Paleomagnetic systematics of ordinary chondrites

Abstract

A pilot systematic study of the remanent magnetic behavior of ordinary chondrites reveals several correlations with their chemical-petrological classification. The intensity of natural remanent magnetization (NRM) decreases in the sequence E→H→L→LL-chondrites. Its relative stability and degree of directional coherence under progressive AF demagnetization increase in the same order. These trends parallel a decrease in average metal content (from ∼20% in E to 2% in LL) and a corresponding Ni enrichment of metal grains (from ∼6%Ni in E to∼35% in LL). Within each chemical group, the stability of NRM is inversely related to petrologic subtype. Shocked meteorites display a distinctive demagnetization curve, an unusually large soft component of NRM, and pronounced directional scatter of NRM components. These characteristics can be reproduced reversibly by brief laboratory heating (to ∼800°C) of unshocked chondrites. Comparative studies of laboratory (saturation isothermal, IRM s, and thermal, TRM) remanence acquisition and behavior, suggest that other magnetic parameters, such as the ratios NRM/TRM and IRM s/NRM, may be useful for classification. Patterns of magnetic behavior are explained in terms of metallography and thermal history. In contrast to the carbonaceous chondrites, very few ordinary chondrites preserve a useful paleomagnetic record. The LL-chondrites hold the best promise in this regard. Paleofield estimates obtained for eight representative chondrites range from 0.01 to 0.3 Oe.