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.
The Earth’s mantle is more oxidized than other planetary mantles. This is seen in the greater abundance of more oxidized forms of iron atoms. In most planetary mantles, metallic (Fe0 found in steel) and ferrous (Fe2+ found in volcanic minerals) iron are the dominant forms of iron while in Earth’s mantle, metallic iron is largely absent and ferrous and ferric (Fe3+ found in rust) dominate. The reason why the Earth is different is not well known but could have involved iron reduction and sequestration of metallic iron in Earth’s core or loss of reduced hydrogen gas to space. The timing of this oxidation process is also not well constrained. The inability to definitely answer those questions stems in part from the difficulty of establishing the oxidation state of mantle rocks. Indeed, upward transport and exposure to oxidative conditions at Earth’s surface can modify the oxidation state of mantle rocks. Therefore, scientists have to rely on indirect proxies of the oxidation state of the mantle. Intellectual Merit. To help understand why the Earth is more oxidized than other planetary bodies and to better trace exchanges between Earth’s mantle and the surface, we have established a new tracer of the oxidation state of iron during melting. It relies on measuring the isotopic composition of iron (i.e., the 56Fe/54Fe ratio) as ferric iron tends to have more of the heavy isotope variety than ferrous iron. The goal of this project was to calibrate this isotopic fractionation by using a novel spectroscopic technique known as nuclear resonant inelastic X-ray scattering (NRIXS). This sophisticated synchrotron technique allows one to measure the strength of the bonds that hold iron in position in a solid, which theory shows directly influence the way that iron isotopes are fractionated. We have made synthetic glasses of compositions similar to magmas. Some glasses contained mostly ferrous iron while other contained mostly ferric iron. We have found a strong relationship between the strength of iron bonds in solids and the oxidation state of iron. We can now relate quantitatively the iron isotopic composition of igneous rocks with the redox conditions in which they formed. Since 2011, this proposal has generated 10 peer-reviewed publications that have accrued 143 citations. Broader impacts. The question of the redox state of the Earth has far reaching implications beyond the confines of Earth sciences. For example, the rise of oxygen in Earth’s atmosphere approximately 2 billion years ago may have been related to a change in the nature of the gases released by volcanoes. Furthermore, our work helped establish a set of equilibrium iron isotope fractionation factors that can be used in a wide variety of contexts, from low temperature aquatic environments to magma ocean differentiation conditions on the Moon. This work is very novel and led us to important theoretical developments that will have important impact in all the fields that use this technique, such as high pressure mineral physics, heme biochemistry, and condensed matter physics. These developments also led to the development of a new software written by the PI, graduate and undergraduate students to do NRIXS data reduction using a GUI interface. This software is called SciPhon and is available to the community. This proposal supported the PhD of female graduate student Corliss Sio (now postdoctoral fellow at Geophysical Lab, CIW) and several undergraduate students (Matthew Go, Erik Baker, Mark Fornace, Magdalena Naziemiec), many of who are now pursuing PhD theses at premier institutions.
