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

Paleomagnetic studies of meteorites constrain the evolution of magnetic fields in the early solar system. These studies rely on the identification of magnetic minerals that can retain stable magnetizations over ≳4.5 billion years (Ga). The ferromagnetic mineral tetrataenite (γ’-Fe0.5Ni0.5) is found in iron, stony-iron and chondrite meteorite groups. Nanoscale intergrowths of magnetostatically-interacting tetrataenite have been shown to carry records of paleomagnetic fields. Tetrataenite can also occur as isolated, non-interacting grains in many meteorite groups. Here we study non-interacting tetrataenite to establish the grain size range over which it can retain magnetization that is stable over solar system history. We present the results of analytical calculations and micromagnetic modelling of isolated tetrataenite grains with various sizes and geometries. We find that tetrataenite forms a stable single domain state for grain lengths between ~10 and 160 nm dependent on its axial ratio. It also possesses a magnetization resistant to viscous remagnetization over the lifetime of the solar system at 293 K. At smaller grain sizes, tetrataenite is superparamagnetic while at larger grain sizes, tetrataenite’s lowest energy state is a lamellar two-domain state that is stable over Ga-scale timescales. Unlike many other ferromagnetic minerals, tetrataenite does not form a single-vortex domain state due to its large uniaxial anisotropy. Our results show that both single domain and two-domain tetrataenite carry extremely stable magnetization and therefore are promising for paleomagnetic studies.

The meteorite paleomagnetic record indicates that differentiated (and potentially, partially differentiated) planetesimals generated dynamo fields in the first 6-20 Myr after the formation of calcium-aluminium-rich inclusions (CAIs). This early period of dynamo activity has been attributed to thermal convection in the liquid cores of these planetesimals during an early period of magma ocean convection. To better understand the controls on thermal dynamo generation in planetesimals, we have developed a 1D model of the thermal evolution of planetesimals from accretion through to the shutoff of convection in their silicate magma oceans for a variety of accretionary scenarios. The heat source of these bodies is the short-lived radiogenic isotope, 26Al. During differentiation, 26Al partitions into the silicate portion of these bodies, causing their magmas ocean to heat up and introducing stable thermal stratifications to the tops of their cores, which inhibits dynamo generation. In ‘instantaneously’ accreting bodies, this effect causes a delay on the order of >10 Myr to whole core convection and dynamo generation while this stratification is eroded. However, gradual core formation in bodies that accrete over >0.1 Myr can minimise the development of this stratification, allowing dynamo generation from ~4 Myr after CAI formation. Our model also predicts partially differentiated planetesimals with a core and mantle overlain by a chondritic crust for accretion timescales >1.2 Myr, although none of these bodies generate a thermal dynamo field. We compare our results from thousands of model runs to the meteorite paleomagnetic record to constrain the physical properties of their parent bodies.