Secondary magnetite in ancient zircon precludes analysis of a Hadean geodynamo

Tang, F. et al. (2019) Secondary magnetite in ancient zircon precludes analysis of a Hadean geodynamo. Proceedings of the National Academy of Sciences of the United States of America, 116(2), pp. 407-412. (doi: 10.1073/pnas.1811074116) (PMID:30598434) (PMCID:PMC6329971)

Abstract

Zircon crystals from the Jack Hills, Western Australia, are one of the few surviving mineralogical records of Earth’s first 500 million years and have been proposed to contain a paleomagnetic record of the Hadean geodynamo. A prerequisite for the preservation of Hadean magnetization is the presence of primary magnetic inclusions within pristine igneous zircon. To date no images of the magnetic recorders within ancient zircon have been presented. Here we use high-resolution transmission electron microscopy to demonstrate that all observed inclusions are secondary features formed via two distinct mechanisms. Magnetite is produced via a pipe-diffusion mechanism whereby iron diffuses into radiation-damaged zircon along the cores of dislocations and is precipitated inside nanopores and also during low-temperature recrystallization of radiation-damaged zircon in the presence of an aqueous fluid. Although these magnetites can be recognized as secondary using transmission electron microscopy, they otherwise occur in regions that are indistinguishable from pristine igneous zircon and carry remanent magnetization that postdates the crystallization age by at least several hundred million years. Without microscopic evidence ruling out secondary magnetite, the paleomagnetic case for a Hadean–Eoarchean geodynamo cannot yet been made.

• Item Type: Articles
• Additional Information: The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (Grant FP/2007-2013)/European Research Council Grant Agreement 320750, Natural Environment Research Council Grant NE/P002498/1, National Science Foundation (NSF) Grant EAR1647504, and Thomas F. Peterson, Jr. The University of California, Los Angeles ion microprobe facility is partly supported by a grant from the Instrumentation and Facilities Program, Division of Earth Sciences, NSF (1339051).
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Einsle, Dr Joshua Franz
• Authors: Tang, F., Taylor, R. J. M., Einsle, J. F., Borlina, C. S., Fu, R. R., Weiss, B. P., Williams, H. M., Williams, W., Nagy, L., Midgley, P. A., Lima, E. A., Bell, E. A., Harrison, T. M., Alexander, E. W., and Harrison, R. J.
• College/School: College of Science and Engineering > School of Geographical and Earth Sciences > 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
• Published Online: 31 December 2018
• Copyright Holders: Copyright © 2019 National Academy of Sciences
• First Published: First published in Proceedings of the National Academy of Sciences of the United States of America 116(2):407-412
• Publisher Policy: Reproduced in accordance with the copyright policy of the publisher