SIMS Iron Isotope Measurements of the Balmat Pyrite Reference Material: A Non‐Unique δ⁵⁶Fe Signature

Abstract

In situ iron isotope ratios (δ56Fe) 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 δ56Fe composition. However, several studies have reported divergent bulk δ56Fe values, which may jeopardise its use. Here, we combined bulk solution MC‐ICP‐MS and in situ SIMS δ56Fe 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 (δ56Fe = ‐0.39 ± 0.05‰, 2s), Balmat‐UNIL is isotopically 'lighter' with a bulk solution MC‐ICP‐MS composition of ‐1.46 ± 0.024‰. Additionally, Balmat‐UNIL has two subpopulations: the first is characterised by δ56Fe values of ‐1.46 ± 0.25‰, 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 δ56Fe 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 δ56Fe.