Diamonds reveal subducted slab harzburgite in the lower mantle

Meyer, N. A., Stachel, T., Pearson, D. G., Stern, R. A., Harris, J. W. and Walter, M. J. (2023) Diamonds reveal subducted slab harzburgite in the lower mantle. Geology, 51(3), pp. 238-241. (doi: 10.1130/g50675.1)

Abstract

Characterizing compositional heterogeneity in Earth’s lower mantle is critical to understanding its dynamics. Three low-nitrogen diamonds from Koffiefontein (South Africa), containing inclusion assemblages of ferropericlase ± orthopyroxene ± magnesite, constrain diamond formation in an Mg-rich lower-mantle environment. Ferropericlase inclusions have Mg# 82.7–88.5 and orthopyroxene inclusions (retrogressed bridgmanite) have Mg# 95.0–95.1 and mantle-like δ18O of +5.6‰ ± 0.2‰. Magnesite included in one diamond implicates carbonated fluids in diamond formation. High Mg# and low Ca, Al, and Na of the assemblage indicate a melt-depleted meta-harzburgitic environment, in contrast to more fertile compositions expected for primitive lower mantle. Extremely low Ca in orthopyroxene inclusions may reflect a combination of melt depletion and low equilibration temperatures at the time of trapping. Inclusion compositions implicate subducted oceanic slab meta-harzburgite as the host for diamond growth. Mantle-like δ18O of the orthopyroxene inclusions indicates unaltered oceanic lithosphere. Similar melt-depleted characteristics in lower-mantle inclusion assemblages worldwide support that meta-harzburgite is the dominant host of lower-mantle diamonds.

• Item Type: Articles
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Harris, Dr Jeff
• Authors: Meyer, N. A., Stachel, T., Pearson, D. G., Stern, R. A., Harris, J. W., and Walter, M. J.
• College/School: College of Science and Engineering > School of Geographical and Earth Sciences
• Journal Name: Geology
• Publisher: Geological Society of America
• ISSN: 0091-7613
• ISSN (Online): 1943-2682
• Published Online: 20 January 2023
• Copyright Holders: Copyright © 2023 The Authors
• First Published: First published in Geology 51(3): 238-241
• Publisher Policy: Reproduced under a Creative Commons license