Pure metallic iron is a common component in extraterrestrial samples (e.g., ordinary chondrite, returned lunar samples), and is thought to be an indicator of extremely reducing conditions. However, tiny pure metallic iron (<1 mm) particles are rarely found in Howardite–Eucrite–Diogenite meteorites, and their formation mechanism on Vesta is poorly understood. Sub-micron sized pure metallic iron particles were identified by transmission electron microscope studies of a melt pocket in the basaltic eucrite meteorite Northwest Africa 11592. Our results demonstrate that the pure metallic iron likely formed through a dissociation reaction of ferrous iron (i.e., 3Fe²⁺ = Fe⁰ + 2Fe³⁺) in pyroxene. Another disproportionation reaction product, ferric iron, was confirmed to be incorporated into Al-rich clinopyroxene in the melt pocket by electron energy loss spectra analyses. The temperature conditions required for the formation of the melt pocket are estimated to be above 1,310°C, as indicated by the crystallization of nano-sized hercynite within acicular plagioclase. Such high temperatures on Vesta could only have been reached if an impact occurred during peak thermal metamorphism, early in the asteroid's evolution. This formation mechanism of these pure metallic iron particles could play a role in redox environment of the Vesta.

中文翻译：

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Eucrite 陨石中铁的歧化证据：对灶神星撞击过程的影响

纯金属铁是外星样品（例如普通球粒陨石、返回的月球样品）中的常见成分，被认为是极端还原条件的指标。然而，在 Howardite-Eucrite-Diogenite 陨石中很少发现微小的纯金属铁 (<1 mm) 颗粒，并且它们在灶神星上的形成机制知之甚少。亚微米级纯金属铁颗粒通过对玄武质晶辉石陨石西北非洲 11592 中熔体口袋的透射电子显微镜研究确定。我们的结果表明，纯金属铁可能是通过亚铁（即 3Fe ^{2 +}  = Fe ⁰  + 2Fe ³⁺) 在辉石中。通过电子能量损失光谱分析，证实另一种歧化反应产物三价铁被结合到熔体中的富铝单斜辉石中。形成熔体袋所需的温度条件估计高于 1,310°C，如针状斜长石内纳米级硬长石的结晶所示。只有在小行星演化早期的热变质峰值期间发生撞击，灶神星才能达到如此高的温度。这些纯金属铁颗粒的这种形成机制可能在灶神星的氧化还原环境中起作用。