Partial self-reversal of TRM in baked soils and ceramics from Ecuador
Résumé
Partial self-reversed thermoremanent magnetizations (SRTRM) have been found in samples of baked soils and hearths from the Rumipamba archeological site near Quito and ceramics from sites near the town of Esmeraldas (Ecuador). The SRTRMs were recognized during thermal demagnetization and in the course of paleointensity determination with the Thellier method where the SRTRM was acquired in the opposed field direction. The SRTRM is often concealed by a stronger normal polarity magnetization on most samples but cooling the samples in liquid nitrogen enhanced the intensity of the SRTRM. The SRTRM at 77°K is often as strong or stronger than the normal component and measurements of the magnetization at 77°K enables the rapid detection of the SRTRM in most samples. AF demagnetization of the SRTRM carried by the baked clays indicate median destructive field (MDF) of the order of 50 mT and thermal demagnetization give principal unblocking temperatures between 280 and 380°C while the normal component of magnetization has higher unblocking temperatures up to 580°C. Anisotropy tensors of SRTRM were determined on eight samples. The high anisotropy degree (>2) observed when the total magnetization is the sum of a normal TRM and a SRTRM is likely amplified because the anisotropy carried by the SRTRM is larger than the one carried by the normal component. Ti-poor titanomagnetite is the main magnetic carrier identified by magnetic measurement in strong field or susceptibility measurements versus temperature. The magnetic carriers of the SRTRM are stable to heating in air or in vacuum up to 600°C suggesting that they are likely not maghemite or titanomaghemite. The baked clays and ceramics from Ecuador carrying the SRTRM share similar magnetic properties (MDF values, unblocking temperatures) with those reported by Bina et al. (1999, Geophys. J. Int. 138, 159-178) for the Pinatubo pumices of the 1991 eruption. In order to test the hypothesis that the SRTRM could be carried by ilmeno-hematite grains, the magnetization of one sample of the self reversed Pinatubo pumices was also measured during warming from liquid nitrogen temperature to room temperature. The similarities in magnetic properties between the SRTRM carried by the baked clays and the self-reversed Pinatubo pumices support the interpretation that detrital hemoilmenite originating from the recent activity of the numerous Ecuadorian volcanoes may be the main carrier of the SRTRM in the archeological samples. A component of SRTRM may be hidden in various types of rock or archeological materials and this may affect the determination of paleointensity of the Earth magnetic field especially when non thermal method are used.