Shock Response and Phase Transitions of MgO at Planetary Impact Conditions

Abstract

The moon-forming impact and the subsequent evolution of the proto-Earth is strongly dependent on the properties of materials at the extreme conditions generated by this violent collision. We examine the high pressure behavior of MgO, one of the dominant constituents in Earth's mantle, using high-precision, plate impact shock compression experiments performed on Sandia National Laboratories' Z Machine and extensive quantum calculations using density functional theory (DFT) and quantum Monte Carlo (QMC) methods. The combined data span from ambient conditions to 1.2 TPa and 42 000 K, showing solid-solid and solid-liquid phase boundaries. Furthermore our results indicate that under impact the solid and liquid phases coexist for more than 100 GPa, pushing complete melting to pressures in excess of 600 GPa. The high pressure required for complete shock melting has implications for a broad range of planetary collision events.

Thermodynamically complete equation of state of MgO from true radiative shock temperature measurements on samples preheated to 1850 K

“…The predicted melting temperature for MgO at the Earth's core-mantle boundary pressure, ≈135 GPa, ranges from 6000 to 9000 K [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] (Fig. 1).…”

“…Conventional dynamic compression experiments on crystalline samples do not provide direct access to the P -T region of interest either. Upon compression along its principal Hugoniot, MgO goes from the B1 (NaCl structure) solid phase into the B2 (CsCl structure) solid phase, then into the B2-liquid coexistence region, and finally reaches the pure liquid phase above approximately 620-700 GPa [18,21]. Interpolation between the melting points measured for B1 phase at the highest attainable static pressure of ≈40 GPa [3,4] and those measured for B2 phase at approximately 470 to 650 GPa [22,23] or ≈700 GPa [21] is subject to large errors comparable to the scatter between various theoretical predictions.…”

“…None of the recently reported shock experiments to approximately 1 TPa pressure [17,18,[21][22][23][24] was designed to probe even indirectly the actual melting of B1 phase MgO. These studies probed decaying [17,22,23], steady [18], or almost steady [21] states along the principal Hugoniot or quasi-isentrope [24] and so they only characterize, at best, the B2 melting curve and the B1-B2 solid-solid phase transition.…”

“…These studies probed decaying [17,22,23], steady [18], or almost steady [21] states along the principal Hugoniot or quasi-isentrope [24] and so they only characterize, at best, the B2 melting curve and the B1-B2 solid-solid phase transition. Moreover, all the laser shock experiments so far published have very large uncertainties on MgO pressure and temperature due to the severe challenges associated with such measurements.…”

“…McWilliams et al [22] had poor constraints on the mechanical state of their MgO samples at shock speeds below ≈17.3 km/s (i.e., pressures below ≈525 GPa) due to loss of VISAR reflectivity, high uncertainties on all pressures due to sparse calibration assuming a linear Hugoniot despite the [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] and conventional shock temperature data [25,26]. Typical uncertainties of the predicted melt lines are 100-200 K. Thicker lines indicate several melting curves that are close enough to be grouped and shown together.…”