根据40Ar/39Ar年龄谱探讨陨石冲击事件的分期

⁴⁰Ar/³⁹Ar年龄谱是研究陨石冲击事件的重要资料。根据对55块陨石⁴⁰Ar/³⁹Ar冲击年龄和陨石暴露年龄的分析,发现陨石的冲击年龄与陨石类型之间存在对应关系。据此,将陨石冲击事件划分为九期。其中3900-4000Ma、470-540Ma和小于65Ma是陨石母体的三个重要演化阶段。阶段Ⅰ、Ⅱ和Ⅲ(冲击年龄大于30亿年)主要涉及高钙型无球粒陨石。所有球粒陨石的冲击年龄均小于30亿年。陨石暴露年龄因类型而异,铁陨石最大,石铁陨石次之,石陨石最小。

AN APPROACH TO COLLISIONAL STAGES ON METEORITES BY ~(40)Ar/~(39)Ar AGE SPECTRA

⁴⁰Ar/³⁹Ar age spectra of meteorites are very useful data for the study on the collisional events in parent bodies of meteorites. Turner(1969)has calculated theoretical 40Ar/89Ar release profiles based upon diffusion loss of 40Ar* from a log normal distribution of spherical grains brought about by the effect of a discrete reheating events subsequent to the original crystallization and cooling of the object. In the ideal case the time of the degassing event is given by the ⁴⁰Ar/³⁹Ar ages found for gas released at the lowest temperatures, and the ⁴⁰Ar/³⁹Ar ages found for gas fractions released at the higher temperatures should provide a minimum age for the original cooling of the object. Many chondrites show disturbed ⁴⁰Ar/³⁹Ar age spectra, similar to those predicted from the Turner model, and Bogard(1976)was gave a summary of such studies. Following the investigations on fifty-five meteorites ⁴⁰Ar/³⁹Ar collisional ages and the exposure ages of meteorites, the relation between collisional ages and types of meteorites is found. And the collisional events are divided into nine stages as follows.Stage Ⅰ: 4400-4100 Ma, Pre-lmbrium Event. It concerns mainly Ca-rich achondrites such as Bununu, Petersburg and Pasamonte, and one enstatite chondrite Abee.Stage Ⅱ: 4000-3900 Ma, Imbrium Event, animportant evolution stage for parent bodies of meteorites.Stage Ⅲ: 3800-3000 Ma, it is related to Ca-rich achondrites mainly, for example, Stan-nern, Kapoeta, Bununu, Cachari, and one iron meteorite El Taoco.Stage Ⅳ: 3000-2100 Ma, concerning H and L chondrites only, such as Ucrea, Monore, Malakal, Rose city, Kirin(Jilin)-1 and Xinyang.Stage Ⅴ: 1600-1300 Ma, it is related to Ca-rich achondrites only, for example, Malven, Lafayette and Nakhla.Stage Ⅵ: 1100-600 Ma, it concerns ten stony meteorites and stony iron meteorites, and the exposure of iron meteorites.Stage Ⅶ: 540-470 Ma, it is a very important collisional event on L and H chondrites. Thirteen L chondrites and two H chondrites(Kirin-2 and Kirin-5)which recorded the collisional event were found.Stage Ⅷ: 420-200 Ma, concerning seven ordinary chondrites and one achondrite Sher-gotty.Stage Ⅸ:<65 Ma, an important exposure event of stony meteorites, and a collisional event of H and L chondrites.The stage Ⅱ, Ⅶ and Ⅸ are the important evolution stages in parent bodies of meteorites. Stage Ⅰ, Ⅱ and Ⅲ(the collisional ages more than 3.0 Ga)concern mainly Ca-rich achondrites. The collisional ages of all chondrites are less than 3.0 Ga. And the exposure ages of iron meteorites are larger than those of stony irons which are in turn larger than those of stony meteorites.