Thermal History and Trapped 40Ar/39Ar Ratio on Xinyang Meteorite
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摘要: 信阳陨石于1977年12月1日18时57分, 陨落在我国河南省信阳县肖王公社。陨落后, 破裂为X-Ⅰ(48kg)及X-Ⅱ(27.5kg)两大块和几小块碎屑。陶克捷等对该陨石做过化学组成、矿物组分及结构特征研究, 指出属H6型。本文试图根据40Ar/39Ar快中子活化资料及原始捕获氩同位素特征, 探讨信阳陨石的演化特点。Abstract: The Xinyang meteorite fell on Dec. 1, 1977, in the Xiaowang Village of Xinyang County, Henan Province, China, It consists of X-Iand X-IIfragments mainly, and the masses are 48 kg and 27.5 kg respectively. Xinyang meteorite is an ordinary chon-drite, classified as belonging to group H6. The 40Ar/39Ar age spectrum of X-II is quite complex, but shows diffusional loss profiles. The gas released at low temperature yields low ages, with the apparent age increasing monotonically with increasing temperature, but the steps 3, 4 and 5 show a plateau which has a age of 4.42±0.1 b.y., followed by a decrease in apparent age, and then rising again to a plateau at 4.46 ±0.1 b.y. for the last 20% for gas release, at temperature greater than 960℃. The low-middle temperature release of abundant 40Ar* is associated with Ca-poor phases, undoubtedly the plagioclase. The high temperature release of 40Ar* is from Ca-rdch phases with low K/Ca ratios, almost certainly the bro-nzite. Thus it is possible that the ages of bronzite and plagioclase are 4.46 and 4.42 b.y. respectively. The cooling rates of Xinyang meteorite are 1.7 to 8.7℃/106 yr. during the early stage of evolution assuming that the closure temperature of 40Ar* of bronzite and plagioclase are 600° and 250℃ respectively, and the Rb-Sr age of this meteorite is 4.55 b.y. and 87Sr* closure temperature is 750℃.The 40Ar/39Ar age spectrum of Xinyang meteorite shows that there was a collisional event at 2.95 b.y. ago. And this meteorite has a exposure age of 6.0 m.y.. Atrapped 40Ar/36Ar ratio of Xinyang meteorite, 1.07±0.36, is obtained. Note that most of trapped 40Ar/36Ar ratios of lunar soil and breccias are about 1, it is possible that the trapped argon of lunar and meteorites originated in the solar wind.
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