1. Department of Applied Geology, Western Australian School of Mines, Curtin University of Technology, Bentley, Western Australia 6102, Australia
2. Laboratory for Isotope Geology, Swedish Museum of Natural History, SE-104 05 Stockholm, Sweden
3. Institut für Mineralogie, Westfälische Wilhelms-Universität, Corrensstrasse 24, 48149 Münster, Germany

Correspondence to: Alexander A. Nemchin¹ Correspondence and requests for materials should be addressed to A.A.N. (Email: nemchina@kalg.curtin.edu.au).

Abstract

The recent discovery of diamond–graphite inclusions in the Earth's oldest zircon grains (formed up to 4,252 Myr ago) from the Jack Hills metasediments in Western Australia¹ provides a unique opportunity to investigate Earth's earliest known carbon reservoir. Here we report ion microprobe analyses of the carbon isotope composition of these diamond–graphite inclusions. The observed ¹³C_PDB values (expressed using the PeeDee Belemnite standard) range between -5 per mil and -58 per mil with a median of -31 per mil. This extends beyond typical mantle values of around -6 per mil to values observed in metamorphic and some eclogitic diamonds that are interpreted to reflect deep subduction of low-¹³C_PDB biogenic surface carbon. Low ¹³C_PDB values may also be produced by inorganic chemical reactions², and therefore are not unambiguous evidence for life on Earth as early as 4,250 Myr ago. Regardless, our results suggest that a low-¹³C_PDB reservoir may have existed on the early Earth.

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