L群普通球粒陨石母体的撞击历史

Impact history of L chondrite parent body

撞击作用发生在太阳系形成和演化的所有阶段,是最基本的地质过程之一。陨石可以从微观尺度记录下这些重要的过程。在所有陨石族群中,L群普通球粒陨石保留了最完备的冲击变质记录,对撞击发生的时间、冲击过程中的物理条件提供了重要制约。矿物学证据表明,在太阳系形成100 Ma内,L群陨石母体可能发生一次撞击裂解事件,并在随后重组。4.48 Ga左右,原始小行星带经历大范围的撞击作用,这一事件也记录于L群普通球粒陨石中,可能是由月球大撞击事件溅射的大量碎屑进入到原始主小行星带引起。约800 Ma,包括L群陨石母体在内的内太阳系部分天体经历了同时期撞击事件,可能由这一时期裂解的大质量小行星产生的溅射物引发。L群陨石母体在~465 Ma发生撞击裂解,这一事件在L群陨石中保留了丰富的矿物学、年代学记录,并在地球全球奥陶纪地层发现相关信息。综合与该事件相关的所有L群陨石冲击变质特征,本文认为该裂解事件是由一颗大直径(18~22 km)石陨石质小行星,以较低速率(5~6 km/s)撞击导致。同位素年代学数据表明,L群普通球粒陨石母体很可能未受到晚期大撞击事件的影响,这难以用L群陨石母体过小予以解释。可能的原因有:(1)L群普通球粒陨石母体在原始主小行星带分布非常有限,导致其受到晚期大撞击事件影响的概率不高;(2)晚期大撞击事件对原始主小行星带的影响可能并没有之前估计的那么严重,一个持续时间更长但更加温和的撞击模型更加符合现阶段的观察。

Impacts are the fundamental process in the formation and evolution of planets and small bodies of the Solar System.These shock process could be recorded by meteorites at microscale levels.Among all meteorite groups,the L group ordinary chondrites retain the most complete record of shock metamorphism,which provides important constraints on the timing and physical conditions of the impacts.Mineralogical evidence indicates that within 100 Ma of the formation of the Solar System,the L chondrite parent body(LCPB)may experience catastrophic distribution and be reassembled shortly after.At around 4.48 Ga,the primordial asteroid belt was widely impacted,speculatively resulting from the peak arrival time of ejecta debris from the Moon-forming giant impact at the primordial main belt.Some celestial bodies in the inner solar system,including the LCPB,experienced a contemporaneous impact period at about 800 Ma,which was possibly caused by the disruption of a large asteroid at this time.Abundant chronological and mineralogical evidence from L chondrites indicate the distribution of LCPB at approximately 465 Ma.This event also affected the Earth by a dramatic increase in the flux of L-chondritic material and left prominent traces in stratigraphic records.Combined with available literature data of L chondrites associated with this catastrophic event,our results suggest that the LCPB suffered a catastrophic collision with a large projectile(with a diameter of at least 18~22 km)at a low impact velocity(5~6 km/s).The chronological record of L chondrite suggests that LCPB was minimally affected by the Late Heavy Bombardment(LHB).This may be because the distribution of L chondrite asteroid source is limited in main asteroid belt,or the primordial asteroid belt was less intensely collided than originally envisaged.Therefore,a more extended and more modest collision rather than a prominent and narrow impact spike at 3.9 Ga of LHB is implied.