A long-lived planetesimal dynamo powered by core crystallization

Abstract:

Some planetesimals partially melted and contained both chondritic and achondritic constituent materials. The internal structures of such partially-differentiated bodies are not well understood. They could have preserved a chondritic crust overlying a differentiated interior made of a metallic core and an achondritic mantle. Alternatively, they could have consisted of patchworks of localized differentiation products resulting from incomplete melting. These different outcomes could potentially be distinguished using paleomagnetic measurements of meteorites, by searching for records of an ancient dynamo magnetic fields that would require the existence of a metallic core. This could even place a constraint on the relative size of the core given that the duration and intensity of dynamo fields scales with the available energy. However, there has so far been no extended record of dynamo activity constrained by radiometric dating on a single body.Toaddress this, we measured the remanent magnetization carried by the IIE iron meteorites Techado, Colomera and Miles. A variety of petrographic, geochemical and magnetic data indicate that the IIE parent body was likely partially differentiated. We used X-ray photoemission electron microscopy to analyze the meteorite cloudy zones (Fig. 1), nanoscale Fe-Ni ferromagnetic structures that can carry highly stable remanent magnetization. Because the temperature at which cloudy zones would have acquired their magnetization (320°C) is close to the ⁴⁰Ar/³⁹Ar closure temperature for the IIE silicates (330 ± 70°C), the ⁴⁰Ar/³⁹Ar ages of Techado, Colomera and Miles [78 ± 13, 97 ± 10 and 159 ± 9 million years (Ma) after CAI-formation, respectively] anchor the time of their remanence acquisition. We find that the three meteorites experienced a magnetic field with surface intensities between ~5 and ~150 µT and persisting over > 80 Ma. The age, duration, and strength of the paleofield indicate that it was most likely a dynamo powered by core crystallization. We estimate that the core radius was at least 30% of the body radius, consistent with the upper size range typically inferred from chondritic metal contents. This favors efficient formation of a significant core on this partially-differentiated planetesimal.