Deformation of olivine‐spinel aggregates in the system (Mg,Ni)2GeO4 deformed to high strain in torsion: Implications for upper mantle anisotropy

Deformation of mantle phases to high strain is fundamental for understanding mantle dynamics. However, it is technically challenging to perform deformation experiments yielding accurate mechanical results at the high pressures required for certain mantle phases, such as wadsleyite and ringwoodite. Deformation experiments on analog materials of such high‐pressure phases can provide insight into the deformation properties of the mantle. The (Mg,Ni) 2 GeO 4 system undergoes the olivine‐spinel transformation at much lower pressures than the silicate counterpart. Hence the high strain deformation properties of an olivine‐spinel rock can be studied at experimentally tractable pressures and temperatures. Experiments were conducted in a gas medium deformation apparatus at constant angular velocity, a temperature of 1473 K, and 300 MPa confining pressure. At these conditions with a Mg/(Mg + Ni) ratio of 0.9, the resulting aggregate comprised 70% olivine, 30% spinel, and <1% orthopyroxene. The applied strain rate ranged from 10 −4 to 10 −5 s −1 , yielding shear stresses supported by the samples of between 100 and 250 MPa. Samples were deformed to a range of shear strains between γ = 1 and γ = 7 in order to study the microstructural development during high strain deformation. In these experiments, the spinel phase did not deform to produce a crystallographic preferred orientation (CPO). The olivine phase produced a CPO which is consistent with slip on the [001](hk0) slip system. If this slip system is dominant in olivine below 250–300 km in the upper mantle, the observed seismic anisotropy can be explained without resorting to changes in flow regime or deformation mechanism at this depth.