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-i; these flyer plates are capable of driving shock waves with amplitudes of 3540 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 yield a shock velocity-particle velocity (Us-Up) relation given by U s -3.06+1.36 Up km s -1 up to values of Up--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 K0s = 24.2 GPa and K t =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 hat the liquid stiffens considerably. This may indicate that the gradual structural changes characteristic of the lower-pressure regime, such as changes of A13+ and Si 4+ coordination by oxygen from fourfold to sixfold, are essentially complete by GPa. These high-pressure Hugoniot data are fit by U s --0.85+2.63 Up km s -•. The high-pressure r gime 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.