Shock compression of molten silicate: Results for a model basaltic composition

A technique has been developed for measurement of the shock wave, pressure-density equation of state of molten silicates initially at temperatures of up to ∼2000 K. A 40-mm propellant gun apparatus accelerates metal flyer plates to speeds of up to 2.5 km s⁻¹; these flyer plates are capable of driving shock waves with amplitudes of 35–40 GPa (350–400 kbar) into molten silicate samples. Modifications to the standard equation of state experiments that are described here include design of a molybdenum sample container for the molten silicate; use of a 10-kW radio frequency induction heater to melt the sample prior to impact; implementation of shuttering systems to protect the optical system and prevent preexposure of the film in the rotating-mirror, continuously writing, streak camera; and reduction of Hugoniot data taking into account the effect of the sample capsule. Data for a model basaltic liquid (36 mol % anorthite, 64 mol % diopside) at an initial temperature of 1673 K and initial density of 2.61 Mg m⁻³, yield a shock velocity-particle velocity (U_S-U_P) relation given by U_S = 3.06 + 1.36 U_P km s⁻¹ up to values of U_P = 1.7 km s⁻¹. The zero-pressure, bulk sound speed is in good agreement with ultrasonic measurements. The best fit Birch-Murnaghan equation of state for this model basaltic liquid is K_0S = 24.2 GPa and K′ = 4.85 based on Hugoniot points at low pressures (<25 GPa). Within the resolution of our data set, density increases smoothly with pressure over the 0–25 GPa pressure range, suggesting that structural rearrangements take place gradually in response to pressure in this pressure interval. At high pressures (≳ 25 GPa) the Hugoniot data suggest that the liquid stiffens considerably. This may indicate that the gradual structural changes characteristic of the lower-pressure regime, such as changes of Al³⁺ and Si⁴⁺ coordination by oxygen from fourfold to sixfold, are essentially complete by ∼25 GPa. These high-pressure Hugoniot data are fit by U_S = 0.85 + 2.63 Up km s⁻¹. The high-pressure regime is similar to that obtained in initially solid silicates upon shock compression. Shock temperature calculations yield values of 2400–2600 K at 25 GPa, and the states achieved are believed to lie metastably in the liquid field.