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.
Highlights
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
The extraction of robust Hadean paleomagnetic signals from zircon single crystals relies on the following assumptions about any given magnetic particle: (i) iron oxide grains became trapped as primary inclusions in igneous zircon; (ii) inclusions within the zircon acquired a primary thermoremanent magnetization (TRM) during postcrystallization cooling and have not been subsequently reheated above the Curie temperature; (iii) armoured magnetic inclusions remained chemically and thermally unaltered by preand postdepositional high-temperature metamorphic and lowtemperature aqueous alteration/recrystallization events; and (iv) the high-temperature component of primary TRM can be separated from overlapping sources of secondary magnetization
Thermal demagnetization of grain A demonstrates that 30–40% of its natural remanent magnetization (NRM) is retained after heating to 550 °C in zero field and that the NRM is fully demagnetized by 580 °C (SI Appendix, section C), confirming the presence of magnetite remanence carriers with high blocking temperatures
Summary
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. We focus primarily on two grains (A and B) that are >3.9 Ga and that passed strict initial selection criteria for potential paleomagnetic targets: lack of evidence for alteration from scanning electron microscopy (SEM) images, concordant U–Pb ages (SI Appendix, section E), and treatment with 6M HCl to remove Fe in cracks [14], and a stable natural remanent magnetization (NRM) component (SI Appendix, section C).
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