Martin R. Lee, Luke Daly, Jennika Greer, Sammy Griffin, Cameron J. Floyd, Levi Tegg, Julie Cairney
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引用次数: 0
摘要
许多CM碳质闪长岩都是碎屑岩,因此应该有大量碰撞加工的证据。这些碎屑岩的组成碎块经常显示出压实岩性;然而,橄榄石的微观结构显示,大多数CM是未受冲击的。为了更好地理解这一矛盾的原因,我们利用 Cold Bokkeveld 陨石寻找 CM 软骨超高速撞击加工的其他证据。我们发现,这块碎屑岩含有罕见的泡状冲击熔体颗粒,其化学成分与块状CM软玉很接近。对一颗熔珠的透射电子显微镜观察表明,它是由硅酸盐玻璃组成的,其中夹杂着彭特兰石、黄铁矿和黑钨矿。通过原子探针断层扫描法对另一颗珠子的碎片进行表征,发现了纳米级的硫磺团块,这代表了在生长早期阶段被截获的硫化物包裹体。这些玻璃微粒在矿物学上可与 Cb 型小行星龙宫的微流星体撞击熔体以及通过脉冲激光照射 CM 目标实验产生的熔体相媲美。这些玻璃可能是在原地冲击熔化形成的,但岩石学证据表明,它们更像是来自远端撞击的弹道抛射物。这块陨石以及更广泛的 CM 软骨中熔体的稀少与含水小行星在高能碰撞后的爆炸性碎裂是一致的。Cold Bokkeveld 的母体很可能是第二代小行星,由一个或多个早期天体的碎片构成,只有一小部分重新生成的物质经过高度震荡和熔化。
Shock melt in the Cold Bokkeveld CM2 carbonaceous chondrite and the response of C-complex asteroids to hypervelocity impacts
Many of the CM carbonaceous chondrites are regolith breccias and so should have abundant evidence for collisional processing. The constituent clasts of these fragmental rocks frequently display compactional petrofabrics; yet, olivine microstructures show that most CMs are unshocked. To better understand the reasons for this contradiction, we have sought other evidence for hypervelocity impact processing of CM chondrites using the Cold Bokkeveld meteorite. We find that this regolith breccia contains rare particles of vesicular shock melt that are close in chemical composition to bulk CM chondrite. Transmission electron microscopy of a melt bead shows that it is composed of silicate glass with inclusions of pentlandite, pyrrhotite, and wüstite. Characterization of shards of another bead by atom probe tomography reveals nanoscale clusters of sulfur that represent sulfide inclusions arrested at an early stage of growth. These glass particles are mineralogically comparable to micrometeoroid impact melt described from the Cb-type asteroid Ryugu and melt that has been experimentally produced by pulsed laser irradiation of CM targets. The glass could have formed by in situ shock-melting, but petrographic evidence is more consistent with an origin as ballistic ejecta from a distal impact. The scarcity of melt in this meteorite, and CM chondrites more broadly, is consistent with the explosive fragmentation of hydrous asteroids following energetic collisions. Cold Bokkeveld's parent body is likely to be a second-generation asteroid that was constructed from the debris of one or more earlier bodies, and only a small proportion of the reaccreted material had been highly shocked and melted.
期刊介绍:
First issued in 1953, the journal publishes research articles describing the latest results of new studies, invited reviews of major topics in planetary science, editorials on issues of current interest in the field, and book reviews. The publications are original, not considered for publication elsewhere, and undergo peer-review. The topics include the origin and history of the solar system, planets and natural satellites, interplanetary dust and interstellar medium, lunar samples, meteors, and meteorites, asteroids, comets, craters, and tektites. Our authors and editors are professional scientists representing numerous disciplines, including astronomy, astrophysics, physics, geophysics, chemistry, isotope geochemistry, mineralogy, earth science, geology, and biology. MAPS has subscribers in over 40 countries. Fifty percent of MAPS'' readers are based outside the USA. The journal is available in hard copy and online.