Magnetic properties of natural pyrrhotite.II. High- and low-temperature behaviour of ~and TRM as function of grain size

Abstract

Twelve grain-size fractions, ranging from 250 ~tm down to <5 ~m, of four natural pyrrhotite samples were homogeneously dispersed in a non-magnetic matrix to study their magnetic behaviour during thermal demagnetization and low-temperature treatment. Normalized decay curves of the isothermal saturation remanence (Jrs) obtained by continuous thermal demagnetization are identical for all samples studied. They are characterized by a continuous decay up to the maximum blocking temperature of 325°C. Small grain-size fractions have a blocking temperature spectrum slightly shifted to lower temperatures. Decay curves for ~rs obtained by stepwise thermal demagnetization yield a different behaviour for coarseand fine-grained fractions. The former showa monotonous decay up to the maximum blocking temperature, whereas the latter show an enhanced decay at 210—245°C, followed by a small remanence increase at 260—300°C. The enhanced decay is tentatively related to the formation of non-magnetic anomalous pyrrhotite; the increase with its upper stability temperature. The impact of pyrrhotite alteration and metastable preservation of high-temperature superstructures upon rapid cooling is discussed. The TRM intensity decreases with grain size, possibly owing to the absence of closure domains in pyrrhotite. Normalized TRM decay curves are virtually identical for all fractions. Stepwise thermal demagnetization indicates narrow blocking temperature spectra for coarse grain-size fractions, slightly broadening with decreasing grain size. Low-temperature cycling of ~rs shows a gradually increasing reversible remanence behaviour with decreasing grain size. This concurs with a gradual approach of the SD state in the smallest grain-size fractions (< 5 ~tm).TRM shows a reversible low-temperature behaviour throughout the investigated grain-size range.