Investigations into the polysomatism of antigorite under pressure

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

The m value is defined as the number of tetrahedra contained in one wavelength 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 MgO.₆ Octahedron and SiO₄ tetrahedron and are 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 high pressure and temperature conditions in the Earth.

In this study, the researchers used the first principles method to calculate the free energy (enthalpy) of antigorites with different m-values ​​(m=14–19) under pressure and compared the stability of antigorites with different m-values. The study was published in 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, with increasing pressure, antigorite gradually dehydrates and adopts a structure with a shorter wavelength.

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

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