Doctor of Philosophy, Case Western Reserve University, 2024, Geological Sciences

Little is understood about the chemical evolution of banded iron formations (BIFs) subducted into the mantle during the Precambrian era. In general, the mantle becomes more reducing with increasing depth, with much of the deep mantle thought to be below the iron-wustite (IW) buffer. At equilibrium, under shallower mantle conditions, the hematite and magnetite in subducted BIFs would reduce wustite. In more deeply subducted BIFs, where the oxygen fugacity buffer is below IW, the wustite would reduce to iron metal. A key question is how rapidly iron oxide reduction reactions proceed at mantle pressures and temperatures. Fast reaction rate would imply that large amounts of wustite and/or metal may have precipitated in the deep mantle. BIFs that reduced to wustite and resisted further reduction could exist in the form of ULVZs (ulta-low velocity zones), as suggested by Dobson and Brodholt (2005). BIFs that fully reduced to iron metal could have produced large volume iron diapirs which would have been capable of sinking into the core and providing an inner core nucleation substrate, as suggested by Huguet et al. (2018). The studies reported here seek to answer these questions by determining the high-pressure, high-temperature reduction rates of iron oxides under mantle conditions. Chapter one describes the various approaches used to recreate banded iron formation subduction at high-pressures and high temperatures. Experiments explore temperatures from 600-1200 oC and pressures from 1.5-15 GPa. Chapter two addresses the first step of BIF reduction—the reduction of hematite and magnetite to wustite in the upper mantle. Experiments explore 14 temperatures from 600-1400 oC and pressures between 2-14 GPa. Chapter three addresses the final step in BIF reduction—the reduction of wustite to iron metal in the lower mantle.

Doctor of Philosophy, The Ohio State University, 2024, Earth Sciences

Knowledge of the distribution of Fe2+ and Mg between olivine and melt (the distribution coefficient, KD) is crucial to understand the origin and evolution of magmas. However, there is disagreement regarding which variables (temperature, melt composition, and oxygen fugacity – fO2) influence the value of KD, as well as the magnitude of their effects. To evaluate the dependence of KD on these variables, data were compiled from literature consisting of equilibrium olivine-melt pairs in experiments at controlled temperature, fO2, and 1 atm pressure. The results confirm that KD is essentially independent of temperature and fO2. However, it is strongly dependent on melt composition (particularly the concentration of silica and alkalis). An evaluation of different published formulations for KD using these data demonstrates that the expression of Gee and Sack (1988) is the most accurate and precise. Furthermore, a new and simpler model based on variation of KD with silica and alkalis has been fit to the olivine-melt database. This reproduces KD with the same accuracy and precision as the Gee and Sack (1988) formulation. The olivine-melt database also illustrates that KD can be used to calculate the proportion of the different valence states of iron in the melt (the Fe3+/ΣFe ratio), which cannot be measured using routine analytical techniques. The melt Fe3+/ΣFe can then be related to fO2 using empirical relationships. This method, referred to as the Olivine-Melt Equilibrium (OME) method, reproduces the fO2 imposed on the experiments within ±0.3 log units. This method was applied to compiled data for natural samples from literature from mid-ocean ridges, ocean islands, back-arc basin spreading centers, and volcanic arcs. Olivine-melt calculated values of fO2 for each location investigated agree with the results of independent techniques. These include compiled measurements of Fe3+/ΣFe ratios using Fe K-edge μ-X-ray Absorption Near Edge Structure (XANES) spectroscopy, as we (open full item for complete abstract)

Doctor of Philosophy, The Ohio State University, 2024, Earth Sciences

In order to connect volcanic rocks to their mantle sources, it is essential to consider redox equilibria and their dependence on temperature, pressure, chemical composition, and oxygen fugacity. Oxygen fugacity (fO2) is an intensive variable that strongly affects the behavior of those elements in magmas that are sensitive to changes in redox state, such as Fe, and therefore Mg-Fe silicates, such as olivine. Since fO2 plays an important role in fractional crystallization, in principle, it is possible to estimate fO2 from analyses of olivine in equilibrium with the melt. This research describes a new method based on this principle called the Olivine-Melt Equilibrium Method. This method first calculates Fe3+ and Fe2+ from a relationship involving the partitioning of Mg and Fe2+ between olivine and melt. The ratio of Fe3+/Fe2+ expresses the change in the valence state of Fe, which is related to the redox state of the magma. The calculated Fe3+ and Fe2+ contents of the melt can then be used to determine the fO2 at which magma crystallized from a model described by Kress and Carmichael (1991). This model expresses a relationship between the Fe3+/Fe2+ ratio of the melt, fO2, temperature, pressure, and melt composition. The Olivine-Melt Equilibrium Method has the advantage that olivine and glass compositions are determined by an Electron Probe Micro Analyzer (EPMA), so analyzed Fe3+ and Fe2+ of the melt is not required. This is useful because glass analyses in literature typically report all Fe as ΣFeO, rather than distinguishing between Fe2O3 and FeO. Therefore, there is no need for scarce and specialized analytical methods, such as synchrotron-based techniques, to distinguish between the different oxidation states of Fe. Additionally, this method takes advantage of the fact that olivine is ubiquitous in basaltic lavas, unlike Fe-Ti oxides used to estimate fO2 from geothermometer-oxybarometers. We have calculated oxygen fugacities from published analyses of coexisti (open full item for complete abstract)

