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Partial self-reversal of TRM in baked soils and ceramics from Ecuador

Abstract : Partial self-reversed thermoremanent magnetizations (SRTRMs) were observed in samples of baked soils, hearths and ceramics from the Rumipamba archeological site near Quito (Ecuador) and ceramics from sites near the town of Esmeraldas (Ecuador). The SRTRMs were recognized at room temperature on few samples but cooling the samples in liquid nitrogen enhanced the intensity of the SRTRM and measurement at 77 K enables its rapid detection in many samples from these sites. Alternating field demagnetization of the SRTRM indicate median destructive field of the order of 50 mT and thermal demagnetization give unblocking temperatures in the temperature range 280-380 C. The magnetic carriers of the SRTRM are stable to heating in air or in vacuum up to 600 C suggesting that titanomaghemite should not be the magnetic carrier of the SRTRM. The studied baked clays and ceramics contain detrital material of mainly volcanic origin. Ti-poor titanomagnetite is the main magnetic carrier identified by strong field data or susceptibility measurements versus temperature. Ilmeno-hematite grains were recognized with microscope observations under reflected light. Scanning electron microscope observations with energy dispersive X-ray spectrometry indicate a Ti/Fe ratio corresponding to an ilmenite content of 0.55. We also compared the magnetic properties of the partially self-reversed baked clays with those of the self-reversed Pinatubo pumices. The SRTRMs were measured upon cooling from room temperature to 20 K with the MPMS. Upon cooling to 20 K the SRTRM show a nearly tenfold increase in intensity with respect to the room temperature measurement. The baked clay and ceramics from Ecuador carrying the SRTRM share similar magnetic properties with the Pinatubo pumices (unblocking temperatures, low temperature behavior) supporting the interpretation that detrital hemoilmenite originating from the Holocene activity of the numerous Ecuadorian volcanoes is the main carrier of the SRTRM in the archeological samples. In some samples, anisotropy tensors of SRTRM show large anisotropy degree (>2). The high anisotropy observed when the total magnetization is the sum of a normal thermoremanent magnetization (TRM) and a SRTRM is likely amplified because the anisotropy carried by the SRTRM is larger than the one carried by the normal component. A component of SRTRM may be hidden in archeological materials containing detrital hemoilmenites related to dacitic/andesitic volcanism and this may affect the determination of paleointensity of the earth magnetic field especially when non thermal method are used.
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Submitted on : Thursday, December 20, 2012 - 11:22:05 AM
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Pierrick Roperch, Annick Chauvin, Francisco Valdez. Partial self-reversal of TRM in baked soils and ceramics from Ecuador. Physics of the Earth and Planetary Interiors, Elsevier, 2012, 210, pp.8-20. ⟨10.1016/j.pepi.2012.08.004⟩. ⟨insu-00767658⟩



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