New magnetic results from Allende C3(V)

Abstract

Detailed characterization of the temperature-dependent behavior of the composition and structure-sensitive magnetic parameters, as well as the record of continuous thermal demagnetization of NRM, provides new insight into the complex magnetic behavior in Allende, a C3(V) chondrite. There is no ∼ 120 K Verwey transition in the Allende specimen studied, indicating that the magnetite present is possibly non-stoichiometric or else is a minor magnetic phase. Magnetic transitions have been identified as 50–75, ∼ 150, ∼ 320, and 590–610°C in careful study of the I S( T) behavior in five separate specimens. Up to 90% of the NRM and irreversible H R( T) behavior ( H R = remanent coercive force) is associated with magnetic phases having transitions or undergoing thermally induced transformation at temperatures ≲320°C. These phases contribute about 15% of the total I S value. Published paleofield estimates (the Thellier-Thellier test or modifications thereof) should be reviewed with caution as the presented experimental data identify complex irreversible thermomagnetic behavior beginning at temperatures as low as 50°C. The thermomagnetic data suggest that phases carrying the dominant part of the NRM are intergrown, magnetically interacting, and thermally unstable. The ratio (REM) of the NRM/SIRM (natural remanence/saturation remanence) ranges from 0.004 to 0.006 for the bulk meteorite and from 0.0004 to 0.005 for chondrules of all sizes. The chondrules studies by Lanoix et al. (1977, 1978) and Sugiura et al. (1979), which yielded large paleofield estimates, have REM>0.1. The chondrules giving the large paleofields have anomalous magnetic properties, different from other groups of chondrules studied by Sugiura et al. (1979) and the present work. The H R( T) behavior suggests coexisting magnetic phases with differing Curie points or with the low temperature transitions being due to transformation of a thermally metastable phase in much the same way as the conversion of γFe 2 O 3 → αFe 2 O 3 in epitaxy with Fe 3O 4 was demonstrated to be responsible for similar thermomagnetic behavior in massive iron ores (Wasilewski, 1979). This behavior is irreversible.