nai

Project Progress

Fe Isotope Biosignatures Group (dm)

Our major effort this year focused on a) bringing new instrumentation on line for ultra-high precision Fe isotope measurements on very small samples, b) continuing Fe isotope studies of natural samples that may have a biologic origin, and c) experimental determination of Fe isotope fractionation in minerals and fluids. A next-generation multi-collector ICP-MS was installed in a new clean lab, in part funded by NAI, and this instrument produces an unprecedented level of precision (+/- 0.05 per mil) for Fe isotope analysis on very small samples (~ 100 ng). Using our new instrumentation, we completed a survey of late Archean Banded Iron Formations, where up to 3 per mil variation in 56Fe/54Fe ratios were found, making it clear that Fe isotope variations can be preserved in the ancient sedimentary record. Work on determining Fe isotope fractionation in the lab led to the discovery of large (2.7 per mil) isotopic fractionation between Fe(III) and Fe(II) in dilute aqueous solution. Work on the hematite-Fe(III) system showed that although kinetic isotope effects can occur during rapid precipitation of hematite from solution, equilibrium isotope fractionation in this system appears to be minimal. In summary, our results demonstrated, for the first time, that Fe isotope anomalies appear to be restricted to low-temperature, aqueous environments, providing a powerful biosignature for detecting conditions on a planet that were favorable for life. In terms of accomplishments related to our original goals, installation and development of our new instrumentation progressed much faster than we had anticipated. The Fe isotope variations we documented in Banded Iron Formations were much larger than we originally thought, as were the isotopic fractionations among aqueous species. These discoveries have clearly influenced our plans for the coming year.