We performed high-precision SIMS (secondary ion mass spectrometry) ²⁶Al-²⁶Mg and oxygen isotope analyses of two unique CAIs, “Mesquite” and “Y24”, found in the CO3.05 chondrites Northwest Africa 7892 and Yamato-81020, respectively. Mesquite is unusually large (∼5 × 3 mm) for a CAI from any CO chondrite and exhibits a layered texture comprising a melilite-rich core surrounded by hibonite- and spinel-rich mantle layers and a semi-continuous spinel-dominated rim. The CAI Y24 stands out because of its distinct mineralogy: grossite, hibonite, and spinel are accompanied by abundant ultra-refractory-element-rich phases such as warkite, kangite, and perovskite. Silicates are absent in Y24.

Negatively fractionated δ²⁵Mg values of phases in the core and mantle layers of Mesquite suggest that the inclusion as a whole was never molten and, hence, represents an aggregate of condensates. The relatively large grain sizes of melilite in the core (up to ∼300 µm) most likely are the result of solid-state recrystallization and coarsening of melilite in the course of a heating event occurring in the solar nebula. This heating event, however, did not disturb the Al-Mg systematics of Mesquite. Regardless of their position within Mesquite and the phases analyzed, spots analyzed for Al-Mg plot on a single isochron characterized by an initial ²⁶Al/²⁷Al of (4.95 ± 0.08) × 10^–5 and a δ²⁶Mg*₀ of –0.14 ± 0.05‰. We suggest that this initial ²⁶Al/²⁷Al ratio corresponds to the formation of Mesquite in the solar nebula that was slightly heterogeneous with respect to Mg isotopes. Spinel in the rim is uniform in Δ¹⁷O (∼–25‰); in contrast, hibonite in the core and mantle layers, albeit also ¹⁶O-rich, show variable oxygen isotope ratios (Δ¹⁷O ∼ –15‰ to –23‰), which would be consistent with hibonite condensation in a gas with quickly-changing oxygen isotope compositions. The ¹⁶O-poor composition of melilite (Δ¹⁷O ∼ –1‰ to 0‰) in the core could be the result of isotope exchange with an ¹⁶O-poor gas, perhaps during the heating event that caused the solid-state recrystallization and coarsening of melilite or the result of oxygen isotope exchange with a fluid on the parent body. Abundant calcite, phyllosilicates, and sodalite are witnesses to late-stage and low-temperature alteration of the Mesquite CAI; calcite and phyllosilicates most likely are of terrestrial origin, but sodalite could have formed in the parent body.

Inclusion Y24 is irregularly-shaped, indicating a condensation origin. Completely enclosing other phases, warkite forms the matrix of Y24, which could be the result of simultaneous condensation and growth of warkite, grossite, and hibonite. Possibly, spinel formed by replacing grossite or hibonite or both minerals in a gradually cooling gas before any silicates condensed. SIMS analyses indicate that condensation occurred in an ¹⁶O-rich gas when ²⁶Al/²⁷Al was (5.4 ± 1.0) × 10^–5. Oxygen isotope exchange with an ¹⁶O-poor fluid in the parent body or with an ¹⁶O-poor gas in a nebular setting caused the ¹⁶O-poor compositions in grossite and kangite.

我们对两个独特的CAI（“豆科灌木”和“ Y24” ）分别进行了高精度的SIMS（二次离子质谱）²⁶ Al- ²⁶ Mg和氧同位素分析，分别在CO3.05球粒陨石西北部7892和Yamato-81020中发现。对于任何CO球粒陨石而言，豆科植物的CAI都异常大（约5×3 mm），并呈现出层状结构，其中包括富含陨石质的岩心，周围包裹着富含褐铁矿和尖晶石的地幔层以及半连续的尖晶石为主的边缘。CAI Y24因其独特的矿物学而脱颖而出：钙铁矿，辉石岩和尖晶石伴随着丰富的超难熔元素富集相，如方铁矿，方铅矿和钙钛矿。在Y24中不存在硅酸盐。

负分馏δ ^25个在Mesquite的核心和地幔层相的镁数值表明，包含作为一个整体从未熔融，因此，表示的缩合物的集合体。在太阳星云中发生加热事件的过程中，固态硅钢中相对较大的晶粒尺寸（最大约300 µm）很可能是固态重结晶和玛氏石粗化的结果。但是，这种加热事件并没有干扰梅斯基特的Al-Mg系统。无论内麦斯基德并进行相它们的位置的分析，斑点分析的Al-Mg情节上的单个等时线，其特征在于初始的²⁶的Al / ²⁷的Al（4.95±0.08）×10 ^-5和δ ²⁶毫克* ₀为–0.14±0.05‰。我们建议，最初的²⁶ Al / ²⁷ Al比值对应于太阳星云中的豆科灌木的形成，该豆科灌木相对于Mg同位素略有异质。尖晶石在轮辋是在Δ均匀¹⁷ O（〜-25‰）; 与此相反，在hibonite芯和地幔层，尽管也¹⁶富含O，显示出不同的氧同位素比率（Δ ¹⁷ O〜-15‰至-23‰），这将是与hibonite缩合一致的气体与quickly-变化的氧同位素组成。的¹⁶黄长石（Δ的O型差组合物¹⁷在芯O〜-1‰至0‰）可以用同位素交换的结果¹⁶贫氧气体，可能是在加热过程中导致固态重结晶和陨石的粗化，或者是氧同位素与母体上的流体交换的结果。方解石，页硅酸盐和方钠石丰富，是豆科灌木CAI后期和低温变化的见证。方解石和层状硅酸盐最有可能是陆地来源的，但钠长石可能在母体中形​​成。

夹杂物Y24为不规则形状，表示凝结起点。完全包围其他相，方铁矿形成Y24的基质，这可能是由于方铁矿，钙铁矿和褐铁矿同时凝结和生长的结果。可能是尖晶石，是在任何硅酸盐冷凝之前，通过在逐渐冷却的气体中替代钙铁矿或锂铁矿或两种矿物而形成的。SIMS分析表明，当²⁶ Al / ²⁷ Al为（5.4±1.0）×10 ^–5时，富含¹⁶ O的气体中会发生冷凝。在母体中与¹⁶ O贫化流体或在星云状环境中与¹⁶ O贫化气体进行氧同位素交换导致了¹⁶ O贫化的成分为钙铁矿和方铅矿。