2. Meteoritic Magnetism: Implications for Parent Bodies of Origin

Abstract

Recent progress in studying and understanding the magnetic record of meteorites is reviewed. Magnetic data are not compatible with the simple picture of a single planetary parent-body with core-mantle structure and a dynamo-generated magnetic field, as earlier envisaged by Soviet colleagues. The strong preterrestrial magnetization of iron meteorites, previously believed to have been acquired during cooling in parent-body fields of -.6 Oe, has now been shown to be probably a spontaneous moment, directionally controlled by the octahedral Ni-Fe structure. For each class of meteorites, the magnetic record is basically in accord with conclusions based on chemical-mineralogical-petrologic characteristics. For example, the complex remanence of brecciated achondrites bears no record of their primary igneous differentiation, but only of multiple brecciation events. Similarly, the unreilites show the expected magnetic imprint of shock-metamorphism at impact. Although systematic trends were found among ordinary chondrites groups, allowing for a rudimentary magnetic classification, only very few appeared to possess a primordial remanence component, which was used to estimate parent body fields in the range 0.1 = .3 oe. Most ordinary chondrites have been magnetically affected by brecciation at formation (e.g., LL), or by metamorphism within the parent body, or individually - by shock at the breakup of the parent body. Only the carbonaceous chondrites have preserved a clear magnetic record of their formation at low-temperature (T < 500° K), in strong magnetic fields (H ≥ 1 oe). The evidence is compatible with cold condensation and aggregation of component grains either in extended, enhanced solar wind fields, or in cometary magnetic fields.