HR: 17:50h
AN: GP24A-07    [Abstracts]
TI: Age of the Xalnene Ash, Central Mexico: A Rock Magnetic Perspective
AU: * Feinberg, J M
AF: Department of Earth Sciences University of Cambridge, Downing Street, Cambridge, CB2 3EQ, United Kingdom
AU: * Feinberg, J M
AF: Department of Geology & Geophysics University of Minnesota, 310 Pillsbury Drive, Minneapolis, MN 55455, United States
AU: Renne, P R
AF: Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709, United States
AU: Renne, P R
AF: Department of Earth and Planetary Science University of California, 310 McCone Hall, Berkeley, CA 94720, United States
AU: Waters, M R
AF: Departments of Anthropology and Geography and the Center for the Study of the First Americans, Texas A & M University, College Station, TX 77843, United States
AU: Arroyo-Cabrales, J
AF: Laboratório de Arqueozoología Subdirección de Laboratórios y Apoyo Académico, Instituto Nacionál de Antropología e Historia, CP 06060, Mexico
AU: Ochoa-Castillo, P
AF: Subdireccíon de Arqueología, Museo Nacional de Antropología, Reforma y Gandhi, CP 11560, Mexico
AU: Perez-Campo, M
AF: Proyecto Cuicuilco, Instituto Nacionál de Antropología e Historia, CP 06700, Mexico
AB: Features interpreted as human footprints ~40 ka old have been reported from the Toluquilla quarry near Valsequillo Reservoir, 15 km south of the city of Puebla in central Mexico (Gonzalez et al., 2006). The indentations were found in an indurated basaltic lapilli tuff informally known as the Xalnene Ash. The ~40 ka age of the tuff was based on optically stimulated luminescence (OSL) analysis of baked quartz-rich materials incorporated into the ash during eruption. If true, this ~40 ka age suggests that humans migrated into the Americas at a far earlier time than is currently believed. In contrast to the conclusions of Gonzalez et al. (2006), the age of the Xalnene ash was determined as 1.30 ± 0.03 Ma (2σ) based on highly reproducible ⁴⁰Ar/³⁹Ar analyses of homogenous basaltic lapilli (Renne et al., 2005). This age is further supported by paleomagnetic data, which shows a reversed polarity for the Xalnene Ash, consistent with deposition during chron C1r.2r (1.77 to 1.07 Ma). The magnetic mineralogy within the Xalnene Ash is not straightforward, and is a reflection of its volcanic origin modified by its subsequent hydrous alteration via prolonged exposure to varying water levels in Lake Valsequillo. Submicron titanomagnetite grains occur within the cores of the lapilli and are characterized by Curie temperatures between 400-500 °C corresponding to unoxidized compositions ranging from Fe_2.82Ti_0.18O₄ to Fe_2.68Ti_0.32O₄. Two components of magnetization were revealed during progressive alternating field (AF) demagnetization of oriented, individual lapilli: a low coercivity, reversed component that is typically removed after 15-20 mT, and a randomly oriented, but well-defined, higher coercivity component. The directions associated with the low coercivity component are consistent with the direction of the bulk rock characteristic remanent magnetization. This low coercivity component may have been acquired immediately after deposition during the final stages of cooling, or during subsequent reheating relating to the deposition of the overlying ash layer. We interpret the high coercivity component to be related to pre- depositional cooling of the lapilli. Hydrous alteration along the margins of the lapilli has formed clay rinds containing a multitude of iron oxides, including ferrihydrite, goethite, and hematite. Although the duration of this magnetic mineral growth is unknown, these minerals record a secondary component of magnetization that is persistent in bulk sample AF demagnetization experiments. The thin ash beds that comprise the Xalnene Ash show varying degrees of induration and hydrous alteration, indicating that the relative magnetic contribution of the hydrous alteration products will vary from layer to layer. Thus, extreme care needs to be taken when interpreting paleodirectional data obtained from the Xalnene Ash.
DE: 1503 Archeomagnetism
DE: 1512 Environmental magnetism
DE: 1519 Magnetic mineralogy and petrology
DE: 1540 Rock and mineral magnetism
DE: 8404 Volcanoclastic deposits
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Joint Assembly