Rheology of Mg2GeO4 olivine and spinel harzburgite: Implications for Earth's mantle transition zone

• Published in: Geophysical research letters Vol. 42; no. 7; pp. 2212 - 2218
• Main Authors: Shi, F., Zhang, J., Xia, G., Jin, Z., Green II, H. W.
• Format: Journal Article
• Language: English
• Published: Washington Blackwell Publishing Ltd 16-04-2015; John Wiley & Sons, Inc
• Subjects: Boundary conditions; Constraints; Deformation; Discontinuity; Dislocation; dislocation creep; Dislocations; Earth; Earth mantle; Geophysics; Grain size; Magnesium germanates; mantle transition zone; Measurement; Mg2GeO4 olivine; Mg2GeO4 spinel; Modelling; Olivine; Phase transitions; Pyroxenes; Rheological properties; Rheology; Size reduction; Solifluction; Spinel; Transition zone; Upper mantle; Viscosity
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1002/2015GL063316

Geophysical modeling requires an upper mantle low‐viscosity zone (LVZ); recent results suggest that the LVZ could be within the mantle transition zone (MTZ) or above it, depending on boundary conditions. The rheology of the olivine‐dominated uppermost mantle is well constrained experimentally but not the MTZ. Here we report first‐time measurement of the rheology of Mg2GeO4 spinel (analogue of ringwoodite, the dominant mineral of the lower MTZ) in the dislocation creep regime and compare it with that of Mg2GeO4 olivine. In both cases, we incorporate 15–20% pyroxene, the second most abundant mineral in the upper mantle to provide a realistic comparison of rock rheologies. Under similar conditions, spinel‐dominated material is about twice as strong as olivine‐dominated material. Our results suggest that unless H2O weakening of ringwoodite or phase‐transformation‐induced grain size reduction occurs in the lower MTZ, it will be more viscous than above the 410 km discontinuity.
Key Points
Deformation mechanism of both phases of Mg2GeO4 dominated by dislocation creep
Mg2GeO4 spinel is approximately twice as strong as the olivine phase
Stronger Mg2GeO4 spinel places constraints on models of mantle transition zone