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Secondary magnetite in ancient zircon precludes analysis of a Hadean geodynamo.

Accepted version
Peer-reviewed

Type

Article

Change log

Authors

Tang, Fengzai 
Borlina, Cauê S 
Fu, Roger R 

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.

Description

Keywords

Hadean, Jack Hills, correlative microscopy, paleomagnetism, zircon

Journal Title

Proc Natl Acad Sci U S A

Conference Name

Journal ISSN

0027-8424
1091-6490

Volume Title

116

Publisher

Proceedings of the National Academy of Sciences
Sponsorship
European Research Council (320750)
Natural Environment Research Council (NE/P002498/1)
Natural Environment Research Council (NE/M011801/2)
Natural Environment Research Council (NE/P020860/1)
NERC ERC NSF