一颗留存的粉红色尖晶石记录了无球粒陨石母体上的早期富铝熔体
A surviving pink spinel records an early aluminous melt on the ureilite parent body
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中文总结 AI 辅助
研究通过对无球粒陨石EET 87720的观测，发现其含罕见富铝粉红色尖晶石，证实该尖晶石为无球粒陨石母体本土物质，记录了早期富铝熔体，为行星分异提供了关键档案。
中文摘要 AI 辅助
无球粒陨石（ureilites）是超镁铁质无球粒陨石，被认为是分异母体的碎片，然而其起源与演化仍存在争议，因为结构平衡与化学原始成分共存。本文报道了多组分无球粒陨石大象冰碛（Elephant Moraine, EET）87720的矿物学、同位素及显微结构观测结果。该样品含有异常镁质的橄榄石（镁原子分数Mg#最高达98.7）、贫钙辉石（钙辉石中钙长石组分Wo低至1.0），以及罕见的粗粒粉红色富铝尖晶石，其氧化铝含量为56.4-58.7 wt%，三氧化二铬含量为11.3-11.8 wt%。对尖晶石及共生镁橄榄石的原位三重氧同位素测量结果沿~1斜率的碳质球粒陨石无水矿物（CCAM）线分布，与整体无球粒陨石一致。这些碎屑还遵循无球粒陨石的铁损失/增加趋势，锰镁比值接近球粒陨石水平。这些观测结果表明该碎屑是无球粒陨石母体的本土物质，并将已知无球粒陨石的氧同位素范围扩展至δ¹⁸O≈9.7‰。三维暗场X射线显微镜揭示了尖晶石的分级变形显微结构，包括分布的晶格曲率、局部滑移带状边界及相干镶嵌畴边界，表明其以多尺度方式容纳冲击诱发的晶体塑性变形。我们提出，该富铝尖晶石是在低氧逸度条件下从局部富铝、贫钙的熔体中结晶形成的。共存尖晶石与橄榄石之间的铝分配关系得出结晶温度为1318±43 K，与热演化程度较高的母体一致。因此，该尖晶石可能保存了早期富铝熔体的罕见结晶产物，而这类熔体已在无球粒陨石记录中基本消失，为早期行星分异提供了档案记录。
英文摘要
Ureilites are ultramafic achondrites interpreted as fragments of a differentiated parent body, yet their origin and evolution remain debated because textural equilibrium coexists with chemically primitive compositions. Here we report mineralogical, isotopic, and microstructural observations from polymict ureilite Elephant Moraine (EET) 87720. The sample contains unusually magnesian olivine (Mg# up to 98.7), Ca-poor pyroxene (Wo as low as 1.0), and rare coarse-grained pink aluminous spinel containing 56.4-58.7 wt% Al2O3 and 11.3-11.8 wt% Cr2O3. In situ triple oxygen isotope measurements of spinel and associated forsteritic olivine plot along the ~1-slope Carbonaceous Chondrite Anhydrous Mineral (CCAM) line, consistent with bulk ureilites. The clasts also follow the ureilitic Fe-loss/addition trend, with near-constant chondritic Mn/Mg ratios. These observations demonstrate that the clasts are indigenous to the ureilite parent body and extend the known ureilite oxygen isotope range to delta18O ~9.7 per mil. Three-dimensional dark-field X-ray microscopy reveals a hierarchical deformation microstructure in the spinel, comprising distributed lattice curvature, localized slip-band-like boundaries, and coherent mosaic-domain boundaries, indicating multiscale accommodation of shock-induced crystal-plastic deformation. We propose that the aluminous spinel crystallized from a locally Al-rich, Ca-poor melt under low oxygen fugacity. Al partitioning between coexisting spinel and olivine yields a crystallization temperature of 1318 +/- 43 K, consistent with a thermally elevated parent body. The spinel may therefore preserve a rare crystallization product of an early aluminous melt that has largely disappeared from the ureilite record, providing an archive of early planetary differentiation.
