K¨CAr ages of meteorites: Clues to parent-body thermal histories

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• 作者：Donald D. Bogard  ; ;   ; ^{bogard@lpi.usra.edu} ;   ; ^{donbogard@comcast.net}
• 关键词：Ar&ndash ; Ar ages ; Meteorites ; Thermal histories ; Impacts ; Metamorphism
• 刊名：Chemie der Erde - Geochemistry
• 出版年：2011
• 出版时间：August 2011
• 年：2011
• 卷：71
• 期：3
• 页码：207-226
• 全文大小：833 K

文摘

Whereas most radiometric chronometers give formation ages of individual meteorites >4.5 Ga ago, the K¨CAr chronometer rarely gives times of meteorite formation. Instead, K¨CAr ages obtained by the ³⁹Ar?sup xmlns="""">40Ar technique span the entire age of the solar system and typically measure the diverse thermal histories of meteorites or their parent objects, as produced by internal parent body metamorphism or impact heating. This paper briefly explains the Ar¨CAr dating technique. It then reviews Ar¨CAr ages of several different types of meteorites, representing at least 16 different parent bodies, and discusses the likely thermal histories these ages represent. Ar¨CAr ages of ordinary (H, L, and LL) chondrites, R chondrites, and enstatite meteorites yield cooling times following internal parent body metamorphism extending over ?00 Ma after parent body formation, consistent with parent bodies of ?00 km diameter. For a suite of H-chondrites, Ar¨CAr and U¨CPb ages anti-correlate with the degree of metamorphism, consistent with increasing metamorphic temperatures and longer cooling times at greater depths within the parent body. In contrast, acapulcoites¨Clodranites, although metamorphosed to higher temperatures than chondrites, give Ar¨CAr ages which cluster tightly at ?.51 Ga. Ar¨CAr ages of silicate from IAB iron meteorites give a continual distribution across ?.53?.32 Ga, whereas silicate from IIE iron meteorites give Ar¨CAr ages of either ?.5 Ga or ?.7 Ga. Both of these parent bodies suffered early, intense collisional heating and mixing. Comparison of Ar¨CAr and I¨CXe ages for silicate from three other iron meteorites also suggests very early collisional heating and mixing. Most mesosiderites show Ar¨CAr ages of ?.9 Ga, and their significantly sloped age spectra and Ar diffusion properties, as well as Ni diffusion profiles in metal, indicate very deep burial after collisional mixing and cooling at a very slow rate of ?.2 ¡ãC/Ma. Ar¨CAr ages of a large number of brecciated eucrites range over ?.4?.1 Ga, similar to ages of many lunar highland rocks. These ages on both bodies were reset by large impact heating events, possibly initiated by movements of the giant planets. Many impact-heated chondrites show impact-reset Ar¨CAr ages of either >3.5 Ga or <1.0 Ga, and generally only chondrites show these younger ages. The younger ages may represent orbital evolution times in the asteroid belt prior to ejection into Earth-crossing orbits. Among martian meteorites, Ar¨CAr ages of nakhlites are similar to ages obtained from other radiometric chronometers, but apparent Ar¨CAr ages of younger shergottites are almost always older than igneous crystallization ages, because of the presence of excess (parentless) ⁴⁰Ar. This excess ⁴⁰Ar derives from shock-implanted martian atmosphere or from radiogenic ⁴⁰Ar inherited from the melt. Differences between meteorite ages obtained from other chronometers (e.g., I¨CXe and U¨CPb) and the oldest measured Ar¨CAr ages are consistent with previous suggestions that the ⁴⁰K decay parameters in common use are incorrect and that the K¨CAr age of a 4500 Ma meteorite should be possibly increased, but by no more than ?0 Ma.