Redox and Structural Controls on Iron Isotopic Variations in Igneous Rocks

University Of Chicago, Chicago IL

Investigators

Abstract

The elevated degree of oxidation present in the Earth is a major condition of its habitability, allowing the existence of free oxygen and other oxidized species used during respiration to sustain activity in human beings and simpler life forms. Earth is more oxidized than other planetary bodies such as Mars and the reason for this is not well understood. For example, Titan's atmosphere (Titan is the largest moon of Saturn) is composed primarily of nitrogen, methane and ethane. In addition, Titan's surface is covered with lakes of hydrocarbons. It is not known whether Earth was born like it is today or whether it started with a Titan-like atmosphere and the oxidized conditions were established during Earth's history through geological processes. In this study, a new tool will be developed to measure the oxidation conditions of Earth through time. The measurements will use and develop cutting edge analytical methods at a national facility; the intense X-ray source located at the Advanced Photon Source (Argonne National Laboratory). This study will provide critical constraints on why our planet is unique and it will help us understand the nature of volcanic emissions in the distant past. On long timescales, the nature of volcanic emissions has played a key role in climate regulation and prevented the Earth from going into a permanent snowball state. The iron oxidation state of magmas (i.e., Fe3+/Fe2+ ratio) is a key parameter to trace the redox evolution of the Earth. Unfortunately, geological processes such as assimilation, degassing, crystallization, and alteration can blur this record. Iron isotopes provide insight into the conditions of mantle melting that are less susceptible to these secondary processes. A team of investigators with expertise in experimental petrology, iron isotope geochemistry, and nuclear resonance vibrational spectroscopy will calibrate the effects of redox and structural conditions on equilibrium isotopic fractionation between ferrous (Fe2+) and ferric (Fe3+) iron in magmas and minerals. This will provide a solid framework for interpreting iron isotopic variations and redox conditions in igneous rocks of all ages. Silicate glass, olivine, and spinel will be studied by the Nuclear Resonant Inelastic X-ray Scattering (NRIXS) technique to get a holistic view of iron isotopic fractionation during mantle and crustal melting, as well as mafic and felsic magma differentiation. Measurements of basalts through rhyolites produced under a range of oxygen fugacities, will allow the parameterization of iron equilibrium fractionation factors of magmas taking into account parameters such as Fe3+/Fetot ratio and NBO/T (nonbridging oxygen per tetrahedrally coordinated cation, a measure of polymerization of a silicate melt) to predict equilibrium Fe isotopic fractionation between minerals and melts.