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Stable-isotope variations exist among inner solar system solids, planets, and asteroids, but their importance is not understood. We report correlated, mass-independent variations of titanium-46 and titanium-50 in bulk analyses of these materials. Because titanium-46 and titanium-50 have different nucleosynthetic origins, this correlation suggests that the presolar dust inherited from the protosolar molecular cloud was well mixed when the oldest solar system solids formed, but requires a subsequent process imparting isotopic variability at the planetary scale. We infer that thermal processing of molecular cloud material, probably associated with volatile-element depletions in the inner solar system, resulted in selective destruction of thermally unstable, isotopically anomalous presolar components, producing residual isotopic heterogeneity. This implies that terrestrial planets accreted from thermally processed solids with nonsolar isotopic compositions.

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We present a method for the chemical separation of Cr from meteorite and terrestrial samples for isotopic analysis by thermal ionization mass spectrometry (TIMS). After sample digestion, separation of Cr(III) is achieved by means of a two-column cation-exchange chromatography procedure using AG 50W-x8 resin. In a first column, Cr(III) is isolated from major elements and the majority of trace elements. In a second column, trace amounts of Fe, Al and Ti are further removed. Total procedural yields are > 80%. Cr isotopes are measured by TIMS in the static multicollection mode. Mn/Cr ratios are obtained by multi-collector inductively coupled plasma source mass spectrometry (MC-ICPMS). The accuracy of our protocol was tested by reference to terrestrial analogs and comparison of Cr isotopic data for samples that underwent Cr purification following the cation-exchange chromatography described here and an alternative separation method employing both a cationic and an anionic chromatography step. Using our technique, Mn/Cr ratios reproduce to <2% (2s) and 53 Cr/ 52 Cr and 54 Cr/ 52 Cr to 6 ppm and 12 ppm, respectively (2s). This highly precise procedure allows the variability of Cr isotopes in the inner solar system objects to be addressed. Our method enabled us to document an initial homogeneity for 50,52,53 Cr isotopes within 10 ppm, while 20-70 ppm deficits in 54 Cr abundances have been resolved for a number of meteorite samples.

High-precision 60Fe-60Ni isotope data show that most meteorites originating from differentiated planetesimals that accreted within 1 million years of the solar system's formation have 60Ni/58Ni ratios that are approximately 25 parts per million lower than samples from Earth, Mars, and chondrite parent bodies. This difference indicates that the oldest solar system planetesimals formed in the absence of 60Fe. Evidence for live 60Fe in younger objects suggests that 60Fe was injected into the protoplanetary disk approximately 1 million years after solar system formation, when 26Al was already homogeneously distributed. Decoupling the first appearance of 26Al and 60Fe constrains the environment where the Sun's formation could have taken place, indicating that it occurred in a dense stellar cluster in association with numerous massive stars.

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