Controls on Iron Isotope Variations in Planetary Magmas

Abstract

Of all documented planetary bodies such as Mars, Vesta, and the angrite parent-body (APB), Earth is the most oxidized [1]. Understanding how and when Earth's mantle acquired its present redox conditions is a major standing question in planetary sciences. Previsous studies have suggested that iron isotopes could be good tracers of redox conditions during melting [2]. Terrestrial basalts, as well as more felsic rocks, tend to have heavy iron isotopic composition relative to chondrites and Earth’s mantle [2, 3 and references therein]. For example, the average MORB δ^(56)Fe value is ~+0.1 ‰ while chondrites have δ^(56)Fe~+0 ‰ (Fig. 1). In contrast, basalts from Mars and Vesta have Fe isotopic compositions identical to chondrites within uncertainty. Three interpretations have been proposed to explain this feature: (1) during the Moon-forming giant impact, some isotopically light Fe was evaporated, leaving a residue enriched in heavy Fe isotopes [4]; (2) equilibriation between metal and high-pressure phases such as ferropericlase and post-perovskite created iron isotopic fractionation in Earth's mantle [5]; or (3) the isotopic composition measured in crustal rocks from Earth was produced by equilibrium or kinetic isotope fractionation between mantle peridotite and melt [2,6,7]. This poses several critical questions. What aspect of the melting process produces Fe isotopic fractionation? Why does melting on Earth or the APB fractionate Fe isotopes while on Mars and Vesta such fractionation is absent?