A magnetic study of the Millbillillie (eucrite) achondrite: Evidence for a dynamo‐type magnetising field

— The magnetic properties of the Millbillillie eucrite have been intensively studied using mutually oriented samples. The magnetic carrier has been identified as single domain/pseudo-single domain kamacite. It is believed that due to clast-rich/clast-poor layering, the rock has been subjected to minimum shock since formation. This is supported by the presence of a stable, unidirectional remanence. Three separate palaeointensity tests (IRM/NRM, ARM-NRM, and Thellier heating) have been used to determine the strength of the palaeofield. The criteria for the suitability of samples to be used in such tests are discussed, as are the three methods; IRM/NRM is deemed the least accurate. The results obtained (IRM/NRM: 6 μT and 25 μT; ARM-NRM: 31 μT, 28 μT, and 15 μT; Thellier: 30 μT, 37 μT, 15 μT) indicate that the rock cooled in a strong magnetic field. Modes of formation, given that Millbillillie has a layered structure, and the nature of the HED parent body, are discussed. A volcanic genesis is preferred over formation in an impact melt. An internal dynamo field is favoured for dynamical reasons, and calculations based on the assumption that 4 Vesta is a possible HED parent body indicate that an iron core may be present in such a body.