Garvie Laurence A J, Knauth L Paul, Morris Melissa A
Center for Meteorite Studies, Arizona State University, Tempe, AZ 85287-6004, USA.
School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-6004, USA.
Icarus. 2017 Aug;292:36-47. doi: 10.1016/j.icarus.2017.03.021. Epub 2017 Apr 2.
Prominent macroscopic sedimentary laminations, consisting of mm- to cm-thick alternating well-sorted but poorly mixed silicate and metal-rich layers cut by faults and downward penetrating load structures, are prevalent in the Isheyevo (CH/CBb) carbonaceous chondrite. The load structures give the up direction of this sedimentary rock that accumulated from in-falling metal- and silicate-rich grains under near vacuum conditions onto the surface of an accreting planetesimal. The Isheyevo meteorite is the end result of a combination of events and processes that we suggest was initiated by the glancing blow impact of two planetesimals. The smaller impactor was disrupted forming an impact plume downrange of the impact. The components within the plume were aerodynamically size sorted by the nebular gas and swept up by the impacted planetesimal before turbulent mixing within the plume could blur the effects of the sorting. This plume would have contained a range of materials including elementally zoned Fe-Ni metal grains that condensed in the plume to disrupted unaltered material from the crust of the impactor, such as the hydrated matrix lumps. The juxtaposition of hydrated matrix lumps, some of which have not been heated above 150 °C, together with components that formed above 1000 °C, is compelling evidence that they were swept up together. Sweep-up would have occurred as the rotating impactor moved through the plume producing layers of material: the Isheyevo sample thus represents material accumulated while that part of the rotating planetesimal moved into the plume. Vibrations from subsequent impacts helped to form the load structures and induced weak grading within the layers via kinetic sieving. Following sweep-up, the particles were compacted under low static temperatures as evidenced by the preservation of elementally zoned Fe-Ni metal grains with preserved martensite cores, distinct metal-metal grain boundaries, and metal-deformation microstructures. This meteorite provides evidence of gentle layer-by-layer accretion in the early Solar System, and also extends the terrestrial sedimentary source-to-sink paradigm to a near vacuum environment where neither fluvial nor aeolian processes operate.
显著的宏观沉积纹层在伊舍耶沃（CH/CBb）碳质球粒陨石中很常见，这些纹层由毫米至厘米厚的交替排列、分选良好但混合不佳的硅酸盐层和富金属层组成，被断层切割并向下穿透负载构造。负载构造确定了这种沉积岩的向上方向，它是在近真空条件下，由富含金属和硅酸盐的颗粒落到正在吸积的小行星表面堆积而成的。伊舍耶沃陨石是一系列事件和过程的最终结果，我们认为这些事件和过程始于两个小行星的 glancing blow 撞击。较小的撞击体被破坏，在撞击的下游形成了一个撞击羽流。羽流中的成分通过星云气体进行空气动力学尺寸分选，并在羽流内的湍流混合模糊分选效果之前被受撞击的小行星扫起。这个羽流会包含一系列物质，包括在羽流中凝聚的元素分带的铁镍金属颗粒，以及来自撞击体地壳的未改变的破碎物质，如水化基质块。一些未加热到150°C以上的水化基质块与在1000°C以上形成的成分并列，这有力地证明了它们是一起被扫起的。扫起过程会在旋转的撞击体穿过羽流时发生，产生物质层：因此，伊舍耶沃样品代表了旋转小行星的该部分移入羽流时积累的物质。后续撞击产生的振动有助于形成负载构造，并通过动力学筛分在层内诱导出微弱的粒级变化。扫起之后，颗粒在低静态温度下被压实，这从保存有马氏体核心、清晰的金属 - 金属晶界和金属变形微观结构的元素分带铁镍金属颗粒中得到证明。这块陨石提供了早期太阳系中逐层缓慢堆积的证据，并且还将陆地沉积源到汇的范式扩展到了一个既没有河流作用也没有风成作用的近真空环境。
