We report new mineralogical, petrographic and noble gas analyses of the carbonaceous chondrite meteorites Y-82162 (C1/2_ung), Y-980115 (CI1), Y-86029 (CI1), Y-86720 (C2_ung), Y-86789 (C2_ung), and B-7904 (C2_ung). Combining our results with literature data we show that these meteorites experienced varying degrees of aqueous alteration followed by short-lived thermal metamorphism at temperatures of >500 °C. These meteorites have similar mineralogy, textures and chemical characteristics suggesting that they are genetically related, and we strongly support the conclusion of Ikeda (1992) that they form a distinct group, the CYs (“Yamato-type”). The CY chondrites have the heaviest oxygen isotopic compositions (δ¹⁷O ˜12‰, δ¹⁸O ˜22‰) of any meteorite group, high abundances of Fe-sulphides (˜10 ‒ 30 vol%) and phosphates, and contain large grains of periclase and unusual objects of secondary minerals not reported in other carbonaceous chondrites. These features cannot be attributed to parent body processes alone, and indicate that the CYs had a different starting mineralogy and/or alteration history to other chondrite groups, perhaps because they formed in a different region of the protoplanetary disk. The short cosmic-ray exposure ages (≤1.3 Ma) of the CY chondrites suggest that they are derived from a near-Earth source, with recent observations by the Hayabusa2 spacecraft highlighting a possible link to the rubble-pile asteroid Ryugu.

我们报告了碳质球粒陨石Y-82162（C1 / 2 _ung），Y-980115（CI1），Y-86029（CI1），Y-86720（C2 _ung），Y-86789的新矿物学，岩石学和稀有气体分析（C2 _ung）和B-7904（C2 _ung）。将我们的结果与文献数据相结合，我们发现这些陨石在> 500°C的温度下经历了不同程度的水蚀变化，然后经历了短暂的热变质作用。这些陨石具有相似的矿物学，质地和化学特征，表明它们与遗传有关，我们强烈支持Ikeda（1992）的结论，即它们形成了一个独特的族群CYs（“大和型”）。所述CY陨石具有最重的氧同位素组成（δ ¹⁷ø〜12‰，δ ¹⁸ ö约22‰任何陨石基，Fe基硫化物（〜10的高丰度的） - 30％（体积））和磷酸盐，和含有方镁石的大晶粒和次生矿物的不寻常的对象不是在其他报碳质球粒陨石。这些特征不能仅仅归因于母体过程，并且表明CY具有与其他球粒陨石群不同的起始矿物学和/或蚀变历史，可能是因为它们形成于原行星盘的不同区域。CY陨石的短宇宙射线暴露年龄（≤1.3Ma）表明它们来自近地地球，Hayabusa2号航天器最近的观测结果突出表明了其与碎石小行星Ryugu的可能联系。