A mineral magnetic study of some pottery samples: possible implications for sample selection in archaeointensity studies

Abstract

We have related the mineral magnetic properties of some archaeological samples to the quality of the paleointensity data that they produce. We find that samples which have broad unblocking temperature distributions and most of their remanences unblocked at intermediate temperatures produce high-quality paleointensity data. This conclusion is based on a comparative study of pottery samples which had previously produced reliable and unreliable paleointensity data. For each sample, we determined the magnetic mineralogy and the magnetic domain state. In all cases, magnetite (or titanomagnetite) is the primary magnetic carrier, with domain states that extend from superparamagnetic to single domain, and maybe even to pseudo-single domain. We believe that the magnetic grain size distribution of these samples produces a broad unblocking temperature distribution. The advantage of having such a distribution is that high-quality data can be obtained before the onset of the mineralogical changes that can occur at high temperatures with the Thellier and Thellier method. The frequency-dependent susceptibility appears to be useful as a non-destructive estimator of the unblocking temperature distribution for samples having only magnetite as the magnetic mineral.