[SIT22-P26] Melting temperatures of MgO up to ~50 GPa determined by micro-texture analysis
Keywords:Melting, Mineral physics, High pressure experiments
Periclase (MgO) is the second most abundant mineral after bridgmanite in the Earth’s lower mantle, and its melting temperature (Tm) under pressure is important to constrain rheological properties and melting behaviors of the lower mantle materials. Significant discrepancies exist between the Tms of MgO determined by Laser-Heated Diamond Anvil Cell (LHDAC) and those based on dynamic compressions and theoretical predictions. We performed a series of LHDAC experiments for measurements of Tm of MgO under high pressure. The melting was detected by using micro-texture observations of the quenched samples.
We found that the laser-heated area of the sample quenched from the Tm in previous LHDAC experiments showed randomly aggregated granular crystals, which was not caused by melting, but by plastic deformation of the sample. This suggests that the Tms of their study were substantially underestimated. On the other hand, the sample recovered from the temperature higher by 1500-1700 K than the Tms in previous LHDAC experiments showed a characteristic internal texture comparable to the solidification texture typically shown in metal casting. We determined the Tms based on the observation of this texture up to ~50 GPa.
Fitting our Tms to the Simon equation yields dTm/dP of 103 K/GPa at zero pressure, which is consistent with those of the theoretical predictions (90~120 K/GPa). Extrapolation of the present melting curve of MgO to the pressure of the CMB (135 GPa) gives a melting temperature of ~7900 K. The high Tms of MgO suggest the subducted cold slabs should have higher viscosities than previously thought, suggesting that the inter-connecting textural feature of MgO would not play important roles for the slab stagnation in the lower mantle. The present results also predict that the ultra-deep magmas produced in the lower mantle are perioditic, which are stabilized near the core-mantle boundary.
We found that the laser-heated area of the sample quenched from the Tm in previous LHDAC experiments showed randomly aggregated granular crystals, which was not caused by melting, but by plastic deformation of the sample. This suggests that the Tms of their study were substantially underestimated. On the other hand, the sample recovered from the temperature higher by 1500-1700 K than the Tms in previous LHDAC experiments showed a characteristic internal texture comparable to the solidification texture typically shown in metal casting. We determined the Tms based on the observation of this texture up to ~50 GPa.
Fitting our Tms to the Simon equation yields dTm/dP of 103 K/GPa at zero pressure, which is consistent with those of the theoretical predictions (90~120 K/GPa). Extrapolation of the present melting curve of MgO to the pressure of the CMB (135 GPa) gives a melting temperature of ~7900 K. The high Tms of MgO suggest the subducted cold slabs should have higher viscosities than previously thought, suggesting that the inter-connecting textural feature of MgO would not play important roles for the slab stagnation in the lower mantle. The present results also predict that the ultra-deep magmas produced in the lower mantle are perioditic, which are stabilized near the core-mantle boundary.