Elasticity of spinels in the system (Mg,Fe2+)Al2O4: The effect of Fe-Mg substitution up to 10 GPa

According to the pyrolite compositional model, spinel-structured compounds are among the most abundant materials of the Earth’s mantle (especially in the transition zone between 410 and 660 km depth). It is widely known that iron is the most abundant transition metal in the minerals of the mantle. However, the exact iron content and its heterogeneity in the mantle is still a matter of debate. Fe concentration plays a crucial role in determining a wide range of physical properties of the main minerals of the mantle, In particular it was recently pointed out that heterogeneity of Fe content controls the shear waves velocity much more than temperature variations. We performed Brillouin-scattering measurements at ambient conditions and at high pressure up to 10 GPa of a set of synthetic oxide spinels in the system MgAl2O4 (Spinel s.s.) – FeAl2O4 (Hercynite) in order to assess how the Fe2+ → Mg substitution affects the elastic properties of spinel-structured oxides at high pressure. Our results reveal a strong deviation from linearity of the elastic tensor components (Cij’s) as a function of divalent iron content already at low Fe2+ region. With increasing Fe content C11 and C44 show an initial increase followed by a decrease at Fe/(Fe+Mg) > 0.1-0.2 (which is comparable with Fe content in the mantle ringwoodite). With our results we have placed, for the first time, quantitative constraints on the effect of Fe-Mg substitution on the elastic properties of spinel-hercynite solid solutions. Given that the iron content strongly affects the elastic properties of spinel-structured oxides, our results raise the important question of which is the role of both the chemical variability and the cations distribution, especially in terms of Fe → Mg substitution, on the elastic properties of the spinel-structured transition zone silicates (i.e. ringwoodite and wadsleyite). This question needs to be addressed in order to put constrains on the interpretation of seismic features (in particular from seismic tomography) of the mantle in terms of chemical composition and heterogeneity.