We report detailed transmission electron microscope (TEM) observations of carbonates from one hydrated fine-grained Antarctic micrometeorite (H-FgMM). These carbonates show the occurrence of complex chemical variations and microstructures that provide important evidence regarding the formation and evolution of rarely analyzed H-FgMMs. The chemical variations were identified at both micrometer and nanometer scales, indicating that these carbonates formed under localized fluid conditions that suggest a variable chemical microenvironment. Individual carbonates grew from isolated reservoirs of fluid. Moreover, these carbonates contain manganese amounts almost twice as high as those measured in CM chondrites but similar to those identified in CI chondrites. Their particular compositions indicate reducing and progressively evolving conditions in the fluid from which these carbonates precipitated, probably due to water consumption during phyllosilicates formation. In addition to the compositional variability, microstructural features are pervasive in these carbonates, similar to those described in heavily shocked meteorites indicating that these carbonates were probably modified during shock processes after their formation. Since carbonates are highly susceptible to shock metamorphism, we suggest that it is essential to investigate their structure in detail before interpreting the isotopic measurements related to the time of their formation. Additionally, associated with carbonates, ubiquitous phosphates were identified in the micrometeorite analyzed. Future studies of these mineral associations will provide us further insight into the formation and evolution of asteroids, especially since they were both identified in the surface materials of Ryugu and Bennu.

中文翻译：

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水合细粒南极微陨石中的白云石：分析二次过程的有效工具

我们报告了对一种水合细粒南极微陨石 (H-FgMM) 中碳酸盐的详细透射电子显微镜 (TEM) 观察结果。这些碳酸盐显示出复杂的化学变化和微观结构，为很少分析的 H-FgMM 的形成和演化提供了重要证据。在微米和纳米尺度上都发现了化学变化，表明这些碳酸盐是在局部流体条件下形成的，这表明化学微环境发生了变化。单独的碳酸盐是从孤立的流体储层中生长出来的。此外，这些碳酸盐中锰的含量几乎是 CM 球粒陨石中测量值的两倍，但与 CI 球粒陨石中发现的锰含量相似。它们的特殊组成表明这些碳酸盐沉淀的流体中的条件正在减少和逐渐演变，这可能是由于层状硅酸盐形成过程中的水消耗所致。除了成分变化外，这些碳酸盐中还普遍存在显微结构特征，与严重冲击陨石中描述的特征类似，表明这些碳酸盐可能在形成后的冲击过程中发生了改变。由于碳酸盐极易受到冲击变质作用的影响，我们建议在解释与其形成时间相关的同位素测量结果之前，必须详细研究其结构。此外，在分析的微陨石中还发现了与碳酸盐相关的普遍存在的磷酸盐。未来对这些矿物组合的研究将使我们进一步了解小行星的形成和演化，特别是因为它们都在龙宫和贝努的表面材料中被发现。