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The initiation of mobile-lid plate tectonics on Earth represented a critical transition towards a more familiar world in terms of surface temperature stabilization, biogeochemical cycling, topography creation, and other processes. Zircon-based estimates of the geomagnetic field intensity have recently been cited as providing evi- dence for the lack of mobile-lid motion between 3.9 and 3.4 billion years ago (Ga). We reanalyze the published dataset of 91 zircon paleointensities from the Jack Hills (Australia) and Green Sandstone Bed (GSB; South Africa) localities within this time interval and, using both analytical and bootstrap resampling approaches, show that the small number of samples result in large uncertainties in implied paleolatitude. Specifically, in more likely sce- narios that do not assume coherent motion for both localities, all latitudinal displacements on Earth are permitted within the 95 % confidence interval. We also examine the less likely scenario that the two landmasses shared a motion history, which increases the data density and presents the best-case scenario for constraining latitudinal motion. In this case, the 95 % confidence interval of the zircon paleointensity data is compatible with the displacements of between 35 % and 52 % of modern continental localities, all of which experience mobile-lid tectonics. Finally, generating expected paleointensity time series for modern continents undergoing mobile-lid motion shows that about two-thirds of these motions would not be resolved by zircon paleointensities, even in the best-case scenario of combining Jack Hills and GSB datasets. All of these analyses assume that these zircons retain a primary paleomagnetic signal, an assertion which is opposed by a number of published zircon magnetism studies. We conclude that Archean zircon paleointensities do not provide evidence for or against mobile-lid plate tectonics prior to 3.4 Ga. Future paleomagnetic investigation of tectonic regime on the early Earth should therefore focus on magnetization directions in well-preserved, oriented whole rocks. 

Abstract Speleothems are mineral deposits capable of recording detrital and/or chemical remanent magnetization at annual timescales. They can offer high‐resolution paleomagnetic records of short‐term variations in Earth's magnetic field, crucial for understanding the evolution of the dynamo. Owing to limitations on the magnetic moment sensitivity of commercial cryogenic rock magnetometers (∼10⁻¹¹ Am²), paleomagnetic studies of speleothems have been limited to samples with volumes of several hundreds of mm³, averaging tens to hundreds of years of magnetic variation. Nonetheless, smaller samples (∼1–10 mm³) can be measured using superconducting quantum interference device (SQUID) microscopy, with a sensitivity better than ∼10⁻¹⁵ Am². To determine the application of SQUID microscopy for obtaining robust high‐resolution records from small‐volume speleothem samples, we analyzed three different stalagmites collected from Lapa dos Morcegos Cave (Portugal), Pau d'Alho Cave (Brazil), and Crevice Cave (United States). These stalagmites are representative of a range of magnetic properties and have been previously studied with conventional rock magnetometers. We show that by using SQUID microscopy we can achieve a five‐fold improvement in temporal resolution for samples with higher abundances of magnetic carriers (e.g., Pau d'Alho Cave and Lapa dos Morcegos Cave). In contrast, speleothems with low abundances of magnetic carriers (e.g., Crevice Cave) do not benefit from higher resolution analysis and are best analyzed using conventional rock magnetometers. Overall, by targeting speleothem samples with high concentrations of magnetic carriers we can increase the temporal resolution of magnetic records, setting the stage for resolving geomagnetic variations at short time scales. 

A potential record of Earth’s magnetic field going back 4.2 billion years (Ga) ago is carried by magnetite inclusions in zircon grains from the Jack Hills. This magnetite may be secondary in nature, however, meaning that the magnetic record is much younger than the zircon crystallization age. Here, we use atom probe tomography to show that Pb-bearing nanoclusters in magnetite-bearing Jack Hills zircons formed during two discrete events at 3.4 and <2 Ga. The older population of clusters contains no detectable Fe, whereas roughly half of the younger population of clusters is Fe bearing. This result shows that the Fe required to form secondary magnetite entered the zircon sometime after 3.4 Ga and that remobilization of Pb and Fe during an annealing event occurred more than 1 Ga after deposition of the Jack Hills sediment at 3 Ga. The ability to date Fe mobility linked to secondary magnetite formation provides new possibilities to improve our knowledge of the Archean geodynamo. 

The time of origin of the geodynamo has important implications for the thermal evolution of the planetary interior and the habitability of early Earth. It has been proposed that detrital zircon grains from Jack Hills, Western Australia, provide evidence for an active geodynamo as early as 4.2 billion years (Ga) ago. However, our combined paleomagnetic, geochemical, and mineralogical studies on Jack Hills zircons indicate that most have poor magnetic recording properties and secondary magnetization carriers that postdate the formation of the zircons. Therefore, the existence of the geodynamo before 3.5 Ga ago remains unknown. 

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.