SIMS Iron Isotope Measurements of the Balmat Pyrite Reference Material: A Non-Unique ⁵⁶Fe Signature

In situ iron isotope ratios (delta Fe-56) in sulfide measured by secondary ion mass spectrometry (SIMS) can provide valuable information on several of Earth's surface processes. SIMS relies on the use of a matrix-matched reference material to correct for instrumental mass fractionation. To date Balmat pyrite has been widely used as a reference material, on the assumption of its homogeneous delta Fe-56 composition. However, several studies have reported divergent bulk delta Fe-56 values, which may jeopardise its use. Here, we combined bulk solution MC-ICP-MS and in situ SIMS delta Fe-56 measurements on two Balmat batches: the Balmat-Original published in Whitehouse and Fedo (2007) and Balmat-UNIL. Despite similar compositions, this study demonstrates the existence of two isotopically distinct Balmat populations. With respect to Balmat-Original (delta Fe-56 = -0.39 +/- 0.05 parts per thousand, 2s), Balmat-UNIL is isotopically 'lighter' with a bulk solution MC-ICP-MS composition of -1.46 +/- 0.024 parts per thousand. Additionally, Balmat-UNIL has two subpopulations: the first is characterised by delta Fe-56 values of -1.46 +/- 0.25 parts per thousand, whereas the second agrees with the original Balmat batch. In each Balmat-UNIL subpopulation, the intra-grain and inter-grain variabilities are sufficient to use Balmat as a reference material for delta Fe-56 isotope measurements by SIMS. This study revealed at least two end-member compositions of Balmat pyrite and calls for a careful batch-specific determination of bulk delta Fe-56.