Paleomagnetic Records of Meteorites and Early Planetesimal Differentiation

Abstract

The large-scale compositional structures of planets are primarily established during early global differentiation. Advances in analytical geochemistry, the increasing diversity of extraterrestrial samples, and new paleomagnetic data are driving major changes in our understanding of the nature and timing of these early melting processes. In particular, paleomagnetic studies of chondritic and small-body achondritic meteorites have revealed a diversity of magnetic field records. New, more sensitive and highly automated paleomagnetic instrumentation and an improved understanding of meteorite magnetic properties and the effects of shock, weathering, and other secondary processes are permitting primary and secondary magnetization components to be distinguished with increasing confidence. New constraints on the post-accretional histories of meteorite parent bodies now suggest that, contrary to early expectations, few if any meteorites have been definitively shown to retain records of early solar and protoplanetary nebula magnetic fields. However, recent studies of pristine samples coupled with new theoretical insights into the possibility of dynamo generation on small bodies indicate that some meteorites retain records of internally generated fields. These results indicate that some planetesimals formed metallic cores and early dynamos within just a few million years of solar system formation.

Differentiation and core formation in accreting planetesimals

“…In contrast to the assumption of Hevey & Sanders (2006) that the parent bodies of differentiated meteorites formed prior to the accretion of the most chondritic parent bodies, our results indicate that both chondritic meteorites and differentiated meteorites can originate from one and the same parent body. This finding is consistent to the results of Weiss et al (2010) and Elkins-Tanton et al (2011), where a differentiated interior is covered with undifferentiated crust to explain the remanent magnetisation of the CV meteorite Allende.…”

Solar nebula magnetic fields recorded in the Semarkona meteorite

“…The very high NRM to saturation IRM ratio of this component (0.23) indicates that it is an IRM acquired during exposure to artificial fields. The maximum unblocking coercivity suggests that the artificial field had a maximum intensity of ~30 mT (84), which is consistent with a weak hand magnet (38). Furthermore, the presence of the LCb overprint only in samples from local area within 2 mm of the sample edge is expected for an origin due to contact with a hand magnet.…”

“…However, because the Lanoix et al (36) study measured unoriented chondrules, the authors were unable to demonstrate a pre-accretional origin for the observed magnetization (see Section 3.4). In fact, the very high paleointensities and the failure of later experiments to duplicate these results strongly indicate that the purported pre-accretional magnetization component studied in Lanoix et al (36) was due to contamination by strong artificial magnetic fields (e.g., from hand magnets) and does not offer constraints on nebular magnetic fields (37,38).…”

Dynamo Scaling Laws and Applications to the Planets

“…The scaling law based on energy flux provides a handle for estimates of the strength of Earth's paleofield (Aubert et al 2009) and for the magnetic fields of now extinct dynamos, for example in early Mars or in large differentiated asteroids (Weiss et al 2008(Weiss et al , 2009. It also allows to estimate the magnetic field strength of extrasolar planets.…”