Investigation of Iron speciation in silicate glasses and its implications for magma oceans

Abstract

As the most abundant multi-valent element in silicate melt, iron plays an important role in many physical and chemical respects. The ratio of Fe3+ and total Fe concentration, Fe3+/ΣFe, not only reflects but also establishes the redox environment in many magmatic processes. The Fe3+/ΣFe ratio increases with oxygen fugacity, but also is affected by chemical composition, temperature and pressure. This thesis presents experimental investigations of the Fe3+/ΣFe ratio changing with pressure in silicate melts and its implications for the redox environments of early magma ocean. Mössbauer spectrum is one of the most common methods to determine Fe3+/ΣFe ratio in glasses. In Chapter 2, two andesitic glasses synthesized at 1 atm, 1400 °C and 3.5 GPa, 1600 °C, were examined with Mössbauer spectra collected from 47-293 K. The recoilless fractions (f) of Fe3+ and Fe2+, can be determined from those variable-temperature Mo ̈ssbauer spectra. The correction number, C, equals f(Fe3+), will be used to f (Fe2+ ) correct the Fe3+/ΣFe ratio of andesite glasses determined through Mössbauer spectra collected at room temperature in the following studies. For 1 atm andesitic glasses equilibrated over a range of oxygen fugacities (logfO2 from -8.63 to -0.68), were examined with Fe K-edge X- ray absorption near-edge structure (XANES) and Mo ̈ssbauer spectra in Chapter 3. XANES spectral features were calibrated as a function of Mössbauer derived Fe3+/ΣFe ratios. The coordination number (CN) of Fe3+ and Fe2+ ions in andesitic glass can be calculated from observations of pre-edge centroid energies and total intensities, combined with independent constraints on Fe3+/ΣFe ratio from spectra. The mean coordination of Fe2+ ions calculated this way is close to 5.5 for reduced and oxidized compositions, and this is consistent with in- ferences from hyperfine features of the Mössbauer spectra. The mean coordination number of Fe3+ inferred from XANES increases from ∼4.5 to ∼5 as andesitic glasses vary from reduced to oxidized; Mössbauer hyperfine parameters also suggest network-forming behavior of Fe3+, but with higher coordination for more reduced glasses. In Chapter 4, the Fe3+/ΣFe ratios in andesitic glasses synthesized from 1 atm to 7 GPa were examined with Mo ̈ssbauer spectra. The Fe3+/ΣFe ratios decrease as pressure increase, from 1 atm to 4 GPa, and become flatten af- terwards. Those glasses were also examined with XANES spectra. Both hyperfine parameters from Mössbauer spectra and mean coordination number calculated from XANES features show that the CNs of Fe3+ in glasses are ∼5 and vary little with pressure changing, while for Fe2+, the CN increases as pressure increasing. A new thermodynamic model is built to explore the relationship between oxygen fugacity and pressure and consequently, for a homogenous magma ocean, the oxidation states are more reduced at shallow part than at depth.