Abstract Two samples of a unique achondritic lithology of the Almahata Sitta meteorite (MS-MU-019 and MS-MU-036) contain three coexisting pyroxene species: orthoenstatite, clinoenstatite and augite. The silicate assemblage appears to be the restite after extraction of melts of broadly basaltic and metal-sulfide composition from an enstatite chondrite protolith. Transmission electron microscopy (TEM) provides evidence that clinoenstatite within the lithology formed from earlier protoenstatite. The absence of pigeonite despite the successful nucleation of augite and the persistence of orthoenstatite during cooling suggests that the sub-solidus formation of the three coexisting pyroxenes occurred at a pressure of about 0.1 GPa. Rapid cooling at >1 K/h below 1260°C is documented by the cessation of augite equilibration, preservation of the 3-pyroxene assemblage and a low volume fraction of nanoscale orthoenstatite lamellae formed during the transformation of protoenstatite to clinoenstatite. The pressure implies a diameter of roughly 500 km of the differentiated parent body, putting petrological constraints on the size of planetesimals that may have contributed to the accretion of the terrestrial planets including Earth. The high cooling rate indicates a catastrophic disruption of this large planetesimal early in its history. The lithology studied here underlines that planetesimals which existed in the inner Solar System were more diverse than previously thought, and included potentially large differentiated bodies with very FeO-poor, enstatite-dominated mantles. Remains of these bodies are poorly represented in meteorite collections, which points to efficient accretion in the inner Solar System or to removal and little re-distribution of material into the present-day asteroid belt.

中文翻译：

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500 公里大小的顽火-球粒陨石家族分化星子的存在和破坏的岩石学证据

摘要 Almahata Sitta 陨石（MS-MU-019 和 MS-MU-036）的两个独特的无球粒陨石岩性样品包含三种共存的辉石种类：正辉石、斜辉石和辉石。硅酸盐组合似乎是从顽火石球粒陨石原岩中提取了广泛的玄武岩和金属硫化物组成的熔体后的残岩。透射电子显微镜 (TEM) 提供的证据表明，岩性中的斜辉石是由早期原顽辉石形成的。尽管辉石成功成核，并且在冷却过程中仍然存在正辉石，但没有变辉石表明三种共存辉石的亚固相线形成发生在大约 0.1 GPa 的压力下。低于 1260°C 以 >1 K/h 的快速冷却记录为辉石平衡的停止，3-辉石组合的保存和在原顽辉石向斜辉石转变过程中形成的低体积分数的纳米级正辉石片层。压力意味着分化的母体的直径大约为 500 公里，这对可能有助于包括地球在内的类地行星的吸积的微星的大小施加了岩石学限制。高冷却速度表明这个大型星子在其历史早期遭受了灾难性的破坏。这里研究的岩性表明，存在于太阳系内部的星子比以前认为的更加多样化，并且包括潜在的大型分化天体，其中包含非常贫乏的 FeO、顽火石占主导地位的地幔。这些尸体的残骸在陨石收藏中的代表性很差，