Evidence for complex iron oxides in the deep mantle from FeNi(Cu) inclusions in superdeep diamond

Anzolini, C., Marquardt, K., Stagno, V., Bindi, L., Frost, D. J., Pearson, D. G., Harris, J. W., Hemley, R. J. and Nestola, F. (2020) Evidence for complex iron oxides in the deep mantle from FeNi(Cu) inclusions in superdeep diamond. Proceedings of the National Academy of Sciences of the United States of America, 117(35), pp. 21088-21094. (doi: 10.1073/pnas.2004269117) (PMID:32817475)

Abstract

The recent discovery in high-pressure experiments of compounds stable to 24–26 GPa with Fe4O5, Fe5O6, Fe7O9, and Fe9O11 stoichiometry has raised questions about their existence within the Earth’s mantle. Incorporating both ferric and ferrous iron in their structures, these oxides if present within the Earth could also provide insight into diamond-forming processes at depth in the planet. Here we report the discovery of metallic particles, dominantly of FeNi (Fe0.71Ni0.24Cu0.05), in close spatial relation with nearly pure magnetite grains from a so-called superdeep diamond from the Earth’s mantle. The microstructural relation of magnetite within a ferropericlase (Mg0.60Fe0.40)O matrix suggests exsolution of the former. Taking into account the bulk chemistry reconstructed from the FeNi(Cu) alloy, we propose that it formed by decomposition of a complex metal M oxide (M4O5) with a stoichiometry of (Fe3+2.15Fe2+1.59Ni2+0.17Cu+0.04)Σ = 3.95O5. We further suggest a possible link between this phase and variably oxidized ferropericlase that is commonly trapped in superdeep diamond. The observation of FeNi(Cu) metal in relation to magnetite exsolved from ferropericlase is interpreted as arising from a multistage process that starts from diamond encapsulation of ferropericlase followed by decompression and cooling under oxidized conditions, leading to the formation of complex oxides such as Fe4O5 that subsequently decompose at shallower P-T conditions.

• Item Type: Articles
• Additional Information: This investigation was financially supported by the project INDIMEDEA (“INclusions in DIamonds: MEssengers from the Deep EArth”), funded by a European Research Council Starting Grant 2012 to F.N. (Grant 307322). C.A. acknowledges financial support through a Canada Excellence Research Chair grant to D.G.P. and the Diamond Exploration and Research Training School. K.M. acknowledges financial support through Grants MA6287/3 and MA6287/6 of the German Science Foundation. V.S. acknowledges financial support through “Fondi di Ateneo 2017 and 2018.” R.J.H. acknowledges support of the A.F. Sloan Foundation through the Deep Carbon Observatory. FEI Scios FIB machine, BGI Bayreuth, is supported through grant INST 91/315-1 FUGG
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Harris, Dr Jeff
• Authors: Anzolini, C., Marquardt, K., Stagno, V., Bindi, L., Frost, D. J., Pearson, D. G., Harris, J. W., Hemley, R. J., and Nestola, F.
• College/School: College of Science and Engineering > School of Geographical and Earth Sciences
• Journal Name: Proceedings of the National Academy of Sciences of the United States of America
• Publisher: National Academy of Sciences
• ISSN: 0027-8424
• ISSN (Online): 1091-6490
• Copyright Holders: Copyright © 2020 National Academy of Sciences
• First Published: First published in Proceedings of the National Academy of Sciences of the United States of America 117(35):21088-21094
• Publisher Policy: Reproduced in accordance with the copyright policy of the publisher
• Data DOI: 10.25430/researchdata.cab.unipd.it. 00000358