The transition zone as a host for recycled volatiles: Evidence from nitrogen and carbon isotopes in ultra-deep diamonds from Monastery and Jagersfontein (South Africa)

Palot, M., Pearson, D.G., Stachel, T., Stern, R.A., Le Pioufle, A., Gurney, J.J. and Harris, J.W. (2017) The transition zone as a host for recycled volatiles: Evidence from nitrogen and carbon isotopes in ultra-deep diamonds from Monastery and Jagersfontein (South Africa). Chemical Geology, 466, pp. 733-749. (doi: 10.1016/j.chemgeo.2017.07.023)

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Abstract

Sublithospheric (ultra-deep) diamonds provide a unique window into the deepest parts of Earth's mantle, which otherwise remain inaccessible. Here, we report the first combined C- and N-isotopic data for diamonds from the Monastery and Jagersfontein kimberlites that sample the deep asthenosphere and transition zone beneath the Kaapvaal Craton, in the mid Cretaceous, to investigate the nature of mantle fluids at these depths and the constraints they provide on the deep volatile cycle. Both diamond suites exhibit very light δ13C values (down to − 26‰) and heavy δ15N (up to + 10.3‰), with nitrogen abundances generally below 70 at. ppm but varying up to very high concentrations (2520 at. ppm) in rare cases. Combined, these signatures are consistent with derivation from subducted crustal materials. Both suites exhibit variable nitrogen aggregation states from 25 to 100% B defects. Internal growth structures, revealed in cathodoluminescence (CL) images, vary from faintly layered, through distinct cores to concentric growth patterns with intermittent evidence for dissolution and regular octahedral growth layers in places. Modelling the internal co-variations in δ13C-δ15N-N revealed that diamonds grew from diverse C-H-O-N fluids involving both oxidised and reduced carbon species. The diversity of the modelled diamond-forming fluids highlights the complexity of the volatile sources and the likely heterogeneity of the deep asthenosphere and transition zone. We propose that the Monastery and Jagersfontein diamonds form in subducted slabs, where carbon is converted into either oxidised or reduced species during fluid-aided dissolution of subducted carbon before being re-precipitated as diamond. The common occurrence of recycled C and N isotopic signatures in super-deep diamonds world-wide indicates that a significant amount of carbon and nitrogen is recycled back to the deep asthenosphere and transition zone via subducting slabs, and that the transition zone may be dominated by recycled C and N.

Item Type:Articles
Status:Published
Refereed:Yes
Glasgow Author(s) Enlighten ID:Harris, Dr Jeff
Authors: Palot, M., Pearson, D.G., Stachel, T., Stern, R.A., Le Pioufle, A., Gurney, J.J., and Harris, J.W.
College/School:College of Science and Engineering > School of Geographical and Earth Sciences
Journal Name:Chemical Geology
Publisher:Elsevier
ISSN:0009-2541
ISSN (Online):1872-6836
Published Online:25 July 2017
Copyright Holders:Copyright © 2017 Elsevier B.V.
First Published:First published in Chemical Geology 466:733-749
Publisher Policy:Reproduced in accordance with the copyright policy of the publisher.

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