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S U M M A R Y Zircons of the Jack Hills of Western Australia are the oldest known terrestrial minerals and as such they hold potential for recording Earth’s oldest geomagnetic field. To preserve records of the most ancient magnetic field, the zircon host rocks must not have been heated to temperatures that resulted in a complete remagnetization. To test this hypothesis, magnetic minerals having very high unblocking temperatures must be present in the host rocks and capable of retaining magnetizations on billion-year timescales. Here, we use scanning electron microscopy analyses and transmission electron microscopy characterization of focused ion beam lift outs to document for the first time through direct imaging the presence of pseudosingle (PSD) to single-domain (SD) magnetite inclusions in Jack Hills quartzite cobbles. We further use focused ion milling to document the 3-D detrital morphology of an inclusion and micromagnetic modelling that suggest a single vortex magnetization. These results, together with recent advances in our understanding of the blocking temperatures and relaxation times of PSD grains, indicate that the Jack Hills host rocks contain magnetic inclusions capable of recording magnetizations as old as the age of the conglomerate (ca. 3 billion years old). These new results confirm that magnetic directions observed retained at high unblocking temperatures (500–580 ◦C) in these rocks identified in two independent laboratory analyses are carried by PSD/SD magnetite. The lack of an overprint direction recorded at these high temperatures excludes pervasive thermal and/or chemical remagnetization and indicates that the oldest zircons from the Jack Hills are potential recorders of the Hadean geodynamo.

geophysical journal international

Primary pseudo-single and single-domain magnetite inclusions in quartzite cobbles of the Jack Hills (Western Australia): implications for the Hadean geodynamo