Abstract We measured oxygen isotope ratios and major elemental compositions of non-porphyritic chondrules and lithic fragments with various textures and chemical compositions in the Asuka-881020 CH chondrite. The oxygen isotope ratios plot along the primitive chondrule mineral line with Δ17O (= δ17O – 0.52 × δ18O) values from ∼ –21‰ to +5‰. The Δ17O values increase with decreasing Mg# (= molar [MgO]/[MgO+FeO]%) from 99.6 to 58.5, similarly to the Δ17O-Mg# trends for the chondrules in other carbonaceous chondrites. Most of the measured objects (non-porphyritic chondrules and lithic fragments) including chondrules analyzed in the previous studies are classified into three groups based on the Δ17O values and chemistry; the –2.3‰ group with FeO-poor compositions (the most abundant group), the +1.4‰ group with FeO-rich compositions, and the –6.3‰ group with FeO-poor compositions. Skeletal olivine and magnesian cryptocrystalline (MgCC) chondrules and MgCC chondrule fragments, which are the –2.3‰ group objects, may have formed via fractional condensation in the isotopically uniform gaseous environment with Δ17O of –2.3‰. When silica-normative materials condensed from gas at ∼ 1200 K, 16O-rich refractory solids, similar to Ca-Al-rich inclusions, were incorporated into the environment. The silica-normative materials that condensed onto the 16O-rich refractory solids were reheated at 1743 – 1968 K and formed cristobalite-bearing chondrules with Δ17O of ∼ –6‰. This scenario can explain the absence of silica-bearing chondrules in the –2.3‰ group and refractory element abundances in the cristobalite-bearing chondrules as high as those in the MgCC chondrules. Refractory element abundances of the +1.4‰ group objects decrease from FeO-Al-rich and ferroan CC (FeCC) chondrules to FeCC chondrule fragments to FeNi metal-bearing to silica-bearing chondrules. This suggests the formation via fractional condensation in the isotopically uniform gaseous environment. The Δ17O values and FeO-rich compositions of this group could be explained by an addition of 16O-poor water ice as an oxidant to the relatively 16O-rich solids with Δ17O of –2.3‰, which may also explain existence of some MgCC chondrules and fragments with intermediate Δ17O values between –2.3‰ and +1.4‰. The immiscibility textures in the silica-bearing chondrules suggest a reheating event at a temperature of > 1968 K after condensation of silica-normative materials. Thus, the non-porphyritic chondrules and fragments in CH and CB chondrites, which are classified into three distinct Δ17O groups, require multiple chondrule-forming environments and heating events. Energy source for the heating events could be either impact plume and/or other dynamical processes in the protoplanetary disk, though a single heating event would not fully explain observed chemical and isotope signatures in these non-porphyritic chondrules.

中文翻译：

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Asuka-881020 CH 球粒陨石 I 的氧同位素研究：非斑状球粒

摘要 我们测量了 Asuka-881020 CH 球粒陨石中具有不同质地和化学成分的非斑状球粒和岩屑碎片的氧同位素比和主要元素组成。氧同位素比沿着原始球粒矿物线绘制，Δ17O (= δ17O – 0.52 × δ18O) 值从 ~ –21‰ 到 +5‰。Δ17O 值随着 Mg#（= 摩尔 [MgO]/[MgO+FeO]%）从 99.6 到 58.5 的降低而增加，类似于其他碳质球粒陨石中球粒的 Δ17O-Mg# 趋势。包括以往研究中分析的球粒在内的大多数测量对象（非斑状球粒和岩屑）根据 Δ17O 值和化学性质分为三组；–2.3‰ 组与 FeO 组成较少（最丰富的组），+1.4‰ 组与 FeO 丰富的组成，以及 –6. 3‰ 组，具有贫铁成分。骨骼橄榄石和镁质隐晶 (MgCC) 球粒和 MgCC 球粒碎片是 –2.3‰ 组物体，可能通过在 Δ17O 为 –2.3‰ 的同位素均匀气态环境中分凝形成。当二氧化硅标准材料从大约 1200 K 的气体中冷凝时，富含 16O 的难熔固体，类似于富含 Ca-Al 的夹杂物，被结合到环境中。凝结在富含 16O 的耐火固体上的二氧化硅标准材料在 1743 – 1968 K 下重新加热，并形成含方石英球粒，Δ17O 约为 –6‰。这种情况可以解释 –2.3‰ 组中没有含二氧化硅的球粒，而含方英石球粒中的难熔元素丰度与 MgCC 球粒中的一样高。+1.4‰ 族天体的难熔元素丰度从富含 FeO-Al 和铁质 CC (FeCC) 球粒到 FeCC 球粒碎片，再到含 FeNi 金属再到含硅球粒。这表明在同位素均匀的气态环境中通过分凝形成。该组的 Δ17O 值和富含 FeO 的成分可以通过将缺乏 16O 的水冰作为氧化剂添加到相对富含 16O 的固体中来解释，Δ17O 为 –2.3‰，这也可以解释一些 MgCC 球粒的存在和Δ17O 值介于 –2.3‰ 和 +1.4‰ 之间的片段。含二氧化硅球粒中的不混溶结构表明，在二氧化硅标准物质凝结后，温度大于 1968 K 时发生了再加热事件。因此，CH 和 CB 球粒陨石中的非斑状球粒和碎片，分为三个不同的 Δ17O 组，需要多个球粒形成环境和加热事件。加热事件的能量来源可能是撞击羽流和/或原行星盘中的其他动力学过程，尽管单个加热事件不能完全解释在这些非斑状球粒中观察到的化学和同位素特征。