CI chondrites are the most significant extra-terrestrial samples for estimating the composition of primordial materials in the Solar System. However, CIs lose many primary features because of heavy parent body aqueous alteration. However, CI and CI-related Ryugu particles contain small amounts of relict anhydrous minerals, indicating primary occurrences of chondrules and refractory inclusions. In this study, we estimated the primordial abundance of chondrules in CIs from calculations of the bulk major element compositions. The constraints for the calculation were as follows: 1) CI chondrites primarily comprised chondrules, refractory inclusions, opaque minerals, and a matrix similar to other carbonaceous (C) chondrites. 2) The chemical compositions of these components were similar to those of the unaltered C chondrites. 3) The primary matrix composition of the CI was close to the mean bulk composition. 4) The alteration occurred isochemically. We used the mean major elemental compositions of chondrules and refractory inclusions in an almost unaltered chondrite, Y-81020, CO3.05. Our results were within the range of previously reported CI bulk chemical compositions in the case where chondrule abundances are ≲10 wt%. We also calculated the bulk chemical composition of Tagish Lake, ungrouped C2, which primarily contained ≲20 wt% chondrules. The CI chondrites and Tagish Lake were formed in the outer Solar System. The low primary abundance of chondrules in CIs is closely related to the formation conditions of chondrules in such regions. We suggest that dust with abundant ice and minor chondrules accreted onto the parent bodies of the CI and Tagish Lake in the outer Solar System. Primordial chondrule abundance is the key to clarifying the physical and chemical conditions and evolution of the early Solar System.

中文翻译：

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CI 球粒陨石中球粒的主要丰度


CI 球粒陨石是用于估计太阳系原始材料成分的最重要的外星样本。然而，由于母体水体的严重改变，CI 失去了许多主要特征。然而，CI 和 CI 相关的 Ryugu 颗粒含有少量残余无水矿物，表明主要存在球粒和难熔包裹体。在这项研究中，我们通过计算大块主要元素成分来估计 CI 中球粒的原始丰度。计算的限制如下：1）CI球粒陨石主要由球粒、难熔包裹体、不透明矿物和类似于其他碳质（C）球粒陨石的基质组成。 2）这些成分的化学成分与未改变的C球粒陨石的化学成分相似。 3) CI 的主要基质成分接近平均本体成分。 4) 改变是等化学发生的。我们使用了几乎未改变的球粒陨石 Y-81020，CO3.05 中球粒和难熔包裹体的平均主要元素组成。在球粒丰度为 ≲10 wt% 的情况下，我们的结果在先前报道的 CI 本体化学成分范围内。我们还计算了塔吉什湖的大量化学成分，即未分组的 C2，其中主要含有 ≲20 wt% 的球粒。 CI 球粒陨石和塔吉什湖形成于太阳系外层。 CIs中球粒原生丰度较低与该区域球粒的形成条件密切相关。我们认为，含有大量冰和小球粒的尘埃积聚在太阳系外的 CI 和塔吉什湖的母体上。 原始球粒丰度是阐明早期太阳系物理化学条件和演化的关键。