Structural Transitions and Strengths of Mg-Silicate Glasses to 80 GPa

Early Earth's mantle may be molten to its deep interior. Therefore, physical processes in the magma ocean determine the initial conditions for the structure of the mantle. Seismologic studies have shown that the present-day mantle has melts atop the transition zone and the core-mantle boundary. Therefore, the properties of silicate melts are important for understanding the evolution of the Earth's mantle. We measured Raman scattering of silicate glasses, frozen forms of melts, with mantle related compositions, MgSiO3, Mg2SiO4, and CaSiO3, in the diamond-anvil cell up to 80 GPa. In MgSiO3 glass, Raman spectra show a structural transition associated with increases in the coordination number at 19-38 GPa and another transition likely related to changes in the Si-O polyhedral connectivity at 65-70 GPa. However, in CaSiO3 and Mg2SiO4 glasses, the former transition occurs at higher pressures (P) by 5-10 GPa and the latter transition does not occur to our maximum P, indicating that a less polymerized Si-O network increases the transition P. The compositional sensitivity of the transition P in silicate melts would result in dense MgSiO3 melt in a high-P structure and less dense Mg2SiO4 and CaSiO3 melts in low-P structures at mid-mantle depths. This density contrast will make Si-rich melts negatively buoyant but Si-poor melts postively buoyant, resulting in the Si-enriched deeper mantle. Therefore, the early magma ocean may be compositionally stratified and therefore contribute to the formation of "hidden geochemical reservoir". Our findings provide a physical mechanism for the early differentiation suggested from geochemical observations of short-lived 142Nd (Boyet and Carlson, 2005). We also measured the yield strength of MgSiO3, Mg2SiO4, and SiO2 glasses to 55 GPa in the diamond cell through the pressure gradients in the samples. The change in thickness with P was monitored visually by measuring the change in focal point from both sides of the sample. Yield strength does not appear to be signficantly different within uncertainties among glasses of different compositions. The measured strength of SiO2 glass is lower than previously reported, but we still observed the same trend from room pressure to 50 GPa (Meade and Jeanloz, 1988).