On the suitability of the Thellier method of palaeointensity determinations on pseudo-single-domain and multidomain grains

We report an experimental and theoretical study Of non-linear Arai-Nagata d iagrams for samples containing pseudo-single-domain (PSD) and multidomain ( MD) magnetite. Our aim is to reveal the physical reasons for the deviation of these plots fi om ideal straight lines. Contrary to expectations, the co ncavity of the Arai-Nagata diagrams is not related to the two most noticeab le violations of tho Thellier laws documented for non-single-domain particl es: the tail of partial thermoremanence and the dependence of the magnitude of pTRM on the thermal history of the sample. Indeed, the contributions fr om these two factors mutually cancel each other. Phenomenologically, the no n-linear Arai-Nagata plots occur because samples during low-temperature sta ges of the Thellier procedure lose too much remanence and recover too littl e of it. The excessive loss of the previously imparted total TRM is due at least partly to some partial demagnetization of high-temperature TRM compon ents and to progressive stabilization of domain structure during the repeti tive heatings to moderate temperatures that form the basis of the Thellier procedure. For natural MD samples a linens fit to the low-temperature data points on the Arai-Nagata plots leads to a palaeointensity overestimated by as much as 60 per cent. The samples containing hydrothermally grown or cru shed and sieved MD magnetite provide low-temperature apparent palaeointensi ties two to three times larger than the correct value. For small PSD sample s the overestimate is less than 10-20 per cent and, in general, PSD samples can be used for the palaeointensity determinations.