The Role of Redox Processes in Determining the Iron Isotope Compositions of Minerals, Melts, and Fluids

Abstract

Stable isotope fractionation is a response to the minimisation of free energy associated with differences in vibrational frequencies during substitution of isotopic masses between two phases. Site properties are dictated by the bonding environment of iron, and influence its 'force constant', a descriptor for bond strength. Because iron readily transitions between its ferrous (Fe 2+ ) and ferric (Fe 3+ ) state over the oxygen fugacities (fO 2 ) of terrestrial magmas, redox processes are presumed to control iron isotope fractionation. In fact, both co-ordination and redox state are key to interpreting iron isotope variations in high-temperature, high-pressure geological systems. Determinations of iron force constants by experimental, theoretical and spectroscopic means in minerals, melts and fluids are reviewed, emphasising the effect of composition in influencing isotopic fractionation. Within this framework, the application of numerical models of partial melting to iron isotope variations in oceanic basalts and subduction-zone (or arc) magmas has shed light on their genesis. The influence of melt composition, mineral mode and fluid exsolution in producing iron isotope fractionation in evolving magmas is explored. Iron isotopic signatures in magmatic rocks are linked to that of their sources, and are influenced by the iron isotopic evolution of subducting slabs and transfer of fluids to the mantle wedge. 2.0. Principles and nomenclature Ratios of two isotopes, n and d of an element, E, are generally expressed relative to their ratio in a standard in delta notation, which is given by:(1) 𝛿 𝑛/𝑑 𝐸 = ( ( 𝑛 𝐸/ 𝑑 𝐸) 𝑠𝑎𝑚𝑝𝑙𝑒

Rejuvenated lavas resolve enriched ponded melt at the lithosphere-asthenosphere boundary

“…for an oceanic island, comparable to those observed in Samoan rejuvenated lavas [3]. Previous modelling shows similar heavy δ 57 Fe signatures in mantle-derived melts cannot be explained solely by fractionation during partial melting from any common mantle source [4,5]. Instead, these high values require a source enriched in heavy Fe isotopes prior to melt generation.…”