南极陨石研究的启示Ⅹ:南极陨石的居地年龄

到目前为止 ,陨石学者已考察了约 2 5 0 0km2 的南极蓝色冰区 ,并在南极冰盖不同的地区回收到约 30 0 0 0个陨石样品。在过去 2 0年 ,主要根据36Cl或1 4 C的浓度测定了几百个南极陨石的居地年龄。每个搁浅区的陨石显示不同的居地年龄分布 ,并提供陨石局部堆积机制的信息 ,在蓝色冰表面南极陨石的密度取决于许多因素 ,如陨石的降落率、冰的消融率、雪的堆积率等。大多数南极陨石的居地年龄范围从小于 1 0ka到 1 0 0 0ka ,Yamato山地冰区陨石的居地年龄可高达 2 0 0ka,而LewisCliff及AllanHills陨石的居地年龄分别高达 5 0 0ka和 1 0 0 0ka,新近发现两个陨石的居地年龄分别为 2Ma及 2 .35Ma,表明它们深埋于靠近冰川底部 ,且冰流率比表面低得多 ,同时也表明陨石堆积进入目前的搁浅区至少始于 2Ma以前 ,这与东南极冰盖为稳定和持续性假设是一致的。陨石的居地年龄是确定陨石历史的一个重要参数 ,同时也可用以估计陨石的搬运时间及平均风化寿命。

INSPIRATION FROM STUDY OF ANTARCTIC METEORITES-X:TERRESTRIAL AGES OF ANTARCTIC METEORITES

Up to date, meteorite scientists have searched about 2500km 2 of blue ice and recovered more than 30000 meteorite specimens from different locations on the Antarctic ice sheet. In the past two decades ,terrestrial ages of a few hundred meteorites were determined, mainly on the basis of the 36 Cl or 14 C concentrations. Each stranding area shows a different terrestrial age distribution and provides the information of the local accumulation mechanisms. The Antarctic meteorite density on the blue ice surface are determined by many factors, such as the meteorite fall rate, the ice ablation rate and the snow accumulation rate. Meteorites from Yamato Mountains Icefields have ages up to 200ka,whereas Lewis Cliff and Allan Hills specimens have ages up to 500ka and 1Ma, respectively. Recently, two meteorites have been discovered with exceptional ages of about 2Ma from Allan Hills and 2.35Ma from Lewis Cliff, which indicate that they are deeply inside the ice, near the base of glacier and ice flow rates are much lower than at surface. Their terrestrial ages also indicate the process of meteorite accumulation into present day stranding areas started at least 2 million years ago and the long terrestrial ages are consistent with an assumption concerning the stability and persistence of the East Antarctic ice sheet. The residence time on the surface is an important parameter in determining the history of the meteorite. In addition, terrestrial ages can be used to estimate the average transport time of meteorites from accumulation to ablation area and the mean weathering lifetime.