Secondary magnetite in ancient zircon precludes analysis of a Hadean geodynamo
Name
407.full.pdf
Description
Published version
Size
1.54 MB
Format
Adobe PDF
Checksum (MD5)
257c5cdf17abefa1d199a1e11665cdc3
Author(s)
Borlina, Cauê S.
Weiss, Benjamin P.
Lima, Eduardo A.
Date Issued
December 31, 2018
Journal
Proceedings of the National Academy of Sciences of the United States of America
Publisher
Proceedings of the National Academy of Sciences
Citation
Tnag, Fengzai et al. "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 (2019): 407-412 © 2019 The Author(s)
Version
Final published version
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.
Subjects
Multidisciplinary
MIT Department
Massachusetts Institute of Technology. Department of Aeronautics and Astronautics
Massachusetts Institute of Technology. Department of Earth, Atmospheric, and Planetary Sciences
Terms of Use
Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use.
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DOI of Published Version
https://doi.org/10.1073/pnas.1811074116
