Cosmic spherules are micrometeorites that melt at high altitude as they enter Earth's atmosphere, and their oxygen isotope compositions are partially or completely inherited from the upper atmosphere, depending on the amount of heating experienced and the nature of their precursor materials. In this study, the three oxygen isotope compositions of 137 cosmic spherules are determined using 277 in situ analyses by ion probe. Our results indicate a possible correlation between increasing average δ¹⁸O compositions of silicate‐dominated (S‐type) spherules along the series scoriaceous < porphyritic < barred < cryptocrystalline < glass < CAT (calcium‐aluminum‐titanium) spherules (~12‰, 20‰, 22‰, 25‰, 26‰, and 50‰). This is consistent with the evolution of oxygen isotopes by mass fractionation owing to increased average entry heating and thus suggests mass fractionation dominates changes in isotopic composition, with atmospheric exchange being less significant. The Δ¹⁷O values of spherules, therefore, are mostly preserved and suggest that ~80% of particles are samples of C‐type asteroids. The genetic relationships between different S‐types can also be determined with scoriaceous, barred, and cryptocrystalline spherules mostly having low Δ¹⁷O values (≤0‰) mainly derived from carbonaceous chondrite (CC)‐like sources, while porphyritic spherules mostly have positive Δ¹⁷O (>0‰) and are largely derived from ordinary chondrite (OC)‐like sources related to S (IV)‐type asteroids. Glass and CAT spherules have variable Δ¹⁷O values indicating they formed by intense entry heating of both CC and OC‐like materials. I‐type cosmic spherules have a narrow range of δ¹⁷O (~20–25‰) and δ¹⁸O (~38–48‰) values, with Δ¹⁷O (~0‰) suggesting their oxygen is obtained entirely from the Earth's atmosphere, albeit with significant mass fractionation owing to evaporative heating. Finally, G‐type cosmic spherules have unexpected isotopic compositions and demonstrate little mass fractionation from a CC‐like source. The results of this study provide a vital assessment of the wider population of extraterrestrial dust arriving on Earth.

中文翻译：

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大量小宇宙球的氧同位素组成：对它们的来源和高层大气同位素组成的影响

宇宙小球是微陨石，当它们进入地球大气层时会在高海拔处融化，其氧同位素组成部分或全部是来自高层大气的，这取决于所经历的加热量及其前体材料的性质。在这项研究中，使用277离子探针进行原位分析，确定了137个宇宙球的三种氧同位素组成。我们的研究结果表明增加的平均δ之间可能的相关¹⁸沿着硅质的<斑状的>条纹的<禁止的<隐晶的<玻璃的<CAT（钙铝钛）球（〜12‰，20‰，22‰，25‰，26‰）系列的硅酸盐为主（S型）球的O组成和50‰）。这与由于平均入口热量增加而导致的质量分馏过程中氧同位素的演化相一致，因此表明质量分馏在同位素组成变化中起着主导作用，而大气交换作用则不那么重要。的Δ ¹⁷小球体的O值，因此，大多是保存和建议颗粒的〜80％是C型小行星样品。不同的S-类型之间的遗传关系也可以与scoriaceous确定，禁止，和隐球粒主要是具有低Δ ¹⁷O值（≤0‰）主要由碳质球粒（CC） -样的来源，而斑状球粒主要是具有正的Δ ¹⁷ O（> 0‰），并在很大程度上与普通球粒陨石（OC）衍生样相关至S源（ IV）型小行星。玻璃和CAT小球具有可变的Δ ¹⁷指示它们通过CC和OC状材料的强烈条目加热形成O值。i型宇宙小球具有的δ范围窄¹⁷ O（〜20-25‰）和δ ¹⁸ O（〜38-48‰）值，其中Δ ¹⁷O（〜0‰）表示它们的氧气完全来自地球大气，尽管由于蒸发加热而导致大量的质量分馏。最后，G型宇宙球具有意想不到的同位素组成，并且几乎没有CC类来源的质量分数。这项研究的结果对地球上更多的外星尘埃提供了至关重要的评估。