11:30 AM - 11:45 AM
[SMP29-09] Contrasting effect of ferric iron on the structure of peridotitic glasses
Keywords:iron valence, melt structure, XANES, magma ocean
The iron (Fe) valence state in peridotitic melt is important in understanding the nature of the magma ocean in the early Earth. The initial magma ocean is expected to be enriched in Fe, but Fe content in the magma ocean would decrease during core formation process. The change in Fe content in peridotitic melt may affect the valence state of Fe in the magma ocean, because the Fe valence ratio is considered to be influenced not only by temperature, pressure, oxygen fugacity but also by chemical compositions. However, previous studies on the valence state of Fe in silicate melts have been carried out mostly on SiO2-rich silicate compositions (andesite-to-basalt melts) (e.g. Kress and Camichael, 1991), while there is only few experimental investigations of the Fe valence state in peridotitic melt, due to experimental difficulties such as high melting temperature and difficulty to vitrify peridotite glass for valence state analysis.
In this study, we used aerodynamic levitation furnace to carry out melting experiments on peridotitic compositions and subsequently recover the glass sample by fast cooling with containerless method. Experiments were conducted under air and Ar+5 wt.% H2 gas environments, respectively. We investigated (Mg,Fe)SiO3 and a peridotite composition glasses with varying Fe content. Fe3+ ratio was determined by Fe L3-edge XANES measurement at BL27SU in SPring-8. The structure of glass was investigated in terms of Qn and Non-Bridging Oxygen per Tetrahedral cation (NBO/T) by Raman spectroscopy.
Results showed Fe3+ ratio tend to increase with increasing Fe content in all compositions. The change in Fe3+ ratio with varying Fe# is much less in glasses synthesized under reducing Ar+5 wt.% H2 gas environment compared to syntheses under air environment. Structure of (Mg,Fe)SiO3 glasses synthesized under air environment is found to depolymerize with increasing Fe#. In contrast, structure of the peridotitic glasses polymerizes with increasing Fe#. These results imply distinct behavior of Fe3+ in the glass structure between (Mg,Fe)SiO3 and peridotitic glasses, suggesting Fe3+ may act as network modifier in (Mg,Fe)SiO3 melt, while it incorporates peridotitic melt as network former. The distinct behavior of Fe3+ in different melt compositions may be important to discuss nature and dynamics of magmas in the Earth's interior.
In this study, we used aerodynamic levitation furnace to carry out melting experiments on peridotitic compositions and subsequently recover the glass sample by fast cooling with containerless method. Experiments were conducted under air and Ar+5 wt.% H2 gas environments, respectively. We investigated (Mg,Fe)SiO3 and a peridotite composition glasses with varying Fe content. Fe3+ ratio was determined by Fe L3-edge XANES measurement at BL27SU in SPring-8. The structure of glass was investigated in terms of Qn and Non-Bridging Oxygen per Tetrahedral cation (NBO/T) by Raman spectroscopy.
Results showed Fe3+ ratio tend to increase with increasing Fe content in all compositions. The change in Fe3+ ratio with varying Fe# is much less in glasses synthesized under reducing Ar+5 wt.% H2 gas environment compared to syntheses under air environment. Structure of (Mg,Fe)SiO3 glasses synthesized under air environment is found to depolymerize with increasing Fe#. In contrast, structure of the peridotitic glasses polymerizes with increasing Fe#. These results imply distinct behavior of Fe3+ in the glass structure between (Mg,Fe)SiO3 and peridotitic glasses, suggesting Fe3+ may act as network modifier in (Mg,Fe)SiO3 melt, while it incorporates peridotitic melt as network former. The distinct behavior of Fe3+ in different melt compositions may be important to discuss nature and dynamics of magmas in the Earth's interior.