Early solar irradiation as a source of the inner solar system chromium isotopic heterogeneity

Different solar system objects display variable abundances of neutron-rich isotopessuch as54Cr,50Ti, and48Ca, which are commonly attributed to a heterogeneous distributionof presolar grains in different domains of the solar system. Here, we show that theheterogeneity of54Cr/52Cr and the correlation of54Cr/52Cr with Fe/Cr in metal fractions ofEH3 chondrites and in inner solar system bodies can be attributed to variable irradiation ofdust grains by solar energetic particles and variable mixing of irradiated material in thedifferent domains of the inner solar nebula. The isotope variations in inner solar systemobjects can be generated by∼300 y long local irradiation of mm- to cm-sized solids withaverage solar energetic particle fluxes of∼105times the modern value. The relativehomogeneity of53Cr/52Cr in inner solar system objects can be a consequence of the productionof53Mn by the early irradiation of dust, evaporation, and nebula-wide homogenization of Mndue to high temperatures, followed by Mn/Cr fractionation within the first few million years ofthe solar system. The54Cr/52Cr of the Earth can be produced by irradiated pebbles and<15 wt%of CI chondrite like material. Alternatively, Earth may contain only a few % of CI chondritelike material but then must have an Fe/Cr ratio 10–15% higher than CI chondrites.