干旱区生态环境知识资源中心

We conducted a detailed paleomagnetic study of 18 ordinary chondrites recovered in a hot desert (the Atacama desert, Chile) to clarify the significance of the natural remanent magnetizations of weathered meteorites. We show that the more weathered meteorites (weathering grade W3) possess a stable magnetization unblocked over two temperature intervals (140-320 degrees C, and 500-550 degrees C). This magnetization was acquired on Earth as indicated by the paleomagnetic directions of large meteorites (>150 g) that are in the direction of the terrestrial field. Rock magnetic data suggest that this stable magnetization is a chemical remanent magnetization acquired by maghemite and magnetite upon their formation as weathering products. On the other hand, the natural remanent magnetization of ordinary chondrites showing little or no weathering (weathering grades W0 and W1) are much weaker, and very unstable upon demagnetization, a similar behavior to the magnetization of meteorite falls. Therefore, up to the weathering grade W1, weathering does not largely overprint the original extraterrestrial magnetization of these meteorites. This is confirmed at microscopic scale by magnetic imaging: tetrataenite and kamacite have randomly oriented magnetization at mineral scale that is canceled out at larger scale (>1 mm(3)), whereas a small fraction of the weathering products has a weak but unidirectional magnetization that masks the weak extraterrestrial magnetization. This suggests that extraterrestrial magnetization can survive mild weathering (below weathering grade W2), and that such meteorites remain worthwhile targets for study of extraterrestrial paleomagnetism. For meteorites with weathering grade W3 or more, the extraterrestrial signal can only be studied at microscopic scale. These data also show that over a time scale of several kyr to several tens of kyr, large stones (>150 g) have remained still at the surface of the Atacama desert whereas smaller ones have moved only by rotation around vertical axes, evidencing the fine scale stability of this desertic surface. (C) 2012 Elsevier B.V. All rights reserved.