The oxygen isotope compositions of large numbers of small cosmic spherules: Implications for their sources and the isotopic composition of the upper atmosphere

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 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. Particles of each different type of cosmic spherule (scoriaceous, porphyritic, cryptocrystalline, barred, glass, calcium aluminium and titanium (CAT), G-type and I-type) in the diameter range ~52–480mm were analysed. The results confirm that the three oxygen isotope compositions of melted micrometeorites reflect a combination of their precursor composition, exchange with the atmosphere and mass fractionation owing to evaporation during entry heating. The data appear to reveal an increase in average δ18O values of silicate dominated (S-type) spherules in the series scoriaceous&lt;...宇宙球粒（cosmic spherules）是进入地球大气层后于高空熔融的微流星体（micrometeorites），其氧同位素组成部分或完全继承自上层大气，具体取决于所经历的加热过程以及前驱物质的性质。本研究通过离子探针开展的277次原位分析，测定了137颗宇宙球粒的三氧同位素组成。本研究分析了直径范围为52~480毫米的各类宇宙球粒颗粒，涵盖多孔状（scoriaceous）、斑状（porphyritic）、隐晶质（cryptocrystalline）、条纹状（barred）、玻璃质（glass）、钙铝钛型（calcium aluminium and titanium, CAT）、G型及I型等类型。研究结果证实，熔融微流星体的三氧同位素组成，是前驱物质组成、与大气的同位素交换以及进入大气层时加热蒸发引发的质量分馏共同作用的结果。数据显示，以硅酸盐为主导的S型球粒的平均δ¹⁸O值在多孔状序列中呈上升趋势<...