Iron-60 as a heat source for melting and differentiation of Earth-forming planetesimals and planetary embryos

太阳星云中放射性60Fe(半衰期：2.62 Myr)的水平，以及它的衰变是否(除了26Al之外)对构成类地行星的小行星的融化和分化做出了重要贡献，仍然是高度不确定的。地球的组成部分已经在多大程度上已经分化，对地球的核心和地幔如何形成目前的形态以及挥发性元素(如H、C、N)和水的丰度有很大的影响，这些元素是建设一个宜居地球的重要组成部分。从60Fe在渐近巨星和超新星中的预测产量来看，60Fe可能存在于太阳星云中，而且丰度很高。这里提出的几种纳米和微量分析技术的组合应用于陨石样品，目的是对后来形成地球的小行星中的60Fe水平施加更好的限制。我们的实验方法依赖于高空间分辨率的Fe-Ni同位素测量和对分析样点的彻底矿物学表征。我们将使用新的氧初级离子源(俄勒冈物理公司的Hyperion II)，该源最近安装在MPI for Chemical的NanoSIMS离子探测器上，以比以前小得多的规模进行Fe-Ni同位素研究。高Fe/Ni比的物体是从60Fe衰变中识别60Ni过剩的先决条件，将根据对几块只经历了最小的水和热蚀变的陨石的联合扫描电子显微镜-电子探针研究来预先选择。FIB/TEM和RIMS(芝加哥大学的CHILI)将对发现的60Ni过剩的物体进行后续研究，以验证推断的60Fe/56Fe比率。FIB/TEM将用于表征矿物学背景，并通过可能的次生蚀变过程以及所分析矿物中存在纳米至亚微米尺寸的金属和硫化物包裹体来限制Fe-Ni同位素体系的干扰程度。对辣椒进行的铁-镍同位素研究将被用来验证观察到的~(60)Ni过量，以排除潜在的不明等压干扰和之前的NanoSIMS研究中没有认识到的极端本征质量分馏效应。

The level at which radioactive 60Fe (half-life: 2.62 Myr) was present in the solar nebula and whether its decay could have made an important contribution (in addition to 26Al) to the melting and differentiation of planetesimals, the building blocks of the terrestrial planets, is still highly uncertain. To which extent the building blocks of the Earth were already differentiated has strong impact on how Earth’s core and mantle came into its present configuration and on the abundances of volatile elements (e.g. H, C, N) and water, important components for building a habitable Earth. In view of the predicted production of 60Fe in asymptotic giant branch stars and supernovae it appears plausible that 60Fe could have been present with significant abundance in the solar nebula. The application of a combination of several nano- and microanalytical analysis techniques to meteoritic samples proposed here is aimed to put better constraints on the level of 60Fe in planetesimals that later formed the Earth. Our experimental approach relies on Fe-Ni isotope measurements with high spatial resolution and a thorough mineralogical characterization of analyzed sample spots. We will use the new oxygen primary ion source (Hyperion II from Oregon Physics), which was recently installed on the NanoSIMS ion probe at MPI for Chemistry, to conduct Fe-Ni isotope studies at much smaller scales than was possible before. Objects with high Fe/Ni ratios, a pre-requisite to identify 60Ni excesses from 60Fe decay, will be pre-selected on the basis of combined SEM-EMPA investigations of several meteorites that experienced only minimal aqueous and thermal alteration. Follow-up studies of identified objects with 60Ni excesses will be conducted by FIB/TEM and by RIMS (CHILI at the University of Chicago) to validate inferred 60Fe/56Fe ratios. FIB/TEM will be used to characterize the mineralogical context and to constrain the degree of disturbance of the Fe-Ni isotope system by possible secondary alteration processes and presence of nanometer- to submicrometer-sized metal and sulfide inclusions within the analyzed minerals. Iron-Ni isotope studies with CHILI will be used to verify observed 60Ni excesses to exclude potential unidentified isobaric interferences and extreme intrinsic mass fractionation effects not recognized in the preceding NanoSIMS studies.