LOW-TEMPERATURE DEMAGNETIZATION ISOLATES STABLE MAGNETIC VECTOR COMPONENTS IN MAGNETITE-BEARING DIABASE

Abstract

It may be difficult to isolate stable palaeomagnetic vectors of different ages if they lie in grain assemblages with overlapping ranges of coercivity or of unblocking temperature. This is because some moments associated with either vector may demagnetize at the same stage of experimental demagnetization. The sharp transition between vector components may be obscured and stable components may appear less linear on the demagnetization plot. Thermal and alternating field (AF) demagnetization techniques remove vector components on a quantitative basis, according to a discrete limiting unblocking temperature or coercivity. Low-temperature demagnetization (LTD) differs in that it removes vector components discretely, wherever there are mobile domain walls. Experiments tested the ability of LTD to improve the effectiveness of AF demagnetization on isothermal and anhysteretic remanent magnetizations (IRM, ARM) in diabase. Multicomponent NRMs were simulated IRMs or ARMs in different, non-overlapping coercivity ranges, along three orthogonal axes or along two non-orthogonal directions. The known directions of the experimentally applied vector components were always more successfully verified by AF demagnetization if LTD was first applied. For the same specimens, LTD reduced the same artificial remanences by ~50 per cent for the coercivity range 0-15 mT, by ~25 per cent for the range 15-30 mT, and negligibly for higher-coercivity fractions. LTD demagnetizes polydomain magnetite as domain walls rearrange on passing through a low-temperature transition, near 120 K.