (BIF) ranging in age from 1,800 to >3,800 Myr. In Neoarchean to Palaeoproterozoic BIF subjected to low-grade metamorphism, 13C-depleted graphitic carbon occurs as inclusions in apatite, and carbonate and arguably represents the remineralisation of syngenetic biomass. In BIF subjected to high-grade metamorphism, 13C-depleted graphite co-occurs with poorly crystalline graphite (PCG), as well as apatite, carbonate, pyrite, amphibole and greenalite. Retrograde minerals such as greenalite, and veins cross-cutting magnetite layers contain PCG. Crystalline graphite can occur with apatite and orthopyroxene, and sometimes it has PCG coatings. Crystalline graphite is interpreted to represent the metamorphosed product of syngenetic organic carbon deposited in BIF, while poorly crystalline graphite was precipitated from C–O–H fluids partially sourced from the syngenetic carbon, along with fluid-deposited apatite and carbonate. The isotopic signature of the graphitic carbon and the distribution of fluid-deposited graphite in highly metamorphosed BIF is consistent with carbon in the fluids being derived from the thermal cracking of syngenetic biomass deposited in BIF, but, extraneous sources of carbon cannot be ruled out as a source for PCG. The results here show that apatite + graphite is a common mineral assemblage in metamorphosed BIF. The mode of formation of this assemblage is, however, variable, which has important implications for the timing of life's emergence on Earth.

Type: Article
Title: Widespread occurrences of variably crystalline 13C-depleted graphitic carbon in banded iron formations
Open access status: An open access version is available from UCL Discovery
DOI: 10.1016/j.epsl.2019.01.054
Publisher version: https://doi.org/10.1016/j.epsl.2019.01.054
Language: English
Additional information: This version is the author accepted manuscript. For information on re-use, please refer to the publisher’s terms and conditions.
Keywords: early life; banded iron formation; carbon isotopes; Raman; graphite; biosignatures
UCL classification: UCL
UCL > Provost and Vice Provost Offices
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Earth Sciences
URI: https://discovery.ucl.ac.uk/id/eprint/10070025
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