Explaining Abnormal Sound Velocities of Carbonates in Earth’s Mantle with Extreme Physics and Chemistry

Earth’s interior is a dynamic mixture of minerals and crystals. Under the influence of extreme temperatures and pressures these mixtures move around, transporting deep reservoirs of carbon throughout the mantle. Understanding how different minerals respond to changes in temperature and pressure requires a combination of theoretical and experimental approaches, and is a focus of DCO’s Extreme Physics and Chemistry Community.

Earth’s interior is a dynamic mixture of minerals and crystals. Under the influence of extreme temperatures and pressures these mixtures move around, transporting deep reservoirs of carbon throughout the mantle. Understanding how different minerals respond to changes in temperature and pressure requires a combination of theoretical and experimental approaches, and is a focus of DCO’s Extreme Physics and Chemistry Community.

In a new paper, published in the journal Physical Review Letters, DCO’s Afu Lin (University of Texas Austin, USA) and his graduate students use a new diamond anvil cell apparatus to probe the elastic properties of iron and magnesium carbonates at conditions of the lower mantle [1].

“We wanted to find out how the properties of these carbon-carrying minerals might change inside Earth,” said Lin. “When combined with seismic data, our results will help us quantify how much carbon is stored in Earth’s mantle, figure out how to look for carbonate rich regions, and refine the overall carbon budget of our planet.”

By exposing magnesiosiderite, a carbonate mineral made up of magnesium, iron, carbon, and oxygen, to pressures up to 70 GPa, the authors noticed obvious anomalies in their sound velocity datasets between 42.4 and 46.5 GPa. They explain the anomalies as changes in the elastic modulus, or compressibility, of the mineral due to an electronic spin transition at these pressures, in which electrons in the mineral are forced to pair up due to the extreme force applied.

The anomalies also create changes in the optical properties of the minerals, making them change color as they move through different regions of the mantle.

The authors suggest the drastic change in elastic modulus is the result of a sharp spin transition in the iron atoms. This forces a change in the mineral’s crystal structure, shrinking the size of the crystal and altering its physical properties.

“As minerals navigate the mantle, they respond to changes in their physical and chemical environments,” added Lin. “While we can look at physical properties of the mantle using seismic measurements, we can now speculate these physical observations with possible chemical reactions taking place inside Earth.”

This paper adds to the growing literature on the importance of carbonate chemistry in controlling mantle dynamics, as well as refining our estimates of precisely how much carbon is stored deep in Earth.

 

Image: Abnormal velocities of magnesiosiderite due to high-spin to low-spin transition at high pressure. The compressional wave velocity (Vp) drops drastically while the shear wave velocity (Vs) increases significantly across the spin transition region (shown as green) where high-spin and low-spin irons co-exist. Credit: Afu Lin

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