Palaeomagnetism and Rock Magnetism

Paleomagnetic studies of meteorites over the past two decades have revealed that the cores of multiple meteorite parent bodies, including those of certain chondritic groups, generated dynamo fields as they crystallised. However, uncertainties in the direction and mode of core solidification in asteroid-sized bodies have meant using the timings and durations of these fields to constrain parent body properties, such as size, is challenging. Here, we use updated equations of state and liquidus relationships for Fe-FeS liquids at low pressures to calculate the locations at which solids form in these cores. We perform these calculations for core-mantle boundary (CMB) pressures from 0–2 GPa, and Fe-FeS liquid concentrations on the iron-rich side of the eutectic, as well as two values of iron thermal expansivity that cover the measured uncertainties in this parameter, and adiabatic and conductive cooling of these cores. We predict inward core crystallisation from the CMB in asteroids due to their low < 0.5 GPa pressures regardless of the uncertainties in other key core parameters. However, due to low internal pressures in these cores, remelting of any iron snow, as proposed to generate Ganymede's present-day field, may be unlikely as the cores are approximately isothermal. Therefore a different mode of inward core solidification is possibly required to explain compositionally-driven dynamo action in asteroids. Additionally, we identify possible regimes at higher > 0.6 − 2 GPa pressures in which crystallisation can occur concurrently at the CMB and the centre.

The thermal and magnetic histories of planetesimals provide unique insights into the formation
and evolution of Earth’s building blocks. These histories can be gleaned from meteorites by
using numerical models to translate measured properties into planetesimal behaviour. In this
paper, we present a new 1D planetesimal thermal evolution and dynamo generation model. This
magnetic field generation model is the first of a differentiated, mantled planetesimal that includes
both mantle convection and sub-eutectic core solidification. We have improved fundamental
aspects of mantle heat transport by including a more detailed viscosity model and stagnant lid
convection parametrisations consistent with internal heating. We have also added radiogenic
heating from 60Fe in the metallic Fe-FeS core. Additionally, we implement a combined thermal
and compositional buoyancy flux, as well as the latest magnetic field scaling laws to predict
magnetic field strengths during the planetesimal’s thermal evolution until core solidification is
complete. We illustrate the consequences of our model changes with an example run for a 500 km
radius planetesimal. These effects include more rapid erosion of core thermal stratification and
longer duration of mantle convection compared to previous studies. The additional buoyancy
from core solidification has a marginal effect on dynamo strength, but for some initial core sulfur
contents it can prevent cessation of the dynamo when mantle convection ends. Our model can
be used to investigate the effects of individual parameters on dynamo generation and constrain
properties of specific meteorite parent bodies. Combined, these updates mean this model can
predict the most reliable and complete magnetic field history for a planetesimal to date, so is a
valuable tool for deciphering planetesimal behaviour from meteorite properties.

Tarda is an ungrouped, hydrated carbonaceous chondrite (C2-ung) that was seen to fall in Morocco in 2020. Early studies showed that Tarda chemically resembles another ungrouped chondrite, Tagish Lake (C2-ung), which has previously been linked to the dark D-type asteroids. Samples of D-type asteroids provide an important opportunity to investigate primitive conditions in the outer Solar System. We show that Tarda contains few intact chondrules and refractory inclusions and that its composition is dominated by secondary Mg-rich phyllosilicates (>70 vol.%), carbonates, oxides, and Fe-sulphides that formed during extensive water-rock reactions. Quantitative assessment of first-order reversal curve (FORC) diagrams shows that Tarda’s magnetic mineralogy (i.e., framboidal magnetite) is comparable to that of the CI chondrites and differs notably from that of most CM chondrites. These traits support a common formation process for magnetite in Tarda and the CI chondrites. Furthermore, Tarda’s pre-terrestrial paleomagnetic remanence is similar to that of Tagish Lake and samples returned from asteroid Ryugu, with a very weak paleointensity (5.4–8.3 AU. An origin in the cold, outer regions of the Solar System is further supported by the presence of distinct, porous clasts enriched in aliphatic-rich organics that potentially retain a pristine interstellar composition. Together, our observations support a genetic relationship between Tarda and Tagish Lake.