Iron isotope systematics in planetary reservoirs

Iron is the only polyvalent major element, and controls reduction-oxidation (redox) reactions in a host of geologic processes and reservoirs, from the mineral- to planetary-scale, on Earth and in space. Mass transfer of Fe is often accompanied by changes in bonding environment, meaning the resultant variation in bond-strength in crystals, liquids and gases induces stable isotope fractionation, even at high temperatures. In the absence of iron exchange, electron transfer can also affect iron's valence state and calculated oxygen fugacity (fO(2)), however its isotope composition remains unchanged. Thus, iron isotopes are a powerful tool to investigate processes that involve mass transfer, redox reactions and changes in bonding environment in planetary systems. Primitive chondritic meteorites show remarkable isotopic homogeneity, delta Fe-57 = -0.01 +/- 0.01 parts per thousand (2SE), over a wide range of Fe/Mg vs Ni/Mg, a proxy for fO(2) in the solar nebula. In chondrites, there are iron isotope differences between metal and silicates that become more pronounced at higher metamorphic grades. However, on a planetary scale, Mars and Vesta overlap with chondrites, preserving no trace of core formation or volatile depletion on these bodies. Upon assessment of pristine lherzolites, the Bulk Silicate Earth is heavier than chondrites (delta Fe-57 = +0.05 +/- 0.01 parts per thousand; 2SE), and similar to or slightly lighter than the Moon. That the mantles of some differentiated inner solar system bodies extend to heavier compositions (+0.2 parts per thousand) than chondrites may principally result from volatile depletion either at a nebular or late accretion stage. Within terrestrial silicate reservoirs, iron isotopes provide insight into petrogenetic and geodynamic processes. Partial melting of the upper mantle produces basalts that are heavier than their sources, scaling with degree of melting and driving the increasingly refractory peridotite to lighter compositions. Mid-Ocean Ridge Basalts (MORBs) are homogeneous to delta Fe-57 = 0.10 +/- 0.01 parts per thousand (2SE) after correction to primary magmas, and can be produced from single stage melt extraction. Conversely, iron isotopes in arc basalts are more varied (-0.2 < delta Fe-57 (parts per thousand) < +0.2) than can be produced from partial melting. Their iron isotope compositions are significantly lighter, suggesting they form from mantle re-enriched in light Fe and/or more depleted than Depleted MORB Mantle (DMM). If arc sources are more oxidised, an agent other than iron is required. Magmatic differentiation drives enrichment in heavy isotopes by partial melting of crustal rocks, fluid exsolution and crystallisation. Iron isotope trajectories in evolving magmas depend on their initial 102 and whether the system is closed or open to oxygen and/or mass exchange. Granite end members carry signatures diagnostic of their tectonic setting, where reduced, anorogenic A-type granites (delta Fe-57 = +0.4%o) are heavier than more oxidised I-types (delta Fe-57 = +0.2 parts per thousand). (C) 2016 Elsevier B.V. All rights reserved.