Coarse-grained, igneous Ca-Al-rich inclusions (CAIs) in CV chondrites formed through multiple melting events. We conducted in situ O-isotope analysis and Al-Mg systematics by secondary ion mass spectrometry of relict and overgrown minerals from a partial melting event in an Allende Type B CAI, Golfball. Golfball has a Type B CAI bulk composition and a unique structure: a fassaite-rich mantle enclosing a melilite-rich core. Many of the blocky melilite crystals in the core have irregularly shaped, Al-rich (Åk_5–15) cores enclosed in strongly zoned (Åk_30–70) overgrowths. Since the Al-rich melilite grains could not have formed from a melt of Golfball, they are interpreted as relict grains that survived later melting events. The O-isotopic compositions of the blocky melilite crystals plot along the carbonaceous chondrite anhydrous mineral line, ranging between Δ¹⁷O ~ −14‰ and −5‰. The Al-rich relict melilite grains and their overgrowths exhibit the same O-isotopic compositions, while the O-isotopic compositions are varied spatially among melilites. We found that the O-isotopic compositions steeply change across several melilite crystals within few tens of micrometers, indicating the O-isotopic compositions of the melt could not have been homogenized during the partial melting in that scale. According to the time scale of O self-diffusivity in the melt, the cooling rate of the partial melting event is calculated to be >6 × 10⁴ K h⁻¹. Al-Mg isotope data for core minerals plot on a straight line on an Al-Mg evolution diagram. A mineral isochron for Golfball gives initial ²⁶Al/²⁷Al of (4.42 ± 0.20) × 10^–5 and initial δ²⁶Mg* of −0.035 ± 0.050‰. The chemical and O-isotopic compositions of melilite and those initial values imply that its precursor consisted of fluffy Type A and/or fine-grained CAIs. The partial melting event for Golfball may have occurred in very short order after the precursor formation.

中文翻译：

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氧和铝镁同位素对阿连德 B 型 CAI、高尔夫球部分熔化的冷却速度和年龄的限制

CV 球粒陨石中的粗粒、火成岩富钙铝包裹体 (CAI) 通过多次熔融事件形成。我们通过二次离子质谱法对阿连德 B 型 CAI 高尔夫球场部分熔化事件中残留和过度生长的矿物进行了原位 O 同位素分析和铝镁系统学分析。高尔夫球具有 B 型 CAI 块状成分和独特的结构：富含铁橄榄石的地幔包围着富含黄长石的核心。核心中的许多块状黄长石晶体具有不规则形状的富含铝 (Åk _{5 – 15} ) 的核心，被封闭在强分区 (Åk _{30 – 70}) 过度生长。由于富铝黄长石颗粒不可能由高尔夫球的熔体形成，因此它们被解释为在后来的熔化事件中幸存下来的残余颗粒。块状黄长石晶体的 O 同位素组成沿着碳质球粒陨石无水矿物线绘制，范围在 Δ ¹⁷O～-14‰和-5‰。富铝残余黄黄长石颗粒及其过度生长表现出相同的 O 同位素组成，而 O 同位素组成在黄长石之间存在空间变化。我们发现 O 同位素组成在几十微米内的几个黄长石晶体中急剧变化，表明熔体的 O 同位素组成在该规模的部分熔化过程中无法均质化。根据熔体中 O 自扩散的时间尺度，部分熔化事件的冷却速率计算为 >6 × 10 ⁴  K h ^-1。核心矿物的 Al-Mg 同位素数据绘制在 Al-Mg 演化图上的一条直线上。高尔夫球的矿物等时线给出初始²⁶ Al/ ²⁷(4.42 ± 0.20) × 10 ^{–5 的}Al 和 -0.035 ± 0.050‰ 的初始 δ ²⁶ Mg*。黄长石的化学和 O 同位素组成以及这些初始值意味着其前体由蓬松的 A 型和/或细粒 CAIs 组成。在前体形成之后，高尔夫球的部分熔化事件可能在很短的时间内发生。