The interior of Earth controls many features and hazards at its surface. These include mountain ranges and ocean basins as well as dynamic effects like volcanoes and earthquakes. It is thus important to understand the behavior of minerals within Earth's interior. However, the available experimental techniques required to produce the elevated pressures and temperatures that occur deep within Earth are limited (e.g., extremely large steel presses, diamond-anvil cells combined with laser heating) that are costly and difficult to use. More importantly, they allow only limited examination of the materials at high-pressure and -temperature conditions. Instead, the products must be removed from the equipment in order to examine them using advanced methods such as electron diffraction and high-resolution imaging using a transmission electron microscope (TEM) that allows the examination of details of structure and reaction. In fact, some materials that form under extreme conditions are unquenchable, meaning they change structure and thus behavior when examined at room pressure and temperature.
This project is designed to solve this problem by developing and refining a way of looking at materials at high pressures and temperatures in place within a TEM, where we can examine them down to the atomic level. It is planned to make use of the remarkable properties of carbon nanotubes (CNTs), which we will use as the containers of the minerals that we wish to study at elevated pressures and temperatures. It was shown recently that CNTs will contract when bombarded with an energetic electron beam. They can thus serve as microscopic high-pressure cells that can be studied within a TEM. Therefore, all the superior capabilities of such microscopy can be fully transplanted to in-situ high-pressure research. The preliminary research to be supported will be devoted to developing methods of placing materials of interest into the tiny CNTs. The team will start with materials such as single elements and simple oxides and salts in order to properly test and calibrate the technique. They will then extend the work to examining more complex minerals that are thought to be abundant in Earth's interior and that influence or control behavior within the deep crust and mantle of the planet. Wüstite, an iron oxide that is only stable at high temperature and is abundant in the deep Earth in the form of magnesiowüstite, forms the long-term focus of this study since it may disproportionate at extreme conditions and have profound effects on the history of our planet. Overall, the results will provide fundamental new knowledge about a high-pressure experimental technique that is new to the earth sciences as well as about the nature of geologically and geophysically important minerals under extreme conditions.
Carbon plays a critical role in the geochemical cycles and environmental conditions at Earth’s surface, but its surface abundance is ultimately controlled by high-pressure and high-temperature processes in Earth’s interior because this is the dominant carbon reservoir on the planet. It has long been accepted that carbon under these extreme conditions is concentrated into particular carbon-rich minerals such as graphite or diamond, leaving the bulk of mantle material essentially carbon-free. However, laboratory studies of synthetic materials has shown that carbon can in certain circumstances be concentrated within atomic-scale defects. In this work, we have shown that minerals with the same structures as those occurring in Earth’s deep interior can, in fact, accommodate carbon in this fashion, thereby demonstrating that a significant fraction of Earth’s deep carbon could plausibly be hosted in common mantle minerals. This result is significant because the behavior of this carbon component during melting of the mantle can differ according to its mode of storage and may accordingly influence transfer of carbon to the surface of Earth. The work is significant technologically because we developed an experimental technique that represents the first in-situ application of electron microscopy to study of Earth interior materials at high pressure (P) and high temperature (T). This was required because experimental materials quenched from high PT conditions to ambient laboratory conditions are frequently highly unstable under the intense electron beams necessary to image structures at the atomic level in a transmission electron microscope (TEM). Our goal of performing such analysis under high PT conditions was accomplished by placing samples inside tiny cages (nano-cages) whose walls consist of sheets of carbon atoms (graphene). Carbon atoms were then displaced from the walls of these cages by electron bombardment within a TEM, causing the cages to shrink in size, thereby squeezing their contents and achieving high pressures. Thus, a key outcome of the research activities sponsored by this grant is the development of this new way to observe minerals and mineral reactions at high PT within an electron microscope so that changes that occur deep within Earth can be studied in great spatial detail, revealing fundamental high-pressure processes at the atomic scale.
