External magnetization of undifferentiated carbonaceous chondrite parent asteroids:Constraints on early Solar System evolution

Magnetic data from the CV (Vigarano type) carbonaceous chondrite Allende have been used to argue for a core dynamo and igneous differentiation of the CV chondrite parent asteroid. New rock magnetic, directional and mineralogical data from Allende define profound magnetic interactions, violating requirements for reliable paleointensity and directional recordings based on thermoremanent magnetization theory. In contrast, the weak and consistent paleofields recorded by other carbonaceous chondrites indicate external magnetization of CV and CM (Mighei type) undifferentiated parent asteroids less than 5 million years after formation of CV Ca,Al-rich inclusions, the first Solar System solids. Theory and ideal magnetohydrodynamic models show that intense solar winds can induce these magnetizations during parent asteroid metasomatic alteration within orbital distances of the present-day C-type asteroids (∼2-4 AU). These rare constraints on process and orbital distance have key implications for early Solar System evolution. Our new orbital constraints imply that scattering of the CV and CM parent bodies and related motion of Jupiter occurred before the magnetization age (ca. 4562 Ma). Moreover, CV and CM external magnetizations suggest that accretion rates had fallen below a critical threshold of ∼10^-10 Solar mass/yr, and that gas and dust were essentially dissipated, before 4562 Ma.