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Investigations of the polysomatism of antigorite under pressure

First-principles investigations of the polysomatism of antigorite under pressure
The crystal structure of antigorite with m=17. The number of tetrahedra within one wavelength along the a-axis represents the m-value. Orange: Mg, Blue: Si, Red: O, Pink: H . Credit: Jun Tsuchiya, Ehime University

Antigorite is a type of serpentine, which is the most abundant hydrated mineral on the Earth. It is widely believed that this mineral is the main carrier of water deep into the Earth in subducting oceanic plates. It has a wavy structure along the a-axis, and in nature, several polysomes with different m-values (m=13–24) have been identified (polysomatism).

The m-value is defined as the number of tetrahedra contained in one , and is controlled by the difference in length between the octahedral layer and the tetrahedral layer.

This length is mainly determined by the size of the MgO6 octahedra and SiO4 tetrahedra, and is therefore expected to vary as a function of pressure and temperature. However, it is not well understood how the m-value of antigorite changes under the and temperature conditions in the Earth.

In this study, researchers used the first-principles method to calculate the (enthalpy) of antigorites with different m-values (m=14–19) under pressure, and compared the stability of antigorites with different m-values. The study is published in the Journal of Geophysical Research: Solid Earth.

As a result, it was found that the m-value of the most stable antigorite gradually decreases. In other words, antigorite gradually dehydrates as pressure increases, changing into a structure with a shorter wavelength.

First-principles investigations of the polysomatism of antigorite under pressure
The present results indicate that the m-value of antigorite tends to decrease under high pressure. Therefore, the decrease in the m-value of antigorite is expected as the depth increases in subduction zones. Credit: Jun Tsuchiya, Ehime University

This suggests that the structure of antigorite in the oceanic lithosphere may gradually evolve into a polysome with a smaller m-value that differs from the antigorite observed under or near-surface pressure conditions (i.e. m=17).

Such changes in m-values are accompanied by minor dehydration reactions. Changes in the amount of water in rocks and minerals due to the polysomatism of antigorite in may affect the distribution of intermediate-depth earthquakes, such as those observed in the double seismic zone.

More information: Jun Tsuchiya et al, First‐Principles Investigations of Antigorite Polysomatism Under Pressure, Journal of Geophysical Research: Solid Earth (2024). DOI: 10.1029/2023JB028060

Journal information: Journal of Geophysical Research

Provided by Ehime University

Citation: Investigations of the polysomatism of antigorite under pressure (2024, July 5) retrieved 23 July 2024 from https://phys.org/news/2024-07-polysomatism-antigorite-pressure.html
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