Densification mechanisms of amorphous silicates: Implications for terrestrial planets

Abstract:

As a means of transporting heat and mass, liquids are the most dynamic phase in the differentiation and evolution of the interiors of terrestrial planets. Understanding the how the physical properties of liquids change as a function of pressure and temperature is critically important, both for interpreting the current physical structure of the Earth from geophysical observables and for understanding the dynamic evolution of planets more generally. However, measuring the physical properties of liquids at extreme conditions is historically experimentally challenging. Many methods for determining high pressure properties of crystalline materials do not translate well to amorphous materials – that is materials that lack long-range atomic ordering. In recent years, two methods for determining the densification of amorphous silicates have been developed at synchrotron facilities, such as Soleil Synchrotron in France: 1) high pressure X-ray microtomography systems that measure densification by bulk volume change and 2) CASEAR X-ray diffraction that measure densification by changes in atomic spacing. Here we present combined in situ X-ray diffraction and microtomography datasets at elevated pressure and temperature in the Rotational Tomography Paris-Edinburgh Cell at Soleil synchrotron for a suite of silicate glasses spanning the SiO₂-MgSiO₄ binary. Simultaneous acquisition of X-ray diffraction on a large Q-range and 3-D tomography were performed under extreme conditions, permitting direct comparison of density from volumetric measurements with changes in atomic structure. We use these combined data to determine the atomistic mechanisms that control densification in silicate liquids, and the implication these mechanisms have on the dynamics of magmatic and planetary differentiation processes.