We report on the petrography and mineralogy of five Yamato polymict eucrites to better constrain the formation and alteration of crustal material on differentiated asteroids. Each sample consists of different lithic clasts that altogether form four dominant textures and therefore appear to originate from closely related petrological areas within Vesta′s crust. The textures range from subophitic to brecciated, porphyritic, and quench‐textured, that differ from section to section. Comparison with literature data for these samples is therefore difficult, which stresses that polymict eucrites are extremely complex in their petrography and investigation of only one thick section may not be representative for the host rock. We also show that sample Y‐793548 consists of more than one lithic unit and must therefore be classified as polymict instead of monomict. The variety and nature of lithic textures in the investigated Yamato meteorites indicate shock events, intense post‐magmatic thermal annealing, and secondary alteration. These postmagmatic features occur in different intensities, varying from clast to clast or among coexisting mineral fragments on a small, local scale. Several clasts within the eucrites studied have been modified by late‐stage alteration processes that caused deposition of Fe‐rich olivine and Fe enrichment along cracks crosscutting pyroxene crystals. However, formation of these secondary phases seems to be independent of the degree of thermal metamorphism observed within every type of clast, which would support a late‐stage metasomatism model for their formation.

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多微卫星的角砾质纹理：南极大和集合中未平衡陨石的岩相学研究

我们报道了五种大和多微晶岩的岩石学和矿物学，以更好地限制分化小行星上地壳物质的形成和蚀变。每个样品由不同的岩性碎屑组成，它们总共形成四个主要的纹理，因此似乎起源于维斯塔地壳内紧密相关的岩石学区域。质地范围从亚次生的到角砾状的，斑状的和淬火的纹理，各节之间有所不同。因此，很难与这些样品的文献数据进行比较，这强调多微晶玉岩的岩石学非常复杂，仅对一个厚壁层的研究可能无法代表基质岩。我们还表明，样本Y‐793548由多个单石单元组成，因此必须归为多poly类而不是单mic类。被研究的大和陨石中的岩性纹理的多样性和性质表明了冲击事件，强烈的后岩浆热退火和二次蚀变。这些后岩浆特征以不同的强度发生，从碎屑到碎屑，或者在小范围局部规模的共存矿物碎片之间变化。后期的蚀变过程已经修饰了所研究的真玉岩中的几块碎屑，这些蚀变过程导致富铁橄榄石的沉积和富铬沿着横穿辉石晶体的裂纹的富集。但是，这些次生相的形成似乎与在每种类型的碎屑中观察到的热变质程度无关，