Equation of State of Amorphous MgSiO3 and (MgFe)SiO3 to Lowermost Mantle Pressures

Abstract:

Melting phenomena have a crucial importance during the Earth’s formation and evolution. For example, a deep magma ocean of 1000 km or more has lead to the segregation of the core. Tomographic images of the Earth reveal ultra-low velocity zones at the core-mantle boundary that may be due to the presence of dense magmas or remnant zones of a deep basal magma ocean [1]. Unfortunately, measurements of amorphous silicate density over the entire pressure regime of the mantle are scarce and the density contrast between solid and liquid are difficult to assess due to the lack of such data. Only few studies have reported density measurements of amorphous silicates at high-pressure, with limitation up to 60 GPa. High-pressure acoustic velocity measurements have been used to calculate the density of MgSiO₃ glass up to 30 GPa [2] but exhibit a large discrepancy compared to recent calculations [3]. SiO2 glass was measured up to 55 GPa using the X-ray absorption method through the diamond anvils [4] and very recently, X-ray diffraction has been used to infer the density of basaltic melt up to 60 GPa [5].

Here we report density measurement of MgSiO₃ glass up to 130 GPa and (MgFe)SiO₃ glass up to 55 GPa using a novel variation of the X-ray absorption method. The sample contained in a beryllium gasket was irradiated with a micro-focus X-ray beam in two directions: perpendicular and parallel to the compression axis to obtain the absorption coefficient and density under pressure. Our data constrain the first experimental EoS for (Mg,Fe)SiO₃ and the first EoS for MgSiO₃ up to lowermost mantle pressures. Technical details and EoS parameters will be shown in the presentation. We will address the implications for melts in the deep Earth based on compressibility, bulk modulus and density contrasts between iron-free and iron-bearing glasses.

[1] Labrosse S. et al. Nature 2007

[2] Sanchez-Valle C. et al. Earth Planet. Sc. Lett. 2010

[3] Ghosh D. et al Am. Mineral. 2014

[4] Sato T. et al Phys. Rev. Lett. 2008

[5] Sanloup C. et al Nature 2013