Chondrites display isotopic variations for moderately volatile elements, the origin of which is uncertain and could have involved evaporation/condensation processes in the protoplanetary disk, incomplete mixing of the products of stellar nucleosynthesis, or aqueous alteration on parent bodies. Here, we report high-precision K and Rb isotopic data of carbonaceous chondrites, providing new insights into the cause of these isotopic variations. We find that the K and Rb isotopic compositions of carbonaceous chondrites correlate with their abundance depletions, the fractions of matrix material, and previously measured Te and Zn isotopic compositions. These correlations are best explained by the variable contribution of chondrules that experienced incomplete condensation from a supersaturated medium. From the data, we calculate an average chondrule cooling rate of similar to 560 +/- 180 K/hour, which agrees with values constrained from chondrule textures and could be produced in shocks induced by nebular gravitational instability or motion of large planetesimals through the the nebula.

The Fe isotopic composition of twenty four glacial diamictite composites with depositional ages ranging from the Mesoarchean to the Palaeozoic serve as proxies of the average upper continental crust (UCC) and can be used to track how delta Fe-56 may have changed in the continental crust through time. The diamictites have elevated chemical index of alteration (CIA) values and other characteristics of weathered regoliths e.g., strong depletion in soluble elements such as Sr), which they inherited from their upper crustal source regions. The delta Fe-56 values in the diamictite composites range from -0.59 parts per thousand to +0.23 parts per thousand. Excluding three samples impacted by the incorporation of materials from Fe formations, the diamictites have an average delta Fe-56 of 0.12 +/- 0.13 parts per thousand (2 sigma), overlapping the recent estimated average delta Fe-56 of 0.09 +/- 0.03 %o (2 s.d.) in the upper continental crust (Dauphas et al., 2017, and references therein). There is no obvious correlation between delta Fe-56 of the glacial diamictites and the CIA. Our data suggest that the Fe isotope composition of the upper continental crust has been relatively constant throughout Earth history and that chemical weathering is not important in producing Fe isotope variations in the upper continental crust. Pre-Great Oxidation Event (GOE) anoxic weathering, when iron was soluble in its divalent state, did not generate different Fe isotopic signatures from the post-GOE oxidative weathering environment in the upper continental crust. Therefore, the large Fe isotopic fractionations observed in various marine sedimentary records are likely due to processes occurring in the oceans (e.g., biological activity) rather than abiotic redox reactions on the continents.

The variability of iron isotopes among rocky bodies in the inner Solar System provides a window onto the diversity of materials and mechanisms from which they formed. The magnitude of isotopic variation in mantle-derived rocks within a given body is similar to that between different planetary bodies. Isotopic signatures arising from primordial events, namely, evaporation/condensation, core formation and melting/crystallization, may be progressively diluted, modified, and redistributed over time by global recycling processes such as plate tectonics. Here, we assess the relative influence of these primordial mechanisms on the iron isotope compositions of igneous rocks and their implications for the structure and accretion histories of rocky planets.