This work was supported by NSF grant EAR-1347056 and NASA grant NNA13AA94A. We thank Prof. Ingo Horn for providing details on the LA-MC-ICP-MS setup in his lab;
and Prof. Martin Whitehouse for providing details on the spatial resolution of Fe isotope analysis by SIMS in his lab;
and Miss Thiruchelvi Reddy for help with data collection;
and two anonymous reviewers for their helpful comments;