eX-cubed: eXploring eXtremes eXperimentally
- Melting in the Earth’s Deep Interior. The origin and subsequent evolution of the Earth and other planets in our own and other solar systems is dictated by several factors including composition and the melt behavior of the constituent phases. However, estimates of the melting temperature of Earth’s mantle—which comprises more than three-quarters of the Earth’s volume and nearly two-thirds of its mass—remain controversial and elusive. The conditions at which the mantle melts and solidifies is especially important for understanding compositional heterogeneity, which constrains deep structure and mixing and is crucial for inferring the thermo-chemical evolution of the Earth, from planetary accretion and the magma ocean, to continental growth, the formation of the oceans and atmosphere, and the current state of the core-mantle boundary. Currently the mantle is nearly entirely solid, but it has not always been this way. During the formation of the Earth, the mantle was likely completely molten—a magma ocean—before it began to solidify as it cooled. How the mantle crystallized, i.e., from the top, the bottom, or somewhere in between, is unknown. Evidence for melt at the core-mantle boundary, at least in pockets, is strong, as it is near the surface. Investigating the melt behavior of the most abundant minerals in the mantle at high pressures will test hypotheses on Earth’s formation and elucidate the geochemistry of the deep mantle. Additionally, this experimental investigation will help constrain current temperatures of the Earth by estimating the temperatures at the core-mantle boundary. The experiments will be studied by 2-dimensional mapping of temperature, composition and texture of sample materials placed under the extreme high-pressure, high-temperature conditions of the Earth’s early mantle as produced by a laser-heated diamond-anvil cell, in order to identify the conditions at which melting occurs and its subsequent influence on the composition and morphology of the molten and surrounding regions. This effort was recently funded by the National Science Foundation.
- Constraining the Earth’s Lower Mantle composition. To better constrain the chemical composition of the Earth’s Mantle, I characterized an undepleted natural peridotite at high pressures and temperatures to determine the plausibility of a homogeneous Mantle. By taking a natural peridotite (an Upper-Mantle rock and direct analog to the pyrolite model) and compressing it to Lower-Mantle conditions, I compare density measurements obtained by high-resolution synchrotron x-ray diffraction with seismological observations of the Preliminary Reference Earth Model (PREM). The surprising finding is that the density of Upper-Mantle peridotite is 1-4% less dense than the Lower Mantle, implying that the Upper and Lower Mantle may be compositionally distinct. See CV for reprints.
We continue to explore alternative Lower Mantle compositions – pyroxenite, enstatite, chondritic, etc. – through experiments, semi-empirical thermodynamic modeling and normal mode comparison. This effort is currently being funded by a CAREER grant awarded by the National Science Foundation.
- Planetary Diversity within and outside of the solar system. With the evergrowing list of exoplanets–planets found outside of the Solar System–understanding what these planets are composed of, how they formed and evolved, is a new frontier. We have recently proposed the possibility of a carbon-rich (“diamond”) exoplanet 55 Cancri e. See CV for reprints.
- Investigating the room-temperature, high-pressure behavior of graphite.We use several complementary experimental techniques in conjunction with theoretical computations to, for the first time, solve the enigma regarding the phase transition of graphite under high pressure and room temperature which has been controversial and disputed for the past half century. Our experimental data clarify that under high pressure the initially hexagonal-structured graphite sluggishly transits into a monoclinic structure, the so called M-carbon phase (Li et al., PRL, 2009) rather than bct C4 (Umemoto et al., PRL, 2010), K4 (Itoh et al., PRL, 2009) T-carbon (Sheng et al., PRL, 2011) or W-carbon (Wang et al., PRL, 2011) structures, and this post-graphite phase with nano-scaled crystal size has high bulk modulus and strength evidenced by its capacity to indent diamond anvils. The information and knowledge derived from the current research introduce a better understanding of the sluggish phase transition and elucidate the unusual properties and phenomena previously observed, for example the large hysteresis of electrical resistance observed in graphite at extreme conditions (e.g., Samara & Drickamer, JCP, 1962; Aust & Drickamer, Science, 1963). This effort was funded in part by the Carnegie/DOE Alliance Center (CDAC). See CV for reprints.
