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

放射性60 Fe（半衰期：2.62百万年）在太阳星云中的含量，以及它的衰变（除了26 Al）是否对类地行星的组成部分--星子的熔化和分化有重要贡献，仍然是高度不确定的。在何种程度上地球的积木已经分化有强烈的影响，地球的核心和地幔如何进入其目前的配置和丰富的挥发性元素（如H，C，N）和水，建设一个可居住的地球的重要组成部分。根据对渐近巨星分支星和超新星中60 Fe的预测，60 Fe可能在太阳星云中以显着的丰度存在似乎是合理的。几个纳米和微量分析技术的组合，陨石样品的应用在这里提出的目的是把更好的约束水平的60 Fe的星子，后来形成了地球。我们的实验方法依赖于具有高空间分辨率的Fe-Ni同位素测量和分析样品点的彻底矿物学表征。我们将使用新的氧一次离子源（俄勒冈州物理公司生产的NATUII），该源最近安装在MPI的NanoSIMS离子探针上，用于化学，以比以前小得多的规模进行Fe-Ni同位素研究。具有高Fe/Ni比的物体是从60 Fe衰变中识别60 Ni过量的先决条件，将根据对几个只经历了最小的水蚀和热蚀变的陨石的SEM-EMPA联合调查进行预选。将通过FIB/TEM和RIMS（芝加哥的辣椒）对已确定的60 Ni过量物体进行后续研究，以验证推断的60 Fe/56 Fe比值。FIB/TEM将用于表征矿物学背景，并限制可能的次生蚀变过程和纳米至亚微米级金属和硫化物夹杂物的存在下分析的矿物内的Fe-Ni同位素系统的干扰程度。使用辣椒进行的铁-镍同位素研究将用于验证观察到的60 Ni过量，以排除先前NanoSIMS研究中未识别的潜在未识别同量异位素干扰和极端固有质量分馏效应。

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.