An early solar system magnetic field recorded in CM chondrites

We present a paleomagnetic study of seven CM carbonaceous chondrites. CM chondrites are believed to be some of the most chemically primitive materials available in our solar system and may sample the continuum of transitional objects between asteroids and comets formed in the outer solar system. As such, CM chondrites can help us to understand primordial aspects of the history of the early solar system including protoplanetary disk and planetesimal magnetism. The ferromagnetic assemblage of CM chondrites is composed of a mixture of primary metallic iron, pyrrhotite, and magnetite. The remanent properties are usually dominated by secondary pyrrhotite. Paleomagnetic analyses using thermal and alternating field demagnetization identified a stable origin-trending component of magnetization in the seven studied CM chondrites. In each meteorite, this component is homogeneous in direction at least at the cm scale and is therefore post-accretional. We interpret this stable component as a pre-terrestrial chemical remanent magnetization acquired during crystallization of magnetite and pyrrhotite during parent body aqueous alteration in a field of at least a few μT(2±1.5 μT). Considering the timescale and intensities of primordial magnetic fields, both internally generated fields from a putative dynamo and external fields, generated in the protoplanetary disk, may have been recorded by CM chondrites. It is presently difficult to discriminate between the two hypotheses. Regardless, CM chondrites likely contain the oldest paleomagnetic record yet identified. •We examine the magnetic properties and paleomagnetism of seven CM chondrites.•CM chondrites possess a post-accretion and pre-terrestrial magnetization.•Magnetization was acquired by magnetite/pyrrhotite formation at ∼4565 Ma.•This paleofield of ∼2 μT could have been of external or internal origin.•CM chondrites likely contain the oldest paleomagnetic record yet identified.