The Fine‐Scale Magnetic History of the Allende Meteorite: Implications for the Structure of the Solar Nebula

Abstract

Magnetic fields in the early solar system may have driven the inward accretion of the protoplanetary disk (PPD) and generated instabilities that led to the formation of planets and ring and gap structures. The Allende carbonaceous chondrite meteorite records a strong early solar system magnetic field that has been interpreted to have a PPD, dynamo, or impact‐generated origin. Using high‐resolution magnetic field imaging to isolate the magnetization of individual grain assemblages, we find that only Fe‐sulfides carry a coherent magnetization. Combined with rock magnetic analyses, we conclude that Allende carries a magnetization acquired during parent body chemical alteration at ~3.0–4.2 My after calcium aluminum‐rich inclusions in an >40 µT magnetic field. This early age strongly favors a magnetic field of nebular origin instead of dynamo or solar wind alternatives. When compared to other paleomagnetic data from meteorites, this strong intensity supports a central role for magnetic instabilities in disk accretion and the presence of temporal variations or spatial heterogeneities in the disk, such as ring and gap structures.The presence of magnetic fields during the formation stage of the solar system may have been critical for planet formation. One way to test this hypothesis is to observe both the strength and the degree of heterogeneity in early solar system magnetic fields; strong fields with significant local variations would suggest a key role for magnetic fields in governing the distribution of planet‐forming materials. We performed both traditional paleomagnetic analyses and high‐resolution magnetic field imaging experiments on the carbonaceous chondrite Allende. By identifying the sub‐regions that carry a magnetization, we infer that the meteorite was magnetized during reactions between Fe‐bearing minerals and aqueous fluids on the parent asteroid at 3.0–4.2 million years after solar system formation. These reactions occurred in an ambient magnetic field of at least 40 µT strength, which is much stronger than inferred from other meteorites of similar age. We therefore find evidence for strong, heterogeneous magnetic fields in the forming solar system, supporting the idea that magnetic fields mediated the concentration of solid materials, potentially leading to planet formation. Strong magnetization in Allende is carried by only a single ferromagnetic mineral‐pyrrhotite‐ while other minerals are weakly magnetized This peculiar pattern of magnetization is best explained by acquisition during aqueous alteration in a strong nebular magnetic field The resulting paleointensity, >40 μT, suggests strong magnetic control on nebular accretion and heterogeneous conditions in the outer disk