IMAGING PERCOLATIVE CORE FORMATION PROCESS BY HIGH-RESOLUTION 3D TOMOGRAPHY

Planet differentiation could proceed through efficient liquid-liquid separation or by percolation of liquid metal in a solid silicate matrix, depending on the size and interior temperature of the planetary bodies. The percolation of liquid metal in the solid silicate matrix is likely a dominant process in the initial differentiation when the temperature is not high enough to melt the entire planetary body. The efficiency of percolation depends on the dihedral angle, determined by the interfacial energies of the solid-solid and solid-liquid interfaces. We have developed a new imaging technique to visualize the distribution of liquid metal in the silicate matrix in three dimension (3D) and precisely determine the dihedral angle by combination of focus ion beam (FIB) milling and high-resolution field-emission SEM imaging, using a dual beam FIB/SEM instrument at the Carnegie Institution of Washington. Typically we mill out a volume of 15x15x15 µm in the recovered sample from a simulation experiment at high pressure and temperature. While milling, we collect about 600 high-resolution SEM images at an interval of 25 nm. The image data files were then used to reconstruct 3D images to visualize the melt distribution and connectivity in the quenched sample. The new imaging technique provides a powerful tool to precisely determine the true dihedral angle. The method is far more superior than the traditional technique based on the measurements of the relative frequency distributions of apparent dihedral angles between the quenched liquid metal and silicate grains on polished 2D cross-sections. It further provides the details of each interface, allowing examination of the wetting ability of liquid in the matrix with multiple crystal phases. Through quantitative calculations, we can obtain volume fraction, surface area ratio, and connectivity. The 3D network through reconstruction can also be used as a realistic import 3D model for other calculations of transport properties such as permeability and conductivity. The new capability is particularly useful to monitor the change in melt distribution across the critical angle (60˚), pinpointing the transition from the non-connected to the interconnected network within a small composition and pressure interval.