Triaxe archeointensity analysis
Résumé
Since 2004, numerous archeomagnetic intensity data have been obtained using the vibrating sample magnetometer Triaxe, which measures full-vector magnetization directly at high temperatures, in either an applied or zero field. Satisfactory comparisons have been made between Triaxe intensity data and results derived from more conventional Thellier-Thellier type techniques, indicating the reliability of Triaxe data. For each specimen analyzed, a Triaxe archeointensity value is obtained from the average of R'(Ti) data. The R'(Ti) parameter is determined every 5 °C and corresponds to the ratio, multiplied by the laboratory field intensity, between the natural remanent magnetization (NRM) and laboratory-thermoremanent magnetization (TRMlab) fractions that are lost between reference temperature T1 and a given temperature Ti between T1 and reference temperature T2. Here, we introduce an additional parameter, based on so-called AutoR'(Ti) data, to facilitate and improve the interpretation of Triaxe measurements. Each individual AutoR'(Ti) datum corresponds to an averaged R'(Ti) value; the AutoR'(Ti) dataset is then obtained by gradually decreasing the temperature range from T1 to T2 to a minimum temperature interval near T2. Several examples of Triaxe measurements show the value of using AutoR'(Ti) data to isolate the most appropriate temperature range for an intensity determination, as well as to characterize the cooling rate effect on TRM acquisition. In particular, these experiments confirm that the Triaxe procedure minimizes this effect because, when it is present, it appears to be largely due to magnetic grains with high unblocking temperatures (> ~ 350 °C). Moreover, the AutoR'(Ti) dataset provides alternatives for estimating mean archeointensity values at both the fragment and fragment-group levels. We show that the simple approach used so far, based on the average of the R'(Ti) data determined over a single temperature interval, provides results as reliable as those derived from other options. Supplementary Fig. S2. Triaxe pseudo-NRM analysis of three additional baked-clay artifacts (see Fig. 5): a) a specimen from Hospices de Beaune, France (Genevey et al., 2009); b) a specimen from a temple in Angkor, Cambodia (not yet published); c) a specimen from Axum, Ethiopia (not yet published). The laboratory field intensities used for pseudo-NRM acquisition were 55 μT (a) and 40 μT (b), (c), while a cooling rate of 2 °C/min was used in all cases. Each diagram includes the R(Ti), R'(Ti), and AutoR'(Ti) datasets (blue filled with white, blue and red symbols, respectively) obtained using successively cooling rates of 25 °C/min (dots), 10 °C/min (triangles), and 2 °C/min (inverted triangles). Supplementary Table S1. Archeointensity results obtained from the R'(Ti) data shown in Figs. 3, 4, 5, 6, S1 and S2. For the data presented in Fig. 5 and S2 obtained from pseudo-NRM analyses using different cooling rates for TRMlab acquisition, the differences from the applied laboratory field intensity are also given in % (right column). See text and Table 1 for the definition of the different parameters T1'-T2, K, S.