A light carbon reservoir recorded in zircon-hosted diamond from the Jack Hills

Nemchin AA, Whitehouse MJ, Menneken M, Geisler T, Pidgeon RT, Wilde SA

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 Australia(1) 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 delta(13) C-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-delta(13) C-PDB biogenic surface carbon. Low delta(13) C-PDB values may also be produced by inorganic chemical reactions(2), and therefore are not unambiguous evidence for life on Earth as early as 4,250 Myr ago. Regardless, our results suggest that a low-delta(13) C-PDB reservoir may have existed on the early Earth.