Bulletin of the Geological Society of Finland (Dec 1983)
Classification of meteorites by petrophysical methods
Abstract
The low field magnetic susceptibility, natural remanent magnetization (NRM) and bulk density were measured for 75 meteorite samples from 54 meteorites ranging from achondrites through chondrites and stony-irons to irons. The results show distinct relations between susceptibility and NRM and between susceptibility and bulk density. The susceptibility and intensity of NRM increase from achondrites to irons as a function of increasing metallic Ni-Fe content. Moreover, subclasses of meteorites appear as different groupings in the petrophysical relation diagrams. The susceptibility - density diagram in particular offers a rapid method for identifying a meteorite group and subgroup. The petrophysical classification method was successfully applied to previously published petrophysical data from the meteorite collections of the U.S.S.R. When meteorites are compared with terrestrial basic igneous rocks, the petrophysical properties of the terrestrial rocks (peridotites, pyroxenites, gabbros, diorites and diabases) are seen to fall outside the area of achondrites in the susceptibility - NRM diagram. Thus it is possible, in principle, to distinguish achondrites from terrestrial rocks by petrophysical measurements. Two factors considerably reduce the apparent susceptibility values measured. First, the shape demagnetization effect becomes very important in the case of stony-irons and irons rich in metallic Ni-Fe. Second, the apparent susceptibility of stony-irons and irons is reduced owing to the high value of the electrical conductivity, because the measurements were done with an alternating current bridge. These effects can be corrected by giving the sample a regular shape and by applying a frequency dependent correction factor for the susceptibility values. The nature of NRM was studied by applying a storage test, which revealed that the NRM is very liable to changes due to spurious laboratory fields. Alternating field demagnetization of the Haverö achondrite (ureilite) and the Orgueil carbonaceous chondrite (CI1) suggest that these meteorites carry mainly a soft and semistable NRM component, although a weak and harder - perhaps extraterrestrial - component may be present.
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