Archaean and Proterozoic diamond growth from contrasting styles of large-scale magmatism

Janne M Koornneef,Hielke A Jelsma,Michael U Gress,Ingrid L Chinn,Gareth R Davies,Jeff W Harris

doi:10.1038/s41467-017-00564-x

Janne M Koornneef, Hielke A Jelsma + Show 4 more

Open Access

https://doi.org/10.1038/s41467-017-00564-x

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Abstract

Precise dating of diamond growth is required to understand the interior workings of the early Earth and the deep carbon cycle. Here we report Sm-Nd isotope data from 26 individual garnet inclusions from 26 harzburgitic diamonds from Venetia, South Africa. Garnet inclusions and host diamonds comprise two compositional suites formed under markedly different conditions and define two isochrons, one Archaean (2.95 Ga) and one Proterozoic (1.15 Ga). The Archaean diamond suite formed from relatively cool fluid-dominated metasomatism during rifting of the southern shelf of the Zimbabwe Craton. The 1.8 billion years younger Proterozoic diamond suite formed by melt-dominated metasomatism related to the 1.1 Ga Umkondo Large Igneous Province. The results demonstrate that resolving the time of diamond growth events requires dating of individual inclusions, and that there was a major change in the magmatic processes responsible for harzburgitic diamond formation beneath Venetia from the Archaean to the Proterozoic.

Highlights

Precise dating of diamond growth is required to understand the interior workings of the early Earth and the deep carbon cycle
To provide constraints on how the tectono-magmatic conditions responsible for harzburgitic diamond formation may have evolved over time, and assess the possible temporal evolution of the Earth’s carbon cycle, we present coupled major and trace element and Sm-Nd isotope data for 26 individual garnet inclusions extracted from 26 individual peridotitic diamonds from Venetia, which were measured for their carbon isotope compositions
The data reported by Richardson et al.[13] on combined harzburgitic garnet inclusions (32, 33, 34, and 42 specimens in each group) show a significantly smaller range in 147Sm/144Nd and 143Nd/ 144Nd compared with the individual inclusions, demonstrating that the pooling masked the original heterogeneity

Summary

Introduction

Precise dating of diamond growth is required to understand the interior workings of the early Earth and the deep carbon cycle. The results demonstrate that resolving the time of diamond growth events requires dating of individual inclusions, and that there was a major change in the magmatic processes responsible for harzburgitic diamond formation beneath Venetia from the Archaean to the Proterozoic. Radiogenic isotope studies of inclusions in combination with compositional data can constrain the conditions and timing of diamond growth, and provide fundamental information about the tectono-magmatic processes that led to the formation and modification of the lithospheric keels that underlie the oldest parts of the Earth’s continents[1]. To provide constraints on how the tectono-magmatic conditions responsible for harzburgitic diamond formation may have evolved over time, and assess the possible temporal evolution of the Earth’s carbon cycle, we present coupled major and trace element and Sm-Nd isotope data for 26 individual garnet inclusions extracted from 26 individual peridotitic diamonds from Venetia, which were measured for their carbon isotope compositions. The data provide a tantalising glimpse of how processes associated with diamond formation may have changed over time

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