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Interest in magnetic fields on the ancient Earth and other planetary bodies has motivated the paleomagnetic analysis of complex rocks such as meteorites that carry heterogeneous magnetizations at <<1 mm scales. The net magnetic moment of natural remanent magnetization (NRM) in such small samples is often below the detection threshold of common cryogenic magnetometers. The quantum diamond microscope (QDM) is an emerging magnetic imaging technology with ~1 μm resolution and can, in principle, recover magnetizations as weak as 10⁻¹⁷ Am². However, the typically 1–100 μm sample‐to‐sensor distance of QDM measurements can result in complex (nondipolar) magnetic field maps, from which the net magnetic moment cannot be determined using a simple algorithm. Here we generate synthetic magnetic field maps to quantify the errors introduced by sample nondipolarity and by map processing procedures such as upward continuation. We find that inversions based on least squares dipole fits of upward continued data can recover the net moment of complex samples with <5% to 10% error for maps with signal‐to‐noise ratio (SNR) in the range typical of current generation QDMs. We validate these error estimates experimentally using comparisons between QDM maps and between QDM and SQUID microscope data, concluding that, within the limitations described here, the QDM is a robust technique for recovering the net magnetic moment of weakly magnetized samples. More sophisticated net moment fitting algorithms in the future can be combined with upward continuation methods described here to improve accuracy.

 

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 paleomagnetic record is an archive of Earth’s geophysical history, informing reconstructions of ancient plate motions and probing the core via the geodynamo. We report a robust 3.25-billion-year-old (Ga) paleomagnetic pole from the East Pilbara Craton, Western Australia. Together with previous results from the East Pilbara between 3.34 and 3.18 Ga, this pole enables the oldest reconstruction of time-resolved lithospheric motions, documenting 160 My of both latitudinal drift and rotation at rates of at least 0.55°/My. Motions of this style, rate, and duration are difficult to reconcile with true polar wander or stagnant-lid geodynamics, arguing strongly for mobile-lid geodynamics by 3.25 Ga. Additionally, this pole includes the oldest documented geomagnetic reversal, reflecting a stably dipolar, core-generated Archean dynamo. 

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. 

The mode and rates of tectonic processes and lithospheric growth during the Archean [4.0 to 2.5 billion years (Ga) ago] are subjects of considerable debate. Paleomagnetism may contribute to the discussion by quantifying past plate velocities. We report a paleomagnetic pole for the ~3180 million year (Ma) old Honeyeater Basalt of the East Pilbara Craton, Western Australia, supported by a positive fold test and micromagnetic imaging. Comparison of the 44°±15° Honeyeater Basalt paleolatitude with previously reported paleolatitudes requires that the average latitudinal drift rate of the East Pilbara was ≥2.5 cm/year during the ~170 Ma preceding 3180 Ma ago, a velocity comparable with those of modern plates. This result is the earliest unambiguous evidence yet uncovered for long-range lithospheric motion. Assuming this motion is due primarily to plate motion instead of true polar wander, the result is consistent with uniformitarian or episodic tectonic processes in place by 3.2 Ga ago. 

Abstract Field evidence from the Pilbara craton (Australia) and Kaapvaal craton (South Africa) indicate that modern tectonic processes may have been operating at ca. 3.2 Ga, a time also associated with a high density of preserved Archaean impact indicators. Recent work has suggested a causative association between large impacts and tectonic processes for the Hadean. However, impact flux estimates and spherule bed characteristics suggest impactor diameters of <100 km at ca. 3.5 Ga, and it is unclear whether such impacts could perturb the global tectonic system. In this work, we develop numerical simulations of global tectonism with impacting effects, and simulate the evolution of these models throughout the Archaean for given impact fluxes. We demonstrate that moderate-size (∼70 km diameter) impactors are capable of initiating short-lived subduction, and that the system response is sensitive to impactor size, proximity to other impacts, and also lithospheric thickness gradients. Large lithospheric thickness gradients may have first appeared at ca. 3.5–3.2 Ga as cratonic roots, and we postulate an association between Earth’s thermal maturation, cratonic root stability, and the onset of widespread sporadic tectonism driven by the impact flux at this time.