NASA Astrobiology Institute (NAI)

The isotopic composition of iron is an excellent signature of past redox processes and the presence of a hydrologic cycle. Moreover, Fe isotope compositions promise to be a unique marker for dissimilatory iron reduction by bacteria, making this isotope system an excellent biosignature. Our goal is to develop analytical methods to make precise Fe isotope analysis on individual Fe-bearing minerals at a 10 micron diameter spot resolution. This technology will allow assessment of the Fe isotope heterogeneity within an individual Fe-bearing mineral and between different mineral grains. Documentation of such inter- and intra-mineral variations is critical to establishing if the Fe isotope variations measured in ancient rocks is a primary signature indicative of the environment in which the rock formed, or if it is a result of later metamorphic or diagenetic processes. Moreover, performing such spot mineral analyses will allow one to correlate Fe isotope variations that are associated with petrographic and mineral/chemical variations that cannot easily be done using conventional techniques.

Analytical methods have been developed that allow ⁵⁶Fe/⁵⁴Fe ratios to be measured to an external (2-SD) precision of 0.25 per mil using a 10 micron diameter primary ion beam on magnetite crystals. We are in the process of assessing the accuracy of these measurements relative to conventional Fe isotope analyses and to evaluate matrix effects associated with chemical and mineralogical changes of different magnetite minerals. These effects can be quite significant due to sublte changes in instrumental mass bias during ion probe measurements, up to 10 per mil.