- Investigating potentially superhard materials. Transition metal dioxides such as TiO2, ZrO2 and HfO2 have long been considered candidates for superhard materials. New experimental results coupled with ab-initio computations suggests that is notion should be reconsidered. Additionally, the room-temperature post-graphite phase transition is shown to be sluggish, yet quenchable. Superhardness is likely. This effort was funded in part by the Carnegie/DOE Alliance Center (CDAC). See CV for reprints and await upcoming publications.
- High-pressure/temperature behavior of potassium. Radioactive decay through the incorporation of 40K into the core could be an important source of energy deep inside the Earth, helping to power the geodynamo and mantle dynamics. We have found that under high pressures and temperature, potassium and iron form a solid solution using both experiments and ab-initio calculations. In addition, we have explored the K partitioning between Fe, perovskite and post-perovskite. Currently we are measuring the melting curve of potassium to investigate a turnaround in the solidus as seen previously in sodium. See CV for reprints.
- Pressure and chemical dependent electron-capture radioactivity. The half-lives of radioactive isotopes are often considered constant and are determined to high precision. For this reason, many radioactive isotopes are used to date geological and astronomical processes at all time and length scales. Among important radioisotopes are those that decay by electron capture, namely 26Al (-> 26Mg, half-life ~ 720 kyr) and 40K (-> 40Ar, half-life ~ 1.28 Gyr). Heat production due to the decay of 26Al and 40K were also important during the Earth’s accretion process and the current heat budget respectively. Because of their widespread use, any change in the decay rate is fundamental to understanding and implementing these relations to the Earth. For electron-capture decay schemes, external forces such as chemical state, pressure, temperature and ionization can affect the half-life. This is a new effort combining experimental measurements of the changes in decay constant with pressure and chemistry of important electron-capture isotopes with theoretical predictions. This effort brings together state-of-the-art ab-initio computations of electron density with that of high-pressure diamond-anvil cell experiments. Experiments and computations will be combined jointly to investigate how chemical composition and pressure affect the electron-capture portion of their half-lives. Computations will be used to study isotopes under variable conditions in both chemistry and compression that are not easily accessible by experiments either due to very long half-lives or lack of isotopically-enriched samples. This effort was funded in part by the Carnegie/DOE Alliance Center (CDAC). See CV for reprints.
- Electrical conductivity at high pressures. We have developed a technique to measure electrical conductivity under high pressures. These difficult measurements will help to determine not only how electrical conductivity behaves under extreme conditions. We have performed these experiments successfully for highly-ordered pyrolitic graphite (HOPG) under high pressures and room temperature. See CV for reprints.
- Pressure-induced siderophile behavior of normally lithophile, chalcophile and atmophile elements. Pressure can make normally lithophile (rock-loving), chalcophile (sulfur-loving) and atmophile (atmosphere-loving) elements into iron-loving elements. Using both ab-initio quantum-mechanical calculations and experiments, I am investigating how pressure affects the electronic character of alkali (K, Rb), alkaline-earth (Ca, Sr) metals and noble gases (e.g., Xe). The possible alloying behavior between iron and these elements is important for understanding the accretion and evolution of the Earth. Partitioning of these elements between iron and rocky-portion of the Earth can also provide insight into the light-element composition of the Earth’s core. See CV for reprints.
- Combining static and dynamic techniques: laser-driven shockwave experiments on pre-compressed (in a diamond cell) samples. In collaboration with our colleagues at Lawrence Livermore National Laboratory, the Commissariat a l’Energie Atomique and Rutherford Appleton Laboratory’sVULCAN Laser Facility, we have developed a novel technique which allow us to shock materials at a higher initial density thereby tracing a different Hugoniot (higher pressures with lower temperatures). This technique works well for very compressible fluids (we’ve tried this on water, nitrogen, hydrogen, and helium) as precompression to even nominal pressures (~1 GPa) produces large increases in density. See CV for reprints.