Oxygen Vacancy Ordering in Aluminous Bridgmanite in the Earth's Lower Mantle. Issue 15 (5th August 2019)

• Record Type: Journal Article
• Authors: Grüninger, Helen; Liu, Zhaodong; Siegel, Renée; Boffa Ballaran, Tiziana; Katsura, Tomoo; Senker, Jürgen; Frost, Daniel J.
• Is Part Of: Geophysical research letters. Volume 46:Issue 15(2019)
• Page Start: 8731
• Page End: 8740
• Publication Date: 2019-08-05
• Subjects: point defects -- 27Al MAS NMR spectroscopy -- perovskite -- high‐pressure silicates; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2019GL083613
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: Oxygen vacancies (OVs), that charge‐balance the replacement of octahedrally coordinated Si 4+ by Al 3+ in the mineral bridgmanite, will influence transport properties in the lower mantle but little is known about their stability and local structure. Using 27 Al nuclear magnetic resonance (NMR) spectroscopy we have characterized OVs within six aluminous bridgmanite samples. In the resulting NMR spectra sixfold, fivefold, and fourfold coordinated Al species are resolved, in addition to near eightfold coordinated Al substituting for Mg. Fivefold coordinated Al is formed by single OV sites but fourfold coordination must result from short range ordering of OVs, producing OV clusters that may form through migration into twin domain walls. Characterizing the occurrence of such OV structures is an important prerequisite for understanding how transport properties change with depth and composition in the lower mantle. Plain Language Summary: The lower mantle encompasses the largest region of the Earth's interior and is mainly composed of the perovskite‐structured mineral (Mg, Fe, Al)(Al, Si)O3 bridgmanite. Its properties, therefore, control both the diffusive transport of elements and solid state flow in the lower mantle, which will be strongly influenced by point defects. We have identified and quantified defects in bridgmanite that arise from the replacement of silicon by aluminum and result in the creation of a vacant oxygen site. These oxygen defects are also found to form clusters in the structure, which in other perovskite structured minerals have been shown to strongly affect physical properties. As defect formation and ordering is dependent on composition and pressure, strong variations in physical properties may be expected within the upper 300 km of the lower mantle. Key Points: Al substitution in bridgmanite is dominated by the oxygen vacancy mechanism for peridotite mantle compositions Short‐range ordering of oxygen vacancies takes place forming oxygen vacancy clusters possibly due to migration into twin domain walls Oxygen vacancy clusters are expected to have a major influence on transport properties of the lower mantle, such as the electrical conductivity