Evidence for a Phase Transition in Silicate Melt at Extreme Pressure and Temperature Conditions

Laser-driven shock compression experiments reveal the presence of a phase transition in ${\mathrm{MgSiO}}_3$ over the pressure-temperature range 300–400 GPa and 10 000–16 000 K, with a positive Clapeyron slope and a volume change of $\sim 6.3$ ($\pm 2.0$) percent. The observations are most readily interpreted as an abrupt liquid-liquid transition in a silicate composition representative of terrestrial planetary mantles, implying potentially significant consequences for the thermal-chemical evolution of extrasolar planetary interiors. In addition, the present results extend the Hugoniot equation of state of ${\mathrm{MgSiO}}_3$ single crystal and glass to 950 GPa.

Figure 1

An example of data from a single experiment performed with crystalline starting material shows simultaneous reversals in shock velocity and temperature as a function of time as the Hugoniot crosses the phase transition (visible between 2 and 4 ns, as indicated by arrows and the dashed line). Inset: arrows indicate the transition in the raw data images, from which the profiles in the main figure are extracted.

Figure 2

Decaying shock measurements on samples of initially single-crystal (green) and glass (red) ${\mathrm{MgSiO}}_3$ identify a phase boundary with positive Clapeyron slope between low- and high-density liquid phases (dot-dashed line). Results from 3 experiments on each material are plotted, with temperature uncertainties indicated by the colored bands: these are consistent with previous shock-wave data for single-crystal and glass samples (green and red diamonds, respectively [22, 23]). Theoretical predictions of melting temperatures are shown as thin, dotted curves [33, 34, 35]. Experimental melting points from laser-heated diamond-cell experiments are shown as blue circles [36, 37]. Inset: Hugoniot equation-of-state data were collected between 400 and 950 GPa in the present study. The density discontinuities near $6.5\mathrm{g}/{\mathrm{cm}}^3$ correspond to the change documented by velocity interferometry in Fig. 1 and the $P/T$ discontinuities in the main figure (see also the Supplemental Material [13]). EOS data below 200 GPa are from Refs. 22, 23.

Figure 3

Optical reflectivity extracted from velocity interferometry data (532 nm wavelength) documents the onset of metallization: reflectivities exceeding $5–10\%$ indicate a gradual change in bonding character that is predominantly temperature-driven. The onset of metallization is thus distinct from the liquid-liquid phase transition (indicated by grey arrows).