Crystal chemistry of metal element substitution in olivine and its high-pressure polymorphs: Implications for the upper-mantle and the mantle transition zone

Abstract

In this article, we review the distributions of metal elements in the crystal structures of olivine and its high-pressure polymorphs (wadsleyite and ringwoodite) and among these major phases in the upper mantle and the mantle transition zone based on reported structure refinements and chemical compositions of both natural and synthetic mineral samples. We discuss the concepts of a potential reservoir of transition metals in the mantle transition zone and the possible alterations of globally observed 410-km and 520-km discontinuities due to metal element substitutions in olivine polymorphs. We also focus on the crystal chemistry of wadsleyite II (the fourth polymorph of olivine) and the effects of this potential constituent mantle phase on the properties of the middle part of the transition zone. A summary of recent studies indicates that, (1) for olivine and wadsleyite, site preference of a transition metal cation in the crystal structure is controlled by multiple factors that include cation size, site geometry, crystal field stabilization energy (CFSE), and oxygen bond-type. These factors may also determine the distributions of transition metal elements among olivine polymorphs; (2) Ni, Cr, and Ti prefer wadsleyite and ringwoodite to olivine, indicating that the mantle transition zone may be a potential reservoir of these transition metal elements, particularly if a small amount of melt is present near phase boundaries; (3) Ni2+, Co2+, and Fe2+ in the crystal structures have significant influences on the thermodynamic properties and phase transitions of olivine polymorphs. The incorporation of minor Ni2+ or Co2+ (>5 at.%) into the octahedral sites in wadsleyite could possibly lead to a shift of the 520-km discontinuity of about 30–80 km to shallower depths. In contrast, the stability field of wadsleyite can be expanded to higher and lower pressures in the presence of Cr3+ and Ti4+. Therefore, the localized high concentrations of these transition metal cations in the mantle transition zone may also contribute to the variations of depth and sharpness in the 410-km and 520-km discontinuities.