The Relative Strength of the (Fe,Mg)2SiO4 Polymorphs

Abstract:

The polymorphs of (Fe,Mg)₂SiO₄ (olivine, wadsleyite and ringwoodite) form the major component of the Earth’s mantle down to depths of 660 km. Olivine is the stable polymorph to depths of around 410 km, wadsleyite between 410 and 510 km and ringwoodite between 510 and 660 km. The rheology of these three phases plays a significant role in controlling the dynamics of the Earth’s mantle and has implications for the origins of deep focus Earthquakes. Each of the three polymorphs has a different crystal structure, along with which comes different physical properties, including rheology.

The importance of olivine rheology is reflected in the large body of work investigating it over multiple decades. The rheology of wasleyite and ringwoodite, while equally important, has not been so comprehensively studied because of the historic difficulty of performing controlled strain-rate deformation experiments under the stability conditions of the higher pressure phases. Recent studies investigating the rheology of wadsleyite and ringwoodite [e.g. 1,2] do not directly compare their strengths to that of olivine. To date there is only one study directly comparing the strengths of the (Fe,Mg)₂SiO₄ polymorphs [3], which demonstrated that at low temperatures (≤800°C) olivine is significantly weaker its higher pressures polymorphs. However, these experiments were stress-relaxation experiments.

I will present results of constant strain-rate deformation experiments performed under in-situ conditions, using the newly developed Deformation T-Cup [4]. Because the (Fe,Mg)₂SiO₄ polymorphs are not stable concurrently, I measure their strength relative to a pyrope garnet standard which is stable within the stability fields of all the polymorphs. The sample of interest and the standard are deformed simultaneously in a single experiment and the ratio of their strains gives the relative strength.

References:

1: Hustoft et al., 2012, Earth Planet. Sci. Lett. 361, 7

2: Kawazoe et al., 2010, J. Earth Sci. 21, 517

3: Chen et al., 1998, Geophys. Res. Lett., 25, 575

4: Hunt et al., 2014, Rev. Sci. Instrum., 81, 085103