Changes in magnetic properties of baked archaeological samples. Implications for palaeointensity determination

Rock magnetic investigations of archaeological materials of burnt clay, fro m Eneolithic ovens (similar to 4500 years BC) showed particular changes wit h time in the magnetic mineralogy of samples, stored under normal condition s. Our results indicate that well-burnt clay from the archaeological materi als contains a significant amount of very,fine magnetic grains, which could notably, influence the rock magnetic properties and behavior at room tempe rature. The main observations after 4 years of storage under laboratory, co nditions are as follows: 1) decrease in the final unblocking temperature of NRM from similar to 600-620 degreesC to similar to 580 degreesC and 2) inc rease in the capacity of laboratory TRM acquisition. The most probable mech anism responsible for the observed changes is supposed to be fast low-tempe rature oxidation of the finest (superparamagnetic) grains and the developme nt of the maghemite shell in coarser single-domain grains. The Thellier pal aeointensity experiments, carried out at the beginning of the study, showed very, good results, which satisfy, all acceptance criteria, applied to eva luation of the results, quite well. Palaeointensity, determinations repeate d 4 Years later on samples from the same material showed the experimental r esults to be of significantly inferior quality. The main difference is the presence of the significant deviation (change in the slope) on the Arai dia gram after T > 350-400 degreesC. The calculated palaeointensity. is either higher than the one obtained before, or similar, but evaluated with large u ncertainty. Therefore, we conclude that the possibility to obtain biased pa laeointensity values increases during short-time storage (i.e. several year s) due to the low-temperature changes of the material.