Plastic flow of Mn2GeO4 I: Toward a rheological model of the Earth's transition zone

Abstract

Knowledge of the rheological properties of the major mineral phases in the Earth's mantle transition zone between 410 and 660 km depths is lacking because quantitative high-T deformation experiments at P = 13-25 GPa are technologically impossible. To provide a quantitative estimate of the rheology of the upper portion of the transition zone, we have initiated a study of the rheology of α- and β-Mn 2 GeO 4 . The α ↔ β phase transition in Mn 2 GeO 4 occurs at 3.5-5.0 GPa, within the P range of our modified Griggs-type apparatus. These two phases are structural analogues of α- and β-(Mg 0.9 Fe 0.1 ) 2 SiO 4 , the most abundant minerals residing on either side of the 410-km seismic discontinuity. Our working hypothesis is that the change in rheological behavior upon the α ↔ β-olivine phase transition in Earth's mantle can be estimated quantitatively from the creep data of α- and β-Mn 2 GeO 4 through normalization of mechanical data against the respective solidus T. Here, we report for the first time rheological data for α-Mn 2 GeO 4 olivine. We also examine the systematics of dislocation creep rheological properties of olivines with respect to homologous T. Polycrystals of α-Mn 2 GeO 4 olivine were deformed at strain rates (∈) of 10 -5 -10 -4 s -1 , T of 1000-1200 K and P of 0.4-3.8 GPa under both dry and wet conditions using both Griggs apparatus and gas-medium deformation apparatus. As observed in mantle olivine, water significantly weakens α-Mn 2 GeO 4 . At similar homologous T and f 01 , the strength of α-Mn 2 GeO 4 agrees with those of the other olivines within a factor of ∼4, suggesting that dislocation creep of olivines is determined to a large extent by common bonding and structural characteristics. Therefore, combining the new data on α-Mn 2 GeO 4 with the well-documented data for α-(Mg 0.9 Fe 0.1 ) 2 SiO 4 olivine should allow calculation of the rheology across the upper mantle-transition zone boundary when rheological data on β-Mn 2 GeO 4 become available from our ongoing studies.