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Earliest microbial trace fossils in Archaean pillow lavas under scrutiny: new micro-X-ray absorption near-edge spectroscopy, metamorphic and morphological constraints

By
Eugene G. Grosch
Eugene G. Grosch
Department of Geology, Rhodes University, Grahamstown, South Africa
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Manuel Muñoz
Manuel Muñoz
Université Grenoble Alpes, ISTerre, F-38041 Grenoble, France
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Olivier Mathon
Olivier Mathon
European Synchrotron Radiation Facility, F-38043 Grenoble, France
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Nicola McLoughlin
Nicola McLoughlin
Department of Geology, Rhodes University, Grahamstown, South AfricaAlbany Museum, Somerset Street, Grahamstown, South Africa
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Published:
January 01, 2017

Abstract

Filament-shaped titanite microtextures in early Archaean (c. 3.47 Ga) metabasalts from the Barberton Greenstone Belt of South Africa have been argued to represent Earth’s oldest microbial trace fossils, but petrological data on the environmental conditions that led to the formation of these microtextures are sparse. We present here new metamorphic constraints on the equilibrium mineral assemblage containing the titanite microtextures, together with Fe speciation redox variations in the chlorite matrix surrounding the microtextures and morphospace analysis of the microtextures to test their biogenicity. Thermodynamic phase diagram modelling using a calculated rock microdomain composition indicated that the titanite mineral assemblage is stable at temperature conditions of T=240–360°C. The high-resolution quantitative mapping results combined with micro-X-ray absorption near-edge spectroscopy at the Fe K-edge in chlorite revealed that the titanite microtextures are located in low-temperature, high XFe3+ chlorite bands, veins and microdomains, supporting an origin as abiotic, retrograde mineral cooling textures. These new data, combined with a late c. 2.9 Ga U–Pb date for the titanite, are incompatible with the existing biological model, which invokes microbial micro-tunnel formation followed by titanite ‘infilling’ and preservation in the early Archaean sub-seafloor. The continuum of titanite morphologies reported herein indicates that titanite morphology cannot be used as reliable evidence in support of a biogenic origin in these metavolcanic rocks. It is proposed that the titanite microtextures of purported biogenic origin from other greenstone belts, such as the Pilbara Craton of Western Australia, also deserve to be scrutinized by high-resolution petrological investigations.

Supplementary material: Representative electron microprobe analyses of retrograde minerals and µXANES spectra and corresponding fitted pre-edge peaks for all spot analyses are available at https://doi.org/10.6084/m9.figshare.c.3498465

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Contents

Geological Society

Earth System Evolution and Early Life: A Celebration of the Work of Martin Brasier

A. T. Brasier
A. T. Brasier
University of Aberdeen, UK
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D. Mcilroy
D. Mcilroy
Memorial University of Newfoundland, Canada
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N. McLoughlin
N. McLoughlin
Rhodes University, South Africa
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Geological Society of London
Volume
448
ISBN electronic:
9781786202932
Publication date:
January 01, 2017

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