We report the in-situ oxygen isotopic distributions corresponding to the petrographic-mineralogical observation on a compact Type A (CTA) Ca-Al-rich inclusion (CAI), KU-N-02, from a reduced CV3 chondrite, Northwest Africa 7865. The CTA has an igneous texture and mainly consists of spinel, melilite, and Al-Ti-rich clinopyroxene (fassaite). Oxygen isotopic compositions of the constituent minerals plot along the carbonaceous chondrite anhydrous mineral line. The spinel grains are poikilitically enclosed in the melilite and fassaite and are uniformly ¹⁶O-rich (Δ¹⁷O = approximately − 23‰). The fassaite is texturally classified into two types: blocky fassaite and intergranular fassaite. The blocky fassaite crystals exhibit growth zoning as they change from Ti-rich to Ti-poor along the inferred directions of crystal growth from core to rim, while the oxygen isotopic compositions change from ¹⁶O-poor (Δ¹⁷O = approximately − 6‰) to ¹⁶O-rich (Δ¹⁷O = approximately − 23‰) with crystal growth. The intergranular fassaite crystals exist between the melilite crystals and exhibit variable Ti abundance and oxygen isotopic compositions. Additionally, their relationships between Ti contents and oxygen isotopic composition are similar to those of the blocky fassaite. The melilite grains are homogeneously ¹⁶O-poor (Δ¹⁷O = approximately − 2‰), irrespective of their åkermanite (Åk) content. Each melilite grain generally exhibits growth zoning with increasing Åk contents from core to rim, although the melilite contains Åk-rich patches within single crystal. Åk-rich patches often include two types of fassaite: small blebby crystals attached to spinel crystals and round crystals. The oxygen isotopic compositions of the Åk-rich patch and blebby fassaite are ¹⁶O-poor (Δ¹⁷O = approximately − 2‰), similar to that of the host melilite. On the other hand, the round fassaite exhibits significant variation in oxygen isotopic compositions ranging from Δ¹⁷O = −23‰ to − 4‰, which are different from those of the host melilite. These petrographic textures and oxygen isotopic variations indicate the presence of a solid precursor with variable oxygen isotopic compositions for the CTA. The spinel and round fassaite grains are relicts of the precursor that melted in the ¹⁶O-poor nebular gas, resulting in the crystallization of the host melilite from the ¹⁶O-poor melt. The Åk-rich patches and blebby fassaite crystallized from melts trapped by the growing host melilite crystals. The blocky and intergranular fassaite crystallized after the melilite did, and the oxygen isotopic composition of the melt changed to ¹⁶O-rich during the crystallization process, suggesting that the oxygen isotopic composition of the surrounding nebular gas could be varied. The inferred oxygen isotopic evolution for CTA is consistent with those inferred for Type B CAIs, suggesting that coarse-grained igneous CAIs formed in a similar nebular environment regardless of precursor chemistry.

我们报告了原位氧同位素分布，其对应于从减少的CV3球粒陨石，西北非洲7865中紧凑的A型（CTA）富含钙铝的夹杂物（CAI），KU-N-02的岩石矿物学观察。 CTA具有火成的质地，主要由尖晶石，陨石和富含Al-Ti的斜辉石（方铁石）组成。组成矿物的氧同位素组成沿碳质球粒陨石无水矿物线分布。尖晶石晶粒poikilitically封闭在黄长石和fassaite和均匀¹⁶ O丰富（Δ ¹⁷O =大约-23‰）。在结构上，该重晶石分为两类：块状重晶石和晶间重晶石。块状fassaite晶体表现出生长分区，因为他们从富Ti的改变对于Ti-差沿着从核心到边缘的晶体生长的推断方向，而氧同位素组成从改变¹⁶ O型差（Δ ¹⁷ O =约- 6‰ ），以¹⁶ O丰富（Δ ¹⁷ O =约- 23‰）与晶体生长。粒间方解石晶体存在于陨石晶体之间，并表现出可变的Ti丰度和氧同位素组成。另外，它们在钛含量和氧同位素组成之间的关系与块状玄武岩相似。富铝红柱石颗粒均匀¹⁶ O型差（Δ ¹⁷ O =约- 2‰），不论其镁黄长石的（AK）的内容。尽管该陨石在单晶内包含富含Åk的斑块，但每个陨石晶粒通常都显示出随着生长区的增加而从Kk含量到核心的增长区带。富含AK的贴片通常包括两种类型的玄武岩：附着在尖晶石晶体上的小块状晶体和圆形晶体。的AK-富贴片和blebby fassaite的氧同位素组成¹⁶ O型差（Δ ¹⁷ O =约- 2‰），类似于主机黄长石的。在另一方面，圆形fassaite表现出氧同位素组成范围为Δ显著变化¹⁷O = −23‰至− 4‰，不同于寄主陨石。这些岩相织构和氧同位素变化表明存在用于CTA的具有可变氧同位素组成的固体前驱体。尖晶石和圆形方铁石晶粒是在¹⁶ O贫的星云气体中融化的前驱体的残余物，导致基质¹⁶贫¹⁶熔体中的莫来石结晶。富含Åk的斑块和不起泡的玄武岩从不断生长的基质陨石晶体捕获的熔体中结晶出来。镍铁矿后结晶成块状和晶间的辉长岩，​​熔体的氧同位素组成变为¹⁶结晶过程中富含O，表明周围星云气体的氧同位素组成可能会发生变化。推断CTA的氧同位素演化与推断B型CAI的氧同位素演化相一致，这表明不管前驱体化学性质如何，粗粒火成CAI都在相似的星云环境中形成。