On apparent mass-independent fractionation (MIF) signatures from phase partitioning at equilibrium

Abstract

ABSTRACTThe chance of apparent isotope anomalies either in closed systems at equilibrium or during Rayleigh processes is investigated. The term ‘apparent’ is chosen to highlight that only mass-dependent fractionation (MDF) processes are considered. A goal is finding analytical expressions that could be used for dealing with the complexity of real systems. For example, the formula for calculating the isotopic fractionation in systems with any desired number of phases involved is identified. Terrestrial samples could exhibit artificially low MIF signatures because Δ is not invariant with the choice of the reference material. An alternative parameter Δ* only depending on β is therefore introduced. The potential transfer of sample points in a three-isotope plot to parallel lines during reversible processes is also investigated. Larger apparent anomalies are expected in systems lacking an infinite reservoir and for large enrichment factors. Pyrite is stable beyond the conditions for a reducing environment and MIF signatures from Archaean sediments could be compatible with reversible processes involving oxidised species. Isotope anomalies in pyrite from the Dresser Formation, Australia, are compatible with post-depositional incorporation of 2–5% sulphur of atmospheric origin. Overall, observed delta values could mainly be related to changes in the local absolute isotope abundances.