Widespread occurrences of variably crystalline C-13-depleted graphitic carbon in banded iron formations
| Autor: | Zhenbing She, Jonathan O'Neil, Franco Pirajno, Matthew S. Dodd, Dominic Papineau, Juha A. Karhu, C. Manikyamba, Hanika Rizo, Yusheng Wan |
|---|---|
| Přispěvatelé: | Department of Geosciences and Geography |
| Jazyk: | angličtina |
| Rok vydání: | 2019 |
| Předmět: |
1171 Geosciences
METASEDIMENTS 010504 meteorology & atmospheric sciences SUPRACRUSTAL ROCKS Geochemistry chemistry.chemical_element Greenalite 010502 geochemistry & geophysics 01 natural sciences Apatite chemistry.chemical_compound RAMAN Geochemistry and Petrology banded iron formation Earth and Planetary Sciences (miscellaneous) early life Graphite 0105 earth and related environmental sciences Mineral Akilia graphite ORIGIN FLUID-DEPOSITED GRAPHITE APATITE 15. Life on land AKILIA LIFE Geophysics chemistry 13. Climate action Space and Planetary Science carbon isotopes visual_art visual_art.visual_art_medium ISOTOPE Carbonate biosignatures Banded iron formation GREENSTONE-BELT Carbon Geology |
| Popis: | 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, C-13-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, C-13-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. (C) 2019 Elsevier B.V. All rights reserved. |
| Databáze: | OpenAIRE |
| Externí odkaz: |
|