AsianScientist (Mar. 3, 2017) – Researchers have used precision cut diamonds to squeeze tiny dust-sized samples into the same immense pressures that exist at the Earth’s core. Their findings, published in Nature, suggest that crystallization of materials at the core might explain the Earth’s magnetic field.
The Earth’s core consists mostly of a huge ball of liquid metal lying at 3,000 km beneath its surface, surrounded by a mantle of hot rock. At such great depths, both the core and mantle are subject to extremely high pressures and temperatures.
Furthermore, research indicates that the slow creeping flow of hot buoyant rocks—moving several centimeters per year—carries heat away from the core to the surface, resulting in a very gradual cooling of the core over geological time. However, the degree to which the Earth’s core has cooled since its formation is an area of intense debate among Earth scientists.
In 2013, Dr. Kei Hirose, who is now Director of the Earth-Life Science Institute (ELSI) at the Tokyo Institute of Technology (Tokyo Tech), reported that the Earth’s core may have cooled by as much as 1,000°C since its formation 4.5 billion years ago.
This large amount of cooling would be necessary to sustain the geomagnetic field, unless there was another as yet undiscovered source of energy. These results were a major surprise to the deep Earth community, and created what Peter Olson of Johns Hopkins University referred to as, the “New Core Heat Paradox,” in an article published in Science.
Core cooling and energy sources for the geomagnetic field were not the only difficult issues faced by the team. Another unresolved matter was uncertainty about the chemical composition of the core.
“The core is mostly iron and some nickel, but also contains about 10 percent of light alloys such as silicon, oxygen, sulfur, carbon, hydrogen, and other compounds,” Hirose, lead author of the new study to be published in the journal Nature. “We think that many alloys are simultaneously present, but we don’t know the proportion of each candidate element.”
To address these questions, Hirose and his team used a laser irradiated through diamonds to replicate the high temperatures of the Earth’s core. By performing experiments with a range of probable alloy compositions under a variety of conditions, the researchers studied different alloy combinations that match the distinct environment that exists at the Earth’s core.
“In the past, most research on iron alloys in the core has focused only on the iron and a single alloy,” says Hirose. “But in these experiments we decided to combine two different alloys containing silicon and oxygen, which we strongly believe exist in the core.”
The researchers were surprised to find that when they examined the samples in an electron microscope, the small amounts of silicon and oxygen in the starting sample had combined together to form silicon dioxide crystals—the same composition as the mineral quartz found at the surface of the Earth.
“This result proved important for understanding the energetics and evolution of the core,” said Dr. John Hernlund of ELSI, a co-author of the study. “We were excited because our calculations showed that crystallization of silicon dioxide crystals from the core could provide an immense new energy source for powering the Earth’s magnetic field.”
The team has also explored the implications of these results for the formation of the Earth and conditions in the early Solar System. Crystallization changes the composition of the core by removing dissolved silicon and oxygen gradually over time. Eventually, the process of crystallization will stop when then core runs out of its ancient inventory of either silicon or oxygen.
“Even if you have silicon present, you can’t make silicon dioxide crystals without also having some oxygen available,” says ELSI scientist Dr. George Helffrich, who modeled the crystallization process for this study. “But this gives us clues about the original concentration of oxygen and silicon in the core, because only some silicon:oxygen ratios are compatible with this model.”
The article can be found at: Hirose et al. (2017) Crystallization of Silicon Dioxide and Compositional Evolution of the Earth’s Core.
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Source: Tokyo Institute of Technology; Photo: Pexels.
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