This award will provide continued support for the Caltech shock wave laboratory to enable the measurement of physical properties of silicate and metallic liquids under conditions matching those in the deepest parts of the Earth. This information is critical to interpreting geophysical observations of the lower mantle and core in terms of composition, temperature, heat flow, and evolution over time. It also allows speculative ideas about an early molten earth (i.e., primordial magma ocean) to be developed into detailed theories. Shock compression is the only experimental method that can simultaneously measure density, pressure, temperature, energy, and sound speed of liquids at such extreme conditions. As part of this study, the team will undertake experimental developments to achieve much higher initial pre-heating temperatures than were previously possible, enabling the direct study of high-melting point liquid compositions most relevant to the primordial magma ocean and lower mantle petrology.
The experimental and science agenda includes: 1) building on advances in shock temperature measurement techniques to look at temperatures of shocked silicate liquids, constraining their heat capacity at lower mantle pressures and thereby completing our ability to describe their equation of state in pressure-temperature-density-energy space; 2) furthering the development of new methods to achieve much higher initial pre-heating temperatures than were previously possible to enable the study of the equation of state of ultramafic liquid compositions most relevant to the early magma ocean and lower mantle petrology; and 3) initiating measurements of the sound speeds and densities of liquid iron alloys along actual outer core temperature profiles in order to make the well-defined observed gradient in sound speed with depth in the outer core a new and critical constraint on outer core composition.
This award funded experimental projects using the Caltech shock wave lab -- a set of large guns able to collide projectiles with targets at high speed -- in order to study the properties of Earth materials at very high pressure. We do this research in order to understand the conditions deep within the Earth; in particular, the conditions under which melting occurs and the implications of such melting events for the chemical and physical evolution of the planet. One major area of work is the melting curve of magnesium oxide (MgO). This is one of the major consistuents of the Earth's lower mantle, and its melting curve provides an endpoint for all models of lower mantle melting. However, it is a highly refractory compound and its melting point is so hot that it has been difficult to study experimentally. Most of the calculated melting curves that have been published are in open conflict with previous experiments. We showed that the previous experiments are wrong, and that the melting curve is at the low end of the range of theoretical predictions. This result will enable more confident descriptions of high-temperature, high-pressure processes both early in planetary history and ongoing today, and also provides an experimental verification of certain methods of calculating high-temperature mineral melting behavior. A second area of work examined the physical properties of very hot magma (molten rock). We developed the capability to heat our samples to as much as 2000 °C before driving shock compressions through them. This enabled us to measure the sound speed in very high-temperature liquids, which has never been done before. The result differs from guesses extrapolated from data on other liquids that exist at lower temperatures. We are working on theoretical models of liquid structure that can account for these data and others, and allow us to predict properties still unmeasured. We also prepared new experimental techniques to carry forward into future projects, including methods of encapsulating molten rock with a transparent window so that we can observe radiant light emerging from a shocked liquid and thereby measure its temperature and methods of preserving an interface between an opaque sample (such as a molten alloy of Fe) and a transparent sample (such as MgO) so that we can observe the velocity of this interface over time during sudden or gradual compression. This method will enable new constraints on the properties of Fe alloy liquids in the Earth's core. Through all this work, we educated three graduate students. One has gone on to teach high school science, one has gone on to a research postdoctoral position, one is still working with us in the lab. We opened our lab to film crews from a number of science-related channels and production companies. And we brought our research and related subjects in Earth science into elementary school classrooms around Southern California.