A defining characteristic of silicate melts is the degree of polymerization (tetrahedral connectivity), which dictates viscosity and affects compressibility. While viscosity of depolymerized silicate melts increases with pressure consistent with the free-volume theory, isothermal viscosity of polymerized melts decreases with pressure up to ~3–5 GPa, above which it turns over to normal (positive) pressure dependence. Here we show that the viscosity turnover in polymerized liquids corresponds to the tetrahedral packing limit, below which the structure is compressed through tightening of the inter-tetrahedral bond angle, resulting in high compressibility, continual breakup of tetrahedral connectivity and viscosity decrease with increasing pressure. Above the turnover pressure, silicon and aluminium coordination increases to allow further packing, with increasing viscosity and density. These structural responses prescribe the distribution of melt viscosity and density with depth and play an important role in magma transport in terrestrial planetary interiors.

Experimental determination of V _P of hcp-Fe can show potential candidates for major light elements in Earth’s inner core.

Diamond is an evidence for carbon existing in the deep Earth. Some diamonds are considered to have originated at various depth ranges from the mantle transition zone to the lower mantle. These diamonds are expected to carry significant information about the deep Earth. Here, we determined the phase relations in the MgCO₃-SiO₂ system up to 152 GPa and 3,100 K using a double sided laser-heated diamond anvil cell combined with in situ synchrotron X-ray diffraction. MgCO₃ transforms from magnesite to the high-pressure polymorph of MgCO₃, phase II, above 80 GPa. A reaction between MgCO₃ phase II and SiO₂ (CaCl₂-type SiO₂ or seifertite) to form diamond and MgSiO₃ (bridgmanite or post-perovsktite) was identified in the deep lower mantle conditions. These observations suggested that the reaction of the MgCO₃ phase II with SiO₂ causes formation of super-deep diamond in cold slabs descending into the deep lower mantle.