陨石撞击成因气凝球粒及撞击烟柱的化学特征

陨石撞击地球表面往往产生灾难性的影响，然而这种影响往往很快地被地表活跃的地质与生物营力所抹去。撞击事件所产生的烟柱，携带大量气体、液滴和尘埃抛溅到距离撞击坑很远的地方，这些烟柱冷凝后成为球形颗粒被保存到地层中，为还原地球早期撞击事件、评估其对环境的影响提供了重要证据。本研究聚焦于撞击烟柱中高温矿物气体冷凝而成的气凝球粒，仔细分析了其中原生尖晶石相的化学成分特征，并以此为窗口推测了烟柱中的气体成分、氧逸度等。我们的研究表明，由于气凝球粒中尖晶石Fe³⁺/Fe^TOT的比值往往很高，而单纯矿物气液平衡本身过于还原，因此气凝球粒的形成需要有撞击烟柱中混杂的空气参与作用。气凝球粒也因而在一定程度上提供了地球早期大气中氧逸度变化以及撞击烟柱不均一性的记录。这一推断与前人依据实验和地球化学分析得到的推测一致，为关于气凝球粒形成机制的数值模拟研究提供了重要启示。

Chemistry of vapor condensed impact spherules and the impact vapor plumes

Meteorite impacts lead to devastating catastrophes on the earth, while petrologic and geomorphological evidences of the impacts can be quickly destroyed by the active weathering, tectonic processes and also the possible biological activities. On the other hand, impact-generated plumes, which are generally composed of a large amount of vaporized impactite and target rocks, melt droplets and debris, can be ejected far away from the source crater and finally settle down as various spherules. These spherules preserved in the sediments are important clues for recovering ancient impacts and evaluating their influences on the environments. We focus on the vapor-condensed spherules in this study by analyzing the chemical compositions of pristine Ni-rich magnesioferrite spinels. The possible species of the coexisting vapor phase and oxygen fugacity in the plume are predicted from the thermodynamic principles. It turns out that the oxygen fugacity without atmospheric contributions would be too reducing to support the ultrahigh ratio of Fe³⁺/Fe^TOT in spinels. Roles of atmospheric entrainment are therefore proved to be important for the formation of vapor-condensed spherules. In other words, the vapor-condensed spherules provide records for the variations of oxygen fugacity of the atmosphere in the early earth history and also evidences for the heterogeneity in the impact plume. This conjecture agrees quite well with previous studies based on experiments and geochemical analysis and should be helpful for the relevant more realistic numerical modelings.