The magnetic effects of brecciation and shock in meteorites. II

The authors have examined the magnetic characteristics of representative urelites, with a view to identify the magnetic effects of shock and to isolate a primary component of the natural remanent magnetization (NRM). As a group, the urelites show remarkably uniform patterns of magnetic behavior, attesting to a common genesis and history. However, a clearly oberved gradation in magnetic properties of the urelites studied with shock level, parallels their classification based on petrologic and chemical fractionation shock-related trends. The urelite meteorites possess a strong and directionally stable NRM. Laboratory thermal modelling of this presumably primordial NRM preserved in Goajpara and Kenna produced reliable paleointensity estimates of order 1 Oe, thus providing evidence for strong early, nebular magnetic fields. This paleofield strength is compatible with values obtained previously from carbonaceous chondrites and supports isotopic evidence for a contemporary origin of these two groups of meteorites in the same nebular region. The mechanism for recording nebular fields manifestly different in carbonaceous chondrite vs. ureilite meteorites, is thus relatively unimportant: violent collisional shock in urelites seems to have only partially altered an original magnetization, by preferential removal of its least stable portion. (Auth.)