The noble gas and nitrogen relationship between Ryugu and carbonaceous chondrites
The noble gas and nitrogen relationship between Ryugu and carbonaceous chondrites
复制标题
龙宫与碳质球粒陨石之间的稀有气体和氮的关系
DOI：
10.1016/j.gca.2023.01.020
复制
发表时间：
2023
影响因子：
5
通讯作者：
et al.
中科院分区：
文献类型：
--
作者：
Broadley M.W.;Byrne D.J.;Fri E.;et al.
碳质陨石被认为起源于C型小行星，是太阳系中最原始的物质。此外，由于碳质陨石可能含有大量的挥发性元素，它们可能在向地球和其他太阳系内天体提供挥发性物质和有机物质方面发挥了关键作用。然而，解开南极陨石的挥发性成分的一个主要挑战是区分哪些特征是从母体继承的，哪些可能是可归因于陆地风化的次要特征。2020年12月，日本宇宙航空研究开发机构（JAXA）的隼鸟2号（Hayabusa 2）使命成功将C型小行星（162173）龙谷的表面物质送回地球。这种物质现在被归类为非常类似CI型陨石，这是化学上最原始的陨石。因此，对来自Ryugu表面的物质进行分析提供了一个独特的机会，可以分析来自CI型小行星的物质的挥发性成分，而不会产生陆地污染带来的复杂情况。由于其高度挥发性，陨石的惰性气体和氮存量对小行星母体的不同蚀变过程和陆地污染非常敏感。在这里，我们研究了从第一次和第二次着陆地点收集的两个颗粒状颗粒的氮气和惰性气体特征（Okazaki等人，2022 a），以提供对Ryugu的形成和蚀变历史的洞察。在Ryugu样品中被捕获的惰性气体的浓度大于先前测量的Cl的平均组合物，并且主要来自Q相，尽管注意到前太阳纳米金刚石Xe-HL的显著贡献。大量的惰性气体浓度加上太阳前纳米金刚石的显著贡献表明，Ryugu样品可能代表了早期太阳系中一些最原始的未加工材料。与稀有气体相比，两个琉球球粒的氮丰度和δ 15 N组成均低于CI球粒陨石的平均值。我们将本研究中测得的较低的氮丰度和δ 15 N归因于在母星子的水蚀变过程中，我们的样品中富含a15 N的相的优先损失。对从Ryugu返回的其他颗粒的分析表明，氮浓度和δ 15 N变化很大，表明蚀变流体与Ryugu表面目前存在的物质发生了不均匀的相互作用。最后，捕获的稀有气体与氮气的比率高于Cl金刚石，并且更接近难熔相Q和纳米金刚石。这表明Ryugu经历了水蚀变，导致氮的显著和可变损失，可能来自可溶性有机物质，而不改变惰性气体预算，这主要是在不溶性有机物质和前太阳金刚石中，因此更耐水蚀变。
Carbonaceous chondrites are considered to have originated from C-type asteroids and represent some of the most primitive material in our solar system. Furthermore, since carbonaceous chondrites can contain significant quantities of volatile elements, they may have played a crucial role in supplying volatiles and organic material to Earth and other inner solar system bodies. However, a major challenge of unravelling the volatile composition of chondritic meteorites is distinguishing between which features were inherited from the parent body, and what may be a secondary feature attributable to terrestrial weathering. In December 2020, the Hayabusa2 mission of the Japan Aerospace Exploration Agency (JAXA) successfully returned surface material from the C-type asteroid (162173) Ryugu to Earth. This material has now been classified as closely resembling CI-type chondrites, which are the most chemically pristine meteorites. The analysis of material from the surface of Ryugu therefore provides a unique opportunity to analyse the volatile composition of material that originated from a CI-type asteroid without the complications arising from terrestrial contamination. Given their highly volatile nature, the noble gas and nitrogen inventories of chondrites are highly sensitive to different alteration processes on the asteroid parent body, and to terrestrial contamination. Here, we investigate the nitrogen and noble gas signature of two pelletized grains collected from the first and second touchdown sites (Okazaki et al., 2022a), to provide an insight into the formation and alteration history of Ryugu. The concentration of trapped noble gas in the Ryugu samples is greater than the average composition of previously measured CI chondrites and are primarily derived from phase Q, although a significant contribution of presolar nanodiamond Xe-HL is noted. The large noble gas concentrations coupled with a significant contribution of presolar nanodiamonds suggests that the Ryugu samples may represent some of the most primitive unprocessed material from the early solar system. In contrast to the noble gases, the abundance of nitrogen and δ15N composition of the two Ryugu pellets are lower than the average CI chondrite value. We attribute the lower nitrogen abundances and δ15N measured in this study to the preferential loss of a15N-rich phase from our samples during aqueous alteration on the parent planetesimal. The analyses of other grains returned from Ryugu have shown large variations in nitrogen concentrations and δ15N indicating that alteration fluids heterogeneously interacted with material now present on the surface of Ryugu. Finally, the ratio of trapped noble gases to nitrogen is higher than CI chondrites, and is closer to refractory phase Q and nanodiamonds. This indicates that Ryugu experienced aqueous alteration that led to the significant and variable loss of nitrogen, likely from soluble organic matter, without modification of the noble gas budget, which is primarily hosted in insoluble organic matter and presolar diamonds and is therefore more resistant to aqueous alteration.