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

摘要

As the most abundant multi-valent element in silicate melt, iron plays an important role in many physical and chemical respects. The ratio of Fe 3+ and total Fe concentration, Fe3+/SigmaFe, not only reflects but also establishes the redox environment in many magmatic processes. The Fe3+/SigmaFe ratio increases with oxygen fugacity, but also is affected by chemical composition, temperature and pressure. This thesis presents experimental investigations of the Fe3+/SigmaFe ratio changing with pressure in silicate melts and its implications for the redox environments of early magma ocean. Mossbauer spectrum is one of the most common methods to determine Fe3+/SigmaFe ratio in glasses. In Chapter 2, two andesitic glasses synthesized at 1 atm, 1400 °C and 3.5 GPa, 1600 °C, were examined with Mossbauer spectra collected from 47-293 K. The recoilless fractions (f) of Fe 3+ and Fe2+, can be determined from those variable-temperature Mossbauer spectra. The correction number, C, equals f(Fe 3+), will be used to f(Fe2+) correct the Fe3+/SigmaFe ratio of andesite glasses determined through Mossbauer 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 Mossbauer spectra in Chapter 3. XANES spectral features were calibrated as a function of Mossbauer derived Fe3+/SigmaFe 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+/SigmaFe 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 Mossbauer spectra. The mean coordination number of Fe3+ inferred from XANES increases from ∼4.5 to ∼5 as andesitic glasses vary from reduced to oxidized; Mossbauer hyperfine parameters also suggest network-forming behavior of Fe3+, but with higher coordination for more reduced glasses. In Chapter 4, the Fe3+/SigmaFe ratios in andesitic glasses synthesized from 1 atm to 7 GPa were examined with Mossbauer spectra. The Fe3+/SigmaFe ratios decrease as pressure increase, from 1 atm to 4 GPa, and become flatten afterwards. Those glasses were also examined with XANES spectra. Both hyperfine parameters from Mossbauer 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.