Advancing Instrumentation to Study Materials and Reaction in Earth’s Deep Interior

In the extreme conditions of Earth’s deep interior, high pressures and temperatures affect basic physical and chemical properties of elements and minerals.

DAC

Understanding how carbon reacts in these conditions is an important DCO decadal goal, and requires theoretical models based on experimental data. In the laboratory, diamond anvil cells generate high pressures, similar to those found in planetary interiors, while laser heating generates relevant temperatures. However, sustaining samples under these conditions is challenging, and performing optical spectroscopy measurements at such conditions was not possible because of strong thermal background radiation.

In pioneering work performed at the Geophysical Laboratory Carnegie Institution for Science, USA, Alexander Goncharov and his colleagues developed a variety of optical spectroscopy tools that enable accurate measurements at extreme conditions of high pressure and high temperature. These instruments, developed with partial DCO support, were installed at the Geophysical Laboratory at the Carnegie Institution for Science, Washington DC, USA in 2015.

In the first of three new publications published in Nature in June 2016, Goncharov and colleagues describe using laser-heated diamond anvil technology combined with a fast temperature detection system to directly address the thermal conductivity of solid iron under the extreme conditions of Earth’s core [1]. The motion of liquid iron in the planet’s outer core, a phenomenon called a geodynamo, generates the magnetic field shielding life on Earth from cosmic radiation. In the paper, Goncharov and co-authors Zuzana Konopkova (DESY Photon Science, Germany), Stewart McWilliams (University of Edinburgh, UK), and Natalia Gomez-Perez (Universidad de Los Andes, Colombia) address how the geodynamo was created, and then sustained throughout Earth’s history, by looking at the thermal conductivity of iron under the temperatures and pressures found inside planets ranging in size from Mercury to Earth. Their data agree with the lower end of previous estimates of thermal conductivity in Earth’s core, between 18 and 44 watts per meter per Kelvin. This means that Earth’s geodynamo existed as many as 4.2 billion years ago, very early in the planet’s history.

The second paper, which appeared in Earth and Planetary Science Letters, investigated the behavior of an iron-bearing carbonate (siderite), which has important structural similarities to the mantle’s most abundant minerals [2]. Previous work showed that siderite exhibits important changes in the arrangement of electrons around iron atoms (i.e. spin transitions) that result in dramatic changes in the color of the mineral at ~ 45 GPa and 300 K. In this study, the team, led by first author Sergey Lobanov (Geophysical Laboratory, Carnegie Institution for Science, USA), used laser-heated diamond anvil cells combined with a pulsed broadband laser (supercontinuum) and a synchronized, time-gated detector to look at the variations in color at combined high pressure and temperature (45-73 GPa and up to 1600 K). This pioneering study reveals that optical properties of minerals are sensitive to temperature, which has important implications for heat transfer by light in deep Earth. Likewise, the novel instrument may be used to determine the electronic structure of mantle minerals at realistic P-T conditions.

Lastly, a paper published in Physical Review Letters looks at the behavior of hydrogen under conditions of high pressure and temperature [3]. Metallic hydrogen is a potential high-temperature superconductor, and is abundant in the interiors of giant planets. Understanding how metallic hydrogen forms, therefore, is important, however hydrogen is challenging to work with because it is highly reactive. In this study, McWilliams et al. used the laser-heated diamond anvil cell to study hydrogen, managing to reach record high temperatures for static compressions experiments and probing hydrogen optical properties at these conditions.

The results show that there is an unexpected intermediate phase between insulating molecular and metallic hydrogen, which is strongly light absorbing, but not metallic or poorly metallic. This unexpected finding suggests that current planetary models should be reassessed, as these models assume metallic conditions under these pressure-temperature conditions (150 GPa and 3000 K). 

Image: llustration of how the diamond anvil cell is used to mimic and study planetary core conditions (click to enlarge). Credit: Stewart McWilliams

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