PhD, University of Cincinnati, 2007, Arts and Sciences : Geology

Three episodes of alkaline volcanism of Quaternary age have been recognized in the Kula Volcanic Province (KVP) in Western Turkey. The alkaline volcanic rocks of the KVP vary in composition from basanite to tephriphonolite and from trachybasalt to basaltic trachyandesite. Measured values for 87Sr/86Sr and 143Nd/144Nd in the rocks of the KVP range from 0.703029 to 0.703534 and 0.512773 to 0.512998, respectively, (Gulec, 1991; Alici, et al., 2002) suggesting that an isotopically depleted mantle component is involved in the genesis of the Kula lavas. This mantle component is also enriched in the most incompatible elements, as shown by OIB-like primitive mantle normalized multi-element patterns, this indicates that enrichment of the mantle source is probably a recent event. The pMELTS algorithm of Ghiorso, et al., (2002) can be used to show that low degrees of partial melts, generated by fractional melting of a spinel peridotite mantle source (Maaloe and Aoki, 1977), can produce the alkaline magmas of the KVP. Assuming an underlying heat source, I used pMELTS to model the melting behavior of an average spinel peridotite mantle composition at melting at 23 kbar (~80 km) in the presence of 0.1% H2O at fO2 = QFM-1. The degree of partial melting is 8% for the KVP rocks and the primary magma composition is then subjected to polybaric fractional crystallization from 23 to 4 kbar. The compositional diversity displayed by the Kula volcanic rocks is reproduced successfully by the pMELTS polybaric fractionation calculations when the dP/dT gradient is set to 42 and 25 in the mantle and crust, respectively, and the oxygen fugacity is increased from one log unit below to one log unit above the QFM buffer at 13 kbar. Generation and evolution of the Kula magmas were simulated successfully by using pMELTS. Eight per cent fractional melting of an average spinel lherzolite composition at 23 kbar generated the primary magma and polybaric fractional crystallization simulations with changin (open full item for complete abstract)

Master of Science, The Ohio State University, 2010, Geological Sciences

The oxidation state of the Earth is an area of great interest in petrology and mineral physics, as it plays a key role in governing mantle mineralogy and determining mineralogical host of elements with multiple valence states, such as iron and carbon. The amount of oxygen available to drive reactions in a system, as measured by oxygen fugacity, dictates a system's mineralogy, as controlled by reactions with oxygen, including the oxidation of iron to form wustite: Fe + 1/2O2 = FeO (iron wustite buffer, IW), or the simultaneous oxidation of iron and diamond to form siderite: Fe + C + 3/2O2 = FeCO3 (siderite diamond iron buffer, SDI). The degree of oxidation of the lower mantle has been the subject of recent interest, particularly in light of the recently reported crystal-chemically controlled, pressure-induced auto-oxidation-reduction reaction in iron (Frost et al., 2004) and debates on the oxidation state of carbon in the mantle (Brenker et al., 2007; McCammon et al., 2004). In that pressure-induced iron self-reduction is independent of oxygen fugacity, it is likely that the coexistence of metallic iron and wustite buffers mantle redox state at or near IW, as well as determines whether the host of carbon is either diamond or a carbonate. Therefore, knowledge of the relationship between the buffer assemblage containing both reduced and oxidized carbon (SDI buffer) and that containing both reduced and oxidized iron (IW buffer) is critical to knowing the mineralogical host of carbon throughout the mantle as a function of redox state. Thermodynamic modeling of iron, carbon, wustite, and siderite suggests the IW buffer lies between 1.5 and 2.5 log units above the SDI buffer across the pressure and temperature range of Earth's mantle, suggesting that FeCO3 (siderite) will reduce to diamond. This model is supported by high-pressure, high-temperature experiments carried out in the laser-heated diamond anvil cell from 21-62 GPa and 2100-2300K, with starting material: Fe m (open full item for complete abstract)