In well-buffered modern soils, higher annual rainfall is associated with enhanced soil ferrimagnetic mineral content, especially of ultrafine particles that result in distinctive observable rock magnetic properties. Hence, paleosol magnetism has been widely used as a paleoprecipitation proxy. Identifying the dominant mechanism(s) of magnetic enhancement in a given sample is critical for reliable inference of paleoprecipitation. Here we use high-resolution magnetic field and electron microscopy to identify the grain-scale setting and formation pathway of magnetic enhancement in two modern soils developed in higher (~580 mm/y) and lower (~190 mm/y) precipitation settings from the Qilianshan Range, China. We find both soils contain 1-30 µm aeolian Fe-oxide grains with indistinguishable rock magnetic properties while the higher-precipitation soil contains an additional population of ultrafine (<150 nm), magnetically distinct magnetite grains. We show that the in situ precipitation of these ultrafine particles, likely during wet-dry cycling, is the only significant magnetic enhancement mechanism in this soil. These results demonstrate the potential for quantum diamond microscope (QDM) magnetic microscopy to extract magnetic information from distinct, even intimately mixed, grain populations. This information can be used to evaluate the contribution of distinct enhancement mechanisms to the total magnetization.

Several paleomagnetic studies have been conducted on five main group pallasites: Brenham, Marjalahti, Springwater, Imilac and Esquel. These pallasites have distinct cooling histories, meaning that their paleomagnetic records may have been acquired at different times during the thermal evolution of their parent body. Here we compile new and existing data to present the most complete time-resolved paleomagnetic record for a planetesimal, which includes a period of quiescence prior to core solidification as well as dynamo activity generated by compositional convection during core solidification. We present new paleomagnetic data for the Springwater pallasite, which constrains the timing of core solidification. Our results suggest that in order to generate the observed strong paleointensities (∼ 65 - 95 μT), the pallasites must have been relatively close to the dynamo source. Our thermal and dynamo models predict that the main group pallasites originate from a planetesimal with a large core (> 200 km) and a thin mantle (< 70 km). The density of our model large-cored planetesimals is similar to the predicted density of the asteroid (16) Psyche. We therefore suggest that it is plausible that the main group pallasites originated from a Psyche-like parent body. 1

Within the young solar system, a strong magnetic field permeated the protoplanetary disc. The solar nebular magnetic field is likely the source of magnetization for some meteorites like the CM and CV chondrites, which underwent aqueous alternation on their parent bodies before the solar nebular field dissipated. Since aqueous alteration produced magnetic minerals (e.g. magnetite and pyrrhotite), the meteorites could have acquired a chemical remanent magnetization from the nebular field while part of their respective parent bodies. However, questions about the formation history of the parent bodies that produced magnetized CM and CV chondrites await answers—including whether the parent bodies exhibit a detectable magnetic field today. Here, we use thermal evolution models to show that a parent body of the CM chondrites could record ancient magnetic fields and, perhaps, exhibit strong present-day crustal remanent fields. An undisturbed planetesimal would experience one of three thermal evolution cases with respect to the lifetime of the nebular field. First, if a planetesimal formed too late for 26Al-driven water ice melting to occur before the solar nebula dissipates, then aqueous alteration would not occur in the presence of the nebular field and result in no magnetization (Fig. panel a). Second, if a planetesimal forms early enough to undergo alteration before the nebula dissipates but not enough to heat beyond the blocking temperature(s) of the magnetic mineral(s), then nearly the entire planetesimal could be magnetized (Fig. panel b). Lastly, if a planetesimal forms early enough to undergo alteration and subsequently heats beyond the blocking temperature, then any magnetization would be erased except for a thin shell near the surface (Fig. panel c). Our thermal model results suggest that planetesimals that formed between ~2.7 and 3.7 Myr after CAIs could acquire large-scale magnetization. Spacecraft missions could detect this magnetization if it is at the strength recorded in CM chondrites and if it is coherent at scales of tens of kilometers. In-situ magnetometer measurements of chondritic asteroids could help link magnetized asteroids to magnetized meteorites. Specifically, a spacecraft detection of remanent magnetization at 2 Pallas would bolster the claim that 2 Pallas is a parent body of CM chondrites.

Our understanding of the past behavior of the geomagnetic field arises from magnetic signals stored in geological materials, e.g. (volcanic) rocks. Bulk rock samples, however, often contain magnetic grains that differ in chemistry, size and shape; some of them record the Earth’s magnetic field well, others are unreliable. The presence of a small amount of adverse behaved magnetic grains in a sample may already obscure important information on the past state of the geomagnetic field. Recently it was shown that it is possible to determine magnetizations of individual grains in a sample by combining X-ray computed tomography and magnetic surface scanning measurements. Here we establish this new Micromagnetic Tomography (MMT) technique and make it suitable for use with different magnetic scanning techniques, and for both synthetic and natural samples. We acquired reliable magnetic directions by selecting subsets of grains in a synthetic sample, and we obtained rock-magnetic information of individual grains in a volcanic sample. This illustrates that MMT opens up entirely new venues of paleomagnetic and rock-magnetic research. MMT’s unique ability to determine the magnetization of individual grains in a nondestructive way allows for a systematic analysis of how geological materials record and retain information on the past state of the Earth’s magnetic field. Moreover, by interpreting only the contributions of known magnetically well-behaved grains in a sample MMT has the potential to unlock paleomagnetic information from even the most complex, crucial, or valuable recorders that current methods are unable to recover.