The Determination of the Equation of State of Molten Silicates at High Pressures Using Shock-Wave Techniques

Citation

Rigden, Sally Miranda (1986) The Determination of the Equation of State of Molten Silicates at High Pressures Using Shock-Wave Techniques. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/5v04-ws14. https://resolver.caltech.edu/CaltechTHESIS:10272023-170924439

Abstract

Shock wave (Hugoniot) equation-of state experiments have been carried out on molten silicates in the petrologically important system CaMgSi₂O₆-CaAl₂Si₂O₈ via the projectile impact method. An RF heating technique was developed to heat silicate samples contained in pure Mo containers to the necessary high initial temperatures (up to 1773 K). Thermocouple techniques, a refractory sample holding system and optical shutter systems, were developed to allow utilization of a propellant gun apparatus at impact velocities ranging from 1.0 to 2.5 km sec⁻¹ corresponding to shock pressures of 4 to 40 GPa. The methodology for taking into account the effect of the Mo container in measuring the equation of state of the molten silicate is explicitly derived.

Results on molten diopside (CaMgSi₂O₆), anorthite (CaAl₂Si₂O₈) and an inter­ mediate composition (36 mole % CaAl₂Si₂O₈, 64 mole % CaMgSi₂O₆: An_(0.36)Di_(0.64)) are presented. Reduction of the Hugoniot data for these materials to third-order Birch-­Murnaghan isentropes yields 1 atm bulk moduli (K_s) in the range 18-24 GPa which are in good agreement with bulk moduli recently measured by ultrasonic methods at 1 atm and similar temperatures. The pressure derivatives of bulk modulus (K') vary from 5-7. Shock temperature calculations for An_(0.36)Di_(0.64) indicate temperatures of 2400-2600 K at ~25 GPa. The Hugoniot states are believed to lie metastably in the liquid field on the basis of measured bulk modulus, calculated Hugoniot density of a solid of the same composition and estimated crystallization times.

The measured equation of state data for molten diopside is used in conjunction with other thermochemical data to constrain the diopside solidus via the Clausius­ Clapeyron equation at pressures up to 20 GPa. The present data are consistent with measured fusion curve data of others to 5 GPa. Above ~10 GPa, a marked shallowing of the solidus is predicted as the difference in volume between crystalline and mol­ten diopside in equilibrium approaches zero.

Comparison of the results for molten diopside with those from the intermediate composition indicates that the liquids exhibit ideal mixing behavior with respect to volume to within ±2% up to ~40 Gpa. Gradual changes in coordination of Al³⁺ and Si⁴⁺ from tetrahedral at low pressures to octahedral at high pressures are believed to occur during compression of these materials. The integrated compressibility as reflected in the values of K_s and K' is related to the proportion of tetrahedrally coordinated cations at low pressure, and the volume at ~40 GPa is from 100-110% of that of a mixture of the dense, high-pressure phases MgSiO₃ (perovskite), CaSiO₃ (perovskite), Al₂P₃ (corundum) and SiO₂ (stishovite).

Important petrological implications of our results include: (1) basic to ultrabasic melts become denser than olivine- and pyroxene-rich mantle at pressures of 6-10 GPa, and (2) there is a maximum depth from which basaltic melt can rise buoyantly within terrestrial planetary interiors.

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