Abstract Most meteorites are believed to be chips from planetesimals and can provide clues to constrain the chemical evolution and dynamic history of the early Solar System. In this study, we report two unique fragments (ALF-1 and ALF-2) enclosed in the CH3 carbonaceous chondrite Sayh al Uhaymir 290. These two fragments are dominated by Ca,Fe-rich olivine with various amounts of Al,Ti-rich augite, anorthite, oxide minerals, Ca-phosphate mineral, FeNi metal, enstatite, and less Al,Ti-rich augite. The Ca-Mg-Fe systematics and Fe/Mn ratios of olivine grains in the two fragments are similar to that of the volcanic angrites. These similar features imply that the parent body of the two fragments might have bulk chemistry, oxygen fugacity, and differentiation resembling the angrite parent body. However, high-precision SIMS measurements reveal oxygen isotope compositions Δ17O = 0.91 ± 0.18‰ (2σ) of olivine in the two fragments are distinctly different from that of known angrite meteorites, possibly representing a new type of basaltic planetesimal. The two fragments also have a few mineralogical features distinct from angrite meteorites. They include: (1) the lack of a typical igneous texture; (2) the coexistence of two spatially associated Al,Ti-rich augites with different contents of CaO, Al2O3, MgO, and P2O5; (3) the presence of an enstatite-dominant rim in ALF-1; (4) the presence of a Cr,Mn-rich margin in ALF-1; and (5) complex microscale heterogeneity in oxide minerals. We argue that these features could be due to complex thermal histories in their parent body and/or after ejection from their parent body. Among these features, the Mn,Cr-rich margin and the enstatite-dominant rim in ALF-1 can be best explained with thermal events in nebular settings. This implies that the differentiation of the parent body of the two fragments might have taken place prior to the dissipation of the nebular gas.

中文翻译：

---

CH3球粒陨石中独特的类似菱镁矿的碎片揭示了一种新的玄武质星子

摘要 大多数陨石被认为是来自小行星的碎片，可以为限制早期太阳系的化学演化和动力学历史提供线索。在这项研究中，我们报告了包含在 CH3 碳质球粒陨石 Sayh al Uhaymir 290 中的两个独特碎片（ALF-1 和 ALF-2）。这两个碎片主要是富含 Ca、Fe 的橄榄石和不同量的 Al、Ti辉石、钙长石、氧化物矿物、Ca-磷酸盐矿物、FeNi金属、顽火石和较少的富含Al、Ti的辉石。两个碎片中橄榄石晶粒的Ca-Mg-Fe体系和Fe/Mn比与火山灰橄榄石相似。这些相似的特征意味着两个碎片的母体可能具有与 angrite 母体相似的整体化学、氧逸度和分化。然而，高精度 SIMS 测量显示，两个碎片中橄榄石的氧同位素组成 Δ17O = 0.91 ± 0.18‰ (2σ) 与已知的石榴石陨石明显不同，可能代表了一种新型的玄武质微星。这两个碎片还具有一些不同于菱镁矿陨石的矿物学特征。它们包括：（1）缺乏典型的火成岩结构；(2) 两种不同CaO、Al2O3、MgO、P2O5含量的富铝、钛辉石共存；(3) ALF-1 中存在以顽火辉石为主的边缘；(4) ALF-1 中存在富铬、锰的边缘；(5) 氧化物矿物中复杂的微观非均质性。我们认为，这些特征可能是由于母体中和/或从母体中弹出后的复杂热历史所致。在这些特征中，Mn、星云环境中的热事件可以最好地解释 ALF-1 中富含铬的边缘和顽火岩占主导地位的边缘。这意味着两个碎片的母体的分化可能发生在星云气体消散之前。