Earth Uppermost Mantle Research Articles

Effects of Composite Rheology on Plate‐Like Behavior in Global‐Scale Mantle Convection

AbstractEarth's upper mantle rheology controls lithosphere‐asthenosphere coupling and thus surface tectonics. Rock deformation experiments and seismic anisotropy measurements indicate that composite rheology (co‐existing diffusion and dislocation creep) occurs in the Earth's uppermost mantle, potentially affecting convection and surface tectonics. Here, we investigate how the spatio‐temporal distribution of dislocation creep in an otherwise diffusion‐creep‐controlled mantle impacts the planform of convection and the planetary tectonic regime as a function of the lithospheric yield strength in numerical models of mantle convection self‐generating plate‐like tectonics. The low upper‐mantle viscosities caused by zones of substantial dislocation creep produce contrasting effects on surface dynamics. For strong lithosphere (yield strength > 35 MPa), the large lithosphere‐asthenosphere viscosity contrasts promote stagnant‐lid convection. In contrast, the increase of upper mantle convective vigor enhances plate mobility for lithospheric strength <35 MPa. For the here‐used model assumptions, composite rheology does not facilitate the onset of plate‐like behavior at large lithospheric strength.

Probability of radial anisotropy in the deep mantle

It is well established that the Earth's uppermost mantle is anisotropic, but observations of anisotropy in the deeper mantle have been more ambiguous. Radial anisotropy, the discrepancy between Love and Rayleigh waves, was included in the top 220 km of PREM, but there is no consensus whether anisotropy is present below that depth. Fundamental mode surface waves, for commonly used periods up to 200 s, are sensitive to structure in the first few hundred kilometers and therefore do not provide information on anisotropy below. Higher mode surface waves, however, have sensitivities that extend to and below the transition zone and should thus give insight about anisotropy at greater depths, but they are very difficult to measure. We previously developed a new technique to measure higher mode surface wave phase velocities with consistent uncertainties. These data are used here to construct probability density functions of a radially anisotropic Earth model down to approximately 1500 km. In the uppermost mantle, we obtain a high probability of faster horizontally polarized shear wave speed, likely to be related to plate motion. In the asthenosphere and transition zone, however, we find a high probability of faster vertically polarized shear wave speed. To a depth of 1500 km in the lower mantle, we see no significant shear wave anisotropy. This is consistent with results from laboratory measurements which show that lower mantle minerals are anisotropic but LPO is unlikely to develop in the pressure–temperature conditions present in the mid-mantle.

Grain growth in the forsterite–diopside system

Grain growth experiments in dunite, clinopyroxenite, and wehrlites with various forsterite/diopside ratios were performed to investigate the effect of modal composition on grain growth kinetics in the Earth's uppermost mantle. The experiments were conducted using a piston-cylinder apparatus at 1200 °C and 1.2 GPa for 2–763 h under dry conditions. Normal grain growth (NGG) occurred in dunite, clinopyroxenite, and relatively forsterite-poor wehrlites (≤70 vol.% of forsterite). Grain growth rates of forsterite and diopside in relatively forsterite-poor wehrlites were much slower than those in dunite and clinopyroxenite. In the forsterite-rich wehrlites (≥80 vol.% of forsterite), NGG of diopside and abnormal grain growth (AGG) of forsterite occurred. The growth rate of diopside was significantly slower than that in clinopyroxenite, while the growth rate of forsterite by AGG was found to be similar to that by NGG in dunite. The presence of ≤20 vol.% diopside had the effect of inhibiting the forsterite grain growth during the initial stage, resulting in AGG of forsterite, thus overtaking the growth rate in dunite. Our experimental results suggest that there would be a significant difference in grain growth rate and consequently in effective viscosity between olivine-rich peridotites (depleted mantle) and relatively olivine-poor peridotites (fertile mantle) in the case of grain size-sensitive creep. Variation of mean grain sizes in the upper mantle would result not only from differences in temperature and phase assemblage, but also from the variation of modal compositions.

Partitioning of Ni between magnesiowüstite and metal at high pressure: implications for core-mantle equilibrium

The partitioning of Ni between metal and magnesiowüstite is experimentally investigated up to 17 GPa. From the thermodynamic analysis of the experimentally determined distribution coefficient, K D(M/MW) = ( X Ni/ X Fe) metal/ X Ni/ X Fe) oxide, we obtain seven thermodynamic parameters relevant to its dependency on pressure, temperature and chemical composition. K D(M/MW) is strongly dependent on pressure, temperature and MgO content in magnesiowüstite. The pressure effect is most effective and K D(M/MW) significantly decreases with increasing pressure (i.e., chemical affinity of Ni with magnesiowüstite is enhanced by high pressure). K D between metal and bulk mantle silicates, K D(M/BMS), is estimated on the basis of compiled data of K D(M/Silicates) and the extrapolated value of K D(M/MW). K D(M/BMS) also decreases with pressure. The chemical reaction between sinking Fe alloy and mantle material at high pressure leads that Ni transfer into the mantle material from Fe alloy. From the comparison K D(M/BMS) and K D(core/mantle), the present high Ni concentration in the earth's uppermost mantle is well explained by the equilibrium partitioning of Ni during core-mantle separation combined with a subsequent mantle differentiation.

Is the olivine–spinel phase transformation martensitic?

Olivine, the dominant mineral in the Earth's uppermost mantle, undergoes one, or possibly a series of phase transformations to adopt a spinel structure at pressures above ∼13 GPa. The mechanism by which the transformation is accomplished could have important implications for understanding mantle dynamics and deep focus earthquakes. Recently, it has been proposed that the transformation may proceed by a martensitic mechanism1–5. We discuss here the characteristic properties of martensitic transformations and present evidence that the analogous transformation in Mg2GeO4 does not proceed by such a process.