PGA data of Antarctic carbonaceous chondrites with CI and CM affinity

At present, carbonaceous chondrites are classified into nine groups based on their chemical characteristics. Among carbonaceous chondrites, CI and CM chondrites are considered the more primitive meteorites. Currently, there are five known non-Antarctic CI chondrites, and four have been identified among Antarctic meteorites. In this study, elemental analysis was conducted on a total of twelve meteorites, focusing primarily on the major element composition, including four Antarctic CI chondrites and eight others having affinities with CI or CM but insufficiently chemically clarified. The analytical method employed in this study was neutron-induced prompt gamma-ray analysis (PGA), which shares analytical advantages similar to wet chemical analysis. Quantitative values were obtained for 15 elements: Fe, Mg, Si, S, Ni, H, Al, Ca, Cr, Mn, Co, Ti, chlorine, B, and Gd, and almost all of the main constituent elementals were quantified. Comparison with literature values, including those obtained by the wet chemical method, confirmed that the values obtained by the PGA method in this study are sufficiently reliable with the exception of B. Oxygen isotopic compositions of five of the 12 meteorites analyzed, including the Y-86029 meteorite classified as CI, have been reported, and based on the results, a new group name CY was proposed for these meteorites. PGA analysis showed that, in common with the 12 meteorites, H and chlorine are more deplete than those in non-Antarctic CI chondrites. Notably, the degree of this depletion varies significantly between meteorites and exhibits a correlation between the two elements. Petrological and mineralogical observations suggest that these meteorites underwent thermal metamorphism after aqueous alteration on their parent body, being consistent with the observed depletion of chlorine and H. Based on these findings, it was deemed appropriate to classify the analyzed twelve meteorites as CY chondrites. Based on the abundance of Mn and S, it was revealed that these twelve meteorites are classified into two distinct groups. Since the abundance of Mn and S is expected to remain unchanged through aqueous alterations and thermal metamorphism on the parent body, the differences in their abundance likely originated during the formation processes of the parent body, reflecting elemental fractionation during condensation and subsequent accumulation. The Mn and S compositions suggest that the CY chondrites originated from two parent bodies (or parent materials), one formed under conditions similar to non-Antarctic CI, and the other formed through a process intermediate between CM and CI. For facilitating further discussion on CY chondrites, it is proposed to refer to these two groups as CYi and CYm, representing those with compositions closer to CI and CM, respectively.

当前，碳质球粒陨石（carbonaceous chondrites）可依据化学特征划分为9个群类。在碳质球粒陨石中，CI型与CM型球粒陨石被认为是较为原始的陨石。目前已知非南极来源的CI型球粒陨石共有5块，南极陨石中已确认4块CI型球粒陨石。本研究共对12块陨石开展元素分析，重点关注其常量元素组成，其中包含4块南极CI型球粒陨石，以及8块与CI或CM型陨石具有亲缘关系但化学特征尚未明确界定的陨石。本研究采用的分析方法为中子诱发瞬发γ射线分析（PGA），该方法具备与湿化学分析（wet chemical analysis）相似的分析优势。本研究对15种元素获得了定量结果：铁（Fe）、镁（Mg）、硅（Si）、硫（S）、镍（Ni）、氢（H）、铝（Al）、钙（Ca）、铬（Cr）、锰（Mn）、钴（Co）、钛（Ti）、氯、硼（B）以及钆（Gd），几乎覆盖了所有主要构成元素。与包括湿化学分析方法在内的文献数据进行对比后证实，除硼元素外，本研究通过PGA方法获得的定量结果具备足够可靠性。在本次分析的12块陨石中，包括被归类为CI型的Y-86029陨石在内的5块陨石的氧同位素组成已有报道，基于该结果，研究人员为这类陨石提出了新的群类名称CY。PGA分析结果显示，与12块陨石整体特征一致，氢与氯的丰度较非南极CI型球粒陨石更低。值得注意的是，这两种元素的贫化程度在不同陨石间存在显著差异，且二者呈现出相关性。岩石学与矿物学观测结果表明，这类陨石在母天体经历水蚀变后又经受了热变质作用，这与观测到的氢与氯贫化现象相符。基于上述发现，本次分析的12块陨石被划归为CY型球粒陨石是合理的。依据锰与硫的丰度，可将这12块陨石划分为两个截然不同的群类。由于锰与硫的丰度在母天体的水蚀变与热变质过程中预计不会发生改变，因此其丰度差异大概率起源于母天体的形成过程，反映了冷凝阶段的元素分馏以及后续的聚集过程。锰与硫的组成特征表明，CY型球粒陨石起源于两个母天体（或母源物质）：一个的形成条件与非南极CI型球粒陨石相似，另一个则形成于介于CM型与CI型球粒陨石之间的演化过程。为便于后续对CY型球粒陨石开展讨论，研究人员提议将这两个亚群分别命名为CYi与CYm，分别代表成分更接近CI型与CM型的亚群。