Thermal History and Trapped ⁴⁰Ar/³⁹Ar Ratio on Xinyang Meteorite

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.