bweiss91125@yahoo.com — Benjamin P. Weiss

THE JACK HILLS

View of the granitoid rocks in the Yilgarn craton, looking southwest from the summit of Mount Hale, Jack Hills, Western Australia.

JACK HILLS ZIRCON MAGNETISM

Map of the vertical component of the magnetic field of the natural remanent magnetization field 0.2 mm above Jack Hills zircons (circled) using the SQUID Microscope at MIT. The zircons have moments ranging from just 6.8e-14 to 4.4e-12 Am2.

SECONDARY MAGNETIC MINERALS IN JACK HILLS ZIRCONS

Left: magnetic field above two zircons from quantum diamond microscopy. Middle: textural and compositional map of zircons using backscattered scanning electron microscopy. Right: Map of iron abundance using wavelength-dispersive spectroscopy.

It currently is unknown when Earth’s dynamo magnetic field originated. The Isua terrane in Greenland hosts some of the oldest, well preserved rocks while detrital zircon crystals in the Jack Hills of Western Australia are the oldest known materials on Earth. We showe that the Isua rocks may contain the oldest known record of Earth’s magnetic field, dating back to 3.7 billion years ago (Nichols et al. 2024). By comparison. the zircons' host rocks have been pervasively remagnetized (Weiss et al. 2015, 2016) and the zircons themselves are contaminated with secondary magnetic minerals younger than 3 billion years old (Glenn et al. 2017, Weiss et al. 2018, Tang et al. 2019, Borlina et a. 2020, Taylor et al. 2023).