The Potassium Isotopic Composition of CI Chondrites and the Origin of Isotopic Variations among Primitive Planetary Bodies

The isotopic analysis of moderately volatile elements such as K have gained significant interest in recent years as they possess the potential to help us better understand solar system formation. Even so, the precise K isotopic composition of CI chondrites, the most chemically primitive chondrite, has remained elusive. As the K elemental composition of CI chondrites matches well with the solar photosphere, it is possible that their K isotopic composition represents the solar system initial value. Here, we investigate the CI chondrite K isotopic composition in order to determine the precise CI chondrite, and thus possibly solar system initial, & delta;41K value. In addition, we investigate the K isotope compositions of several other chondrite groups, evaluate all available chondrite K isotope data together, and use these data along with data from a range of other isotope systems to assess if nucleosynthetic variations, volatility related processes, or parent body processes can best explain the range of isotope variations. The & delta;41K composition of all nine CI chondrite pieces analyzed in this study show limited variation, ranging from -0.29%o to -0.17%o. When combined with the previous CI analysis, an overall mean CI & delta;41K value of -0.21 & PLUSMN; 0.05%o (2SE) is obtained. This K isotope composition is distinct from the Bulk Silicate Earth value of -0.43 & PLUSMN; 0.17%o (2SD), heavier than almost all other chondrite groups, and may represent the solar system initial K isotope composition. When comparing all chondrites broadly, ordinary chondrites show the lightest mean K isotope composition of -0.76 & PLUSMN; 0.06%o (H = -0.71 & PLUSMN; 0.12%o, L = -0.77 & PLUSMN; 0.04%o, LL = -0.81 & PLUSMN; 0.12%o), enstatite chondrites the middle composition of -0.39 & PLUSMN; 0.11%o (EH = -0.34 & PLUSMN; 0.05%o, EL = -0.45 & PLUSMN; 0.20%o), and carbonaceous chondrites the heaviest composition of -0.31 & PLUSMN; 0.08%o. For the carbonaceous chondrite groups CK (-0.42 & PLUSMN; 0.11%o), CR (-0.46 & PLUSMN; 0.05%o), and CV (-0.38 & PLUSMN; 0.07%o) chondrites show lighter & delta;41K compositions compared to CO (-0.20 & PLUSMN; 0.10%o), CM (-0.23 & PLUSMN; 0.11%o), and CI (-0.21 & PLUSMN; 0.05%o) chondrites. When these K isotope group averages are compared against the averages for other mass-dependent moderately volatile element isotope systems (& delta;87Rb, & delta;66Zn, & delta;71Ga, & delta;128Te) and mass-independent isotope systems (& epsilon;54Cr, & epsilon;64Ni, & epsilon;50Ti, & UDelta;17O, & epsilon;40K, and & epsilon;66Zn,), a range of correlations are observed. Across all chondrite groups & delta;41K shows correlations with & delta;87Rb, & delta;66Zn, and & delta;71Ga, and correlations with & epsilon;54Cr, & epsilon;64Ni, & epsilon;50Ti, & epsilon;40K, and & epsilon;66Zn. When comparing the CCs only, correlations are observed between & delta;41K and all four of the other moderately volatile elements assessed, while the mass-independent isotope systems show no strong correlations. Regarding the K isotope variations, these observations, along with other textural and chemical data, can be best explained by inherited isotopic variations form different precursor reservoirs (the cause of which is difficult to conclusively determine, though most likely related to the NC-CC dichotomy), and volatility related fractionation processes for the carbonaceous chondrite groups (most likely due to component mixing).