嫦娥五号月壤Rb-Sr同位素组成及其记录的复杂热改造过程

Rb-Sr isotopes record complex thermal modification of Chang’e-5 lunar soils

嫦娥五号月壤是迄今为止返回的最为年轻的月球样品,它为研究月球表面20亿年以来的改造历史提供了绝佳的机会.Rb-Sr同位素体系常被用作定年和示踪的工具,同时由于其放射性体系具有较低的封闭温度以及Rb具有较强的挥发性,该体系可以更好地记录月表的改造细节.本文报道了嫦娥五号返回月壤和玄武岩角砾的全岩Sr同位素以及凝结集块、岩屑和玄武质角砾的原位Sr同位素数据.斜长石具有较为均一的Sr同位素组成,代表了嫦娥五号玄武岩的初始Sr同位素在0.69971±0.0033(2SD,n=22).月壤、玄武岩岩屑和矿物晶屑的Rb-Sr同位素体系不同程度地偏离了20亿年等时线,作者认为这表明这些角砾记录了不同程度的热改造过程,可能与不同程度和不同阶段的Rb丢失有关.这些结果表明嫦娥五号月壤经历了复杂的元素挥发和热改造过程,月壤各组分具有不均一的Rb-Sr同位素组成,与嫦娥五号着陆区不同阶段的陨石撞击事件有关.

Lunar soil is produced by fragmentation-comminution-mixing during numerous impacts of(micro)meteorites and is of great importance to understanding the surface condition and modification history of the Moon and other airless parent bodies[1].As the youngest sample returned from the Moon,the Chang’e-5(CE-5)lunar soil offers us a good opportunity to study the modification history of the~2.0 Ga-old lunar surface[2,3].The Rb-Sr isotope system is widely used in dating and tracing the sources of terrestrial and extraterrestrial samples and is also susceptible to thermal events due to its low closure temperature and high volatility of Rb.It is thus an important database for returned samples from the Moon.Disturbance of Rb-Sr systematics in Apollo basalts has been widely reported(e.g.,[4]).However,to what extent the impact events and relevant thermal metamorphism modified the CE-5 lunar soil,including its components,is still poorly understood.