Effect of Fe3+ on Phase Relations in the Lower Mantle: Implications for Redox Melting in Stagnant Slabs. Issue 12 (4th December 2019)

• Record Type: Journal Article
• Authors: Sinmyo, Ryosuke; Nakajima, Yoichi; McCammon, Catherine A.; Miyajima, Nobuyoshi; Petitgirard, Sylvain; Myhill, Robert; Dubrovinsky, Leonid; Frost, Daniel J.
• Is Part Of: Journal of geophysical research. Volume 124:Issue 12(2019)
• Page Start: 12484
• Page End: 12497
• Publication Date: 2019-12-04
• Subjects: lower mantle -- redox state -- melting -- bridgmanite -- ferric iron -- stagnant slab; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1029/2019JB017704
• Languages: English
• ISSNs: 2169-9313

Abstract

Abstract: Recent studies have revealed that Earth's deep mantle may have a wider range of oxygen fugacities than previously thought. Such a large heterogeneity might be caused by material subducted into the deep mantle. However, high‐pressure phase relations are poorly known in systems including Fe 3+ at the top of the lower mantle, where the subducted slab may be stagnant. We therefore conducted high‐pressure and high‐temperature experiments using a multi‐anvil apparatus to study the phase relations in a Fe 3+ ‐bearing system at 26 GPa and 1573–2073 K, at conditions prevailing at the top of the lower mantle. At temperatures below 1923 K, MgSiO3 ‐rich bridgmanite, an Fe 3+ ‐rich oxide phase, and SiO2 coexist in the recovered sample. Quenched partial melt was observed above 1973 K, which is significantly lower than the solidus temperature of an equivalent Fe 3+ ‐free bulk composition. The partial melt obtained from the Fe 3+ ‐rich bulk composition has a higher iron content than coexisting bridgmanite, similar to the Fe 2+ ‐dominant system. The results suggest that strong mantle oxygen fugacity anomalies might alter the subsolidus and melting phase relations under lower mantle conditions. We conclude that (1) a small amount of melt may be generated from an Al‐depleted region of a stagnant slab, such as subducted former banded‐iron‐formation, and (2) Fe 3+ is not transported into the deep part of the lower mantle because of its incompatibility during melting. Key Points: We determined phase relationships of Fe3+‐rich system at top of the lower mantle Melting temperature of Fe3+‐rich system is lower than Fe2+‐dominant system Fe3+ may not be transported into the deep lower mantle because of its incompatibility