Davemaoite, a key mineral in the subduction of oceanic crust

On the surface of the Earth, the age of the oldest oceanic crust does not exceed 200 million years, which is a small percentage on the scale of geological times. And for good reason: it is recycled in the subduction zones, the limit between lithospheric plates at the level of which the old oceanic lithosphere, cold and dense, sinks into the underlying asthenosphere.

If the subduction mechanism is well described, the behavior of subduction in the lower mantle (between 660 and 2,900 km depth) remains poorly understood. For example, it is not yet known why some deciduous plates stop at the boundary between the lower and upper mantle, while others continue to descend into the core. Julia Emmore, from the University of Bayreuth in Germany, and her colleagues studied the properties of a mineral that plays a key role in the mechanism of oceanic subduction: davimoite.

Davemaoite is a mineral with the formula CaSiO3 The cubic structure belongs to the perovskite family. Synthesized in the laboratory in 1975, it was first observed in the natural state at the end of 2021 in a diamond formed in the lower mantle, and named “davemaoite” in honor of Ho-Kwang (Dave) Mao, an experiment pioneer under high pressures and high temperature degrees Davemaoite is a major component of the combined oceanic crust (25%) and mantle (10%). It is very difficult to study this mineral, since it is stable only under conditions of high pressure and high temperature (it begins to form at a depth of 550 kilometers).

Using a Massey anvil cell, Julia Emmore and her colleagues made davimwit. They deformed it and discovered that this mineral is much softer, ie less resistant to deformation than bridgemanite and ferrobriclase, the two other minerals more abundant in the lower mantle. Using the calculations, they estimated that under lower mantle conditions, daphemaite has a viscosity (characteristic of resistance to fluid movement) 1.4 to 4 orders of magnitude lower than that of bridgemanite and 1 to 3 volume times lower than that of ferrobriclase.

According to Julia Emmore and her colleagues, this very low viscosity has vast consequences on the behavior of the lithosphere plate during subduction. Davemaoite is formed between 550 and 700 kilometers downstream of Majorite and is a much harder mineral. Relieves oceanic crust from the ‘dip plate’. When bridgemanite and ironclase in turn form about 660 km away (particularly in the part of the mantle associated with the collapse of oceanic crust), the large variation in viscosity can cause the crust to separate from the rest of the sinking plate. Thus, the crust will accumulate at this depth, at the lower and upper limit of the mantle.

But if the crust does not thin out and continues to sink into the lower mantle, at a depth of about 720 kilometers it undergoes new critical mineral metamorphism, making it even denser than the mantle. Then it sinks to the boundary between the mantle and the core, at a depth of 2,900 km. The presence of plates of oceanic crust rich in daphemia at these depths could in particular explain the presence of hysteresis zones of seismic shear waves, as at the level of two megafauna observed under Africa and the Pacific.

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