Experimental Evidence for the Existence of Metallic Fe-Rich Alloy in the Earth's Lower Mantle.
Abstract
It is known that aluminous silicate perovskite, the major mineral of Earth's lower mantle, can accommodate significant amounts of ferric Fe. We have performed multianvil experiments to determine the influence of oxygen fugacity and temperature on ferric iron solubility in aluminous perovskite. High-pressure melting and sub-solidus experiments were performed on synthetic perovskite and peridotite compositions between 21-27 GPa. Low oxygen fugacities were imposed by the presence of powdered metallic Fe and adding Re and ReO2 produced more oxidising conditions. These experiments show that perovskite ferric Fe solubility is positively correlated with the aluminium content but independent of oxygen fugacity. High ferric iron concentrations are present in Al-bearing perovskite even when it is in chemical equilibrium with metallic Fe and at the peridotite solidus. For typical mantle Al-contents we calculate that over 60% of iron in perovskite will be ferric. This requires the bulk ferric iron content of the lower mantle to be over ten times that estimated for the upper mantle. The lower mantle must consequently be either enriched in ferric iron compared to the upper mantle or perovskite must sequester oxygen by the reduction of ferrous iron to metallic iron. If whole mantle convection occurs then the latter is the more likely scenario and the lower mantle must contain approximately 1 weight % of a metallic Fe rich phase. This metallic phase would form as material enters the perovskite stability field and would remain trapped in the lower mantle assemblage. If minor amounts of this metallic Fe have separated to the core since the lower mantle formed this could have raised the oxygen content of the mantle from its early reduced state after core separation, to the level reflected in the present day upper mantle. This amount of metallic iron will not significantly influence the elastic properties of the lower mantle, however, the substitution mechanism of Al and ferric iron is likely to have a strong influence on the elastic properties of perovskite.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2003
- Bibcode:
- 2003AGUFM.T22D..05F
- Keywords:
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- 1025 Composition of the mantle;
- 3630 Experimental mineralogy and petrology;
- 3924 High-pressure behavior