Experimental results of pressure and oxygen fugacity effects on metallic Fe formation from (Mg, Fe)O
Abstract
Ferropericlase (Mg0.8, Fe0.2) O and bridgmanite (Mg, Fe) SiO3 are primary minerals in the lower mantle. Fe in Earth's mantle is mostly either ferrous Fe or ferric Fe. However, Frost et al. (2004) showed the likely existence of metallic iron in the bridgmanite at P (pressure) ~24 GPa. If metallic Fe were present in the lower mantle, it would affect a number of properties of materials including the melting behavior.
Our observations on samples deformed at the shallow lower mantle conditions (P=24-28 GPa; Girard et al., 2016) showed that a large fraction of them occurs in or next to (Mg,Fe)O and that there is a marked variation in the metallic Fe content in a sample: high metallic Fe in the low-pressure region compared to the high-pressure region. These observations suggest the importance of (Mg,Fe)O, and the goal of this study is to investigate the formation of metallic Fe in (Mg,Fe)O. In order to investigate the influence of pressure and oxygen fugacity on the formation of metallic Fe in (Mg, Fe)O, we performed high-pressure experiments in which a sample of (Mg0.8,Fe0.2)O without metallic Fe was sandwiched with a buffer material to control the oxygen fugacity and annealed it for 0.5 - 20 hours at P=3 to 23 GPa and T=1773 K. Buffer materials provide lower oxygen fugacity and metallic Fe is produced in a sample. We study how the amount of metallic Fe changes with the physical and chemical (oxygen fugacity) conditions. For buffer materials, we used several metal-metal oxide buffers (Cr, Ti , TiC ) as well as (MgxFe1-x)O with x smaller than 0.2 (x is 0.2 for the sample). In most of the runs we found the formation of metallic Fe in the sample, but the role of buffer materials is rather complicated. When metal-metal oxide buffer was used, alloys are formed between Fe and the metal used as a buffer but also a few compounds were formed by the reactions between the buffer and the sample. The use of (MgxFe1-x)O was more successful than metal-metal buffer but we also found a complication caused by the slow kinetics of reduction that is comparable to the kinetics of Mg-Fe diffusion. After making corrections for these complexities, we found that the amount of metallic Fe formed in (Mg0.8,Fe0.2)O is reduced by pressure. Possible implications of this finding will be discussed. </gdiv>- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMDI33A..01X
- Keywords:
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- 1038 Mantle processes;
- GEOCHEMISTRY;
- 7270 Tomography;
- SEISMOLOGY;
- 8115 Core processes;
- TECTONOPHYSICS;
- 8120 Dynamics of lithosphere and mantle: general;
- TECTONOPHYSICS