Zircon grains from the Jack Hills of Western Australia preserve crystallization ages dating back to 4.404 Ga. Previous researchers have proposed that these zircon grains contain primary ferromagnetic inclusions that record evidence of an active magnetic dynamo in the Earth at the time of their crystallization. Benjamin Weiss from MIT led an international team that reassessed these zircons and their results suggest that the inclusions are of a secondary origin, forming along cracks and on grain boundaries at a time subsequent to the initial crystallization of the Hadean zircons.

Link to article: https://pubs.geoscienceworld.org/gsa/geology/article/529025/secondary-magnetic-inclusions-in-detrital-zircons