Secondary magnetite in ancient zircon precludes analysis of a Hadean geodynamo

Tang, Fengzai and Taylor, Richard J. M. and Einsle, Joshua F. and Borlina, Cauê S. and Fu, Roger R. and Weiss, Benjamin P. and Williams, Helen M. and Williams, Wyn and Nagy, Lesleis and Midgley, Paul A. and Lima, Eduardo A. and Bell, Elizabeth A. and Harrison, T. Mark and Alexander, Ellen W. and Harrison, Richard J. (2018) Secondary magnetite in ancient zircon precludes analysis of a Hadean geodynamo. Proceedings of the National Academy of Sciences, 116 (2). pp. 407-412. ISSN 0027-8424 DOI https://doi.org/10.1073/pnas.1811074116

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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:	Article
Uncontrolled Keywords:	2018AREP; IA74
Subjects:	03 - Mineral Sciences
Divisions:	01 - Climate Change and Earth-Ocean Atmosphere Systems 08 - Green Open Access
Journal or Publication Title:	Proceedings of the National Academy of Sciences
Volume:	116
Page Range:	pp. 407-412
Identification Number:	https://doi.org/10.1073/pnas.1811074116
Depositing User:	Sarah Humbert
Date Deposited:	21 Feb 2019 18:00
Last Modified:	21 Feb 2019 18:00
URI:	http://eprints.esc.cam.ac.uk/id/eprint/4428

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