Magnetic evidence for a partially differentiated carbonaceous chondrite parent body

Abstract

The textures of chondritic meteorites demonstrate that they are not the products of planetary melting processes. This has long been interpreted as evidence that chondrite parent bodies never experienced large-scale melting. As a result, the paleomagnetism of the CV carbonaceous chondrite Allende, most of which was acquired after accretion of the parent body, has been a long-standing mystery. The possibility of a core dynamo like that known for achondrite parent bodies has been discounted because chondrite parent bodies are assumed to be undifferentiated. Resolution of this conundrum requires a determination of the age and timescale over which Allende acquired its magnetization. Here, we report that Allende's magnetization was acquired over several million years (Ma) during metasomatism on the parent planetesimal in a > ∼ 20 μT field up to approximately 9-10 Ma after solar system formation. This field was present too recently and directionally stable for too long to have been generated by the protoplanetary disk or young Sun. The field intensity is in the range expected for planetesimal core dynamos, suggesting that CV chondrites are derived from the outer, unmelted layer of a partially differentiated body with a convecting metallic core.differentiation | planetesimal | magnetic field | early solar system | paleointensity A llende is an accretionary breccia from near the surface of the CV parent planetesimal (1). Following accretion, Allende experienced minor aqueous alteration and moderate thermal metamorphism and metasomatism (2) but has remained essentially unshocked (<5 GPa) (3). Its major ferromagnetic minerals are pyrrhotite, magnetite, and awaruite, with an average pseudo single-domain crystal size (4-8). We conducted alternating-field (AF) and thermal demagnetization, rock magnetic, and paleointensity measurements on 71 mutually oriented bulk subsamples of Allende sample AMNH5056 (approximately 10-cm diameter and 8-mm thick slab surrounded by fusion crust). Of these, 51 subsamples were taken from the interior of the meteorite (>1 mm from fusion crust), whereas 20 contained some fusion crust.The differing magnetization directions of interior and fusioncrusted samples demonstrate that >95% of the natural remanent magnetization (NRM) in interior samples is preterrestrial (Figs. 1 and 2 and SI Appendix). AF demagnetization revealed that the interior samples have at least two components: a weak, low-coercivity, nonunidirectional component blocked up to 5 or 10 mT and a high-coercivity (HC) component blocked from approximately 10 to >290 mT (Fig. 1). In agreement with previous studies (4, 9, 10), the HC magnetization is unidirectionally oriented throughout the meteorite's interior (Fig. 2 and SI Appendix, Table S1). Thermal demagnetization (Figs. 1 and 2 and SI Appendix) indicates that interior samples have a low-temperature (LT) component blocked up to approximately 190°C, a dominant middle-temperature (MT) component blocked between approximately 190-300°C and oriented similarly to the HC component isolated by...

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