Temporal constraints on Fe mobility in Jack Hills zircon

Abstract

Ancient detrital zircon grains containing magnetite inclusions have the potential to record the Earth’s magnetic field as far back as the Hadean. However, this requires magnetite to be either a primary inclusion or to be a secondary inclusion that forms shortly after zircon crystallization. Microstructural and TEM analysis of Jack Hills zircon show that magnetite, the magnetic recorder in these zircon crystals, is secondary in nature and is associated with the mobility of Fe. However, the timing of Fe mobility within Jack Hills zircon is poorly constrained. Here we undertake nanoscale characterization of highly magnetic zones of zircon, identified by quantum diamond microscopy (QDM), using atom probe tomography (APT). The APT data show the presence of Pb-bearing nanoclusters and these record isotopic compositions consistent with formation during two discrete thermal events at 3.4 Ga and < 2 Ga. The older population of clusters contain no detectable Fe. However, the younger population of clusters are Fe-bearing. This result shows that the Fe required to form secondary magnetite was not present in the zircon prior to 3.4 Ga and that remobilization of Pb and Fe, the latter associated with magnetite formation, took place after 2 Ga, during an annealing event that took place more than one billion years after deposition of the Jack Hills sediment at 3 Ga. This use of APT to date Fe mobility provides a novel approach to temporally constrain the formation of intragranular secondary magnetite inclusions in highly magnetic areas of zircon grains.