HR: 15:25h
AN: GP43B-06    [Abstracts]
TI: Nanoparticles in Obsidians Characterized by AC Susceptibility
AU: * Fukuma, K
EM: kfukuma@doshisha.ac.jp
AF: Doshisha University, Tatara-Miyakodani 1-3, Kyotanabe, 610-0394 Japan
AB: Particle size distributions of superparamagnetic (SP) - single domain (SD) particles in obsidians were estimated using alternating current (AC) susceptibility. Magnetic properties of nanoparticles near SP-SD boundary are highly sensitive to particle size variation especially in AC susceptibility. Obsidians are essentially quenched glassy materials but contain abundant nanosize Ti-poor titanomagnetite particles. AC susceptibilities of obsidians samples, which were collected at five source sites of archeological stone tools, were measured using Quantum Design MPMS at 20-300 K in temperature and at 0.1-1000 Hz in AC frequency. These obsidians have Curie temperatures of 540-580 deg.C showing Ti-free or -poor titanomagnetites. Hysteresis parameters of these samples plotted on a Day diagram suggest that some samples contain mixtures of single-domain and multidomain particles or pseudo-single-domain particles and others have high fractions of SP particles. Samples possessing Ti-free titanomagnetites (stoichiometric magnetites) showed a peak of in-phase AC susceptibility at about 120 K irrespective of AC frequency. Two samples of Ti-poor titanomagnetites indicated gradual increases of in-phase susceptibilities with temperature, little frequency dependence and almost no out-of-phase susceptibilities, suggesting that there are no significant SP fractions present in these samples. In contrast, large out-of-phase susceptibilities comparable to in-phase susceptibilities and strong frequency dependence were observed for another two samples. Temperature dependencies both of in-phase and out-of-phase susceptibilities exhibit systematic variations in the peak temperatures according to AC frequency. In addition, susceptibilities continue to decrease even above room temperature and do not show significant Hopkinson peaks below the Curie temperatures. Magnetic characterization of nanoparticles in obsidians should be useful for sourcing stone tools, dating with the viscous remanence, and selecting samples suitable for paleointensity measurements.
DE: 1503 Archeomagnetism
DE: 1519 Magnetic mineralogy and petrology
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005