On the Earth’s surface, 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 recycles in subduction zones, the boundary between lithospheric plates at whose level 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 in depth) remains poorly understood. For example, it is not yet clear why some deciduous plates stop at the boundary between the lower and upper mantle, while others continue to descend into the core. Julia Emmore, of the University of Bayreuth in Germany, and her colleagues studied the properties of a mineral that plays a key role in the oceanic subduction mechanism: 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 its 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, a pioneering experiment under high pressures and degrees high temperature 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 colleagues made davimwit. They deformed it and discovered that this mineral is much softer, that is, less resistant to deformation than bridgemanite and ferrobriclase, the other two more abundant minerals 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 times of volume lower than that of ferrobriclase.
According to Julia Emmore and colleagues, this very low viscosity has wide-ranging consequences for the behavior of the lithosphere plate during subduction. Davemaoite is formed between 550 and 700 kilometers downstream from Majorite, and is a much harder mineral. Relieves the oceanic crust of the ‘dip plate’. When bridgemanite and ironclase in turn form about 660 km away (especially in the part of the mantle associated with oceanic crust collapse), the large variation in viscosity may 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 mantle boundary.
But if the crust does not clear out and continues to plunge into the lower mantle, at a depth of about 720 kilometers, it undergoes a new critical mineral metamorphism, making it even denser than the mantle. Then it sinks to the boundary between the mantle and core, at a depth of 2,900 km. The presence of patches of daphemia-rich oceanic crust at these depths could explain in particular the presence of seismic shear-wave hysteresis zones, such as at the level of two megafauna observed under Africa and the Pacific.
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