Can plate tectonics be reproduced in the laboratory?

• Moving the plates results from vast convection movements that drive the Earth's mantle and allow the Earth to shed its internal heat, according to our partner

  The Conversation

  .

• During the second half of the 20th century, great efforts were made to develop experiments capable of reproducing the pressure and temperature conditions of the Earth's interior.

• This analysis was conducted by

  Patrick Cordier

  , professor of physics, specialist in mineral physics at the University of Lille.

On January 11, 2022, we published a scientific article in the scientific journal

presenting the results of our modeling in order to better understand the movements of the Earth's mantle at the origin of plate tectonics.

The plate tectonics that characterize the dynamics of our planet is difficult to date but it would have existed for at least 2 billion years.

The supercontinent called Gondwana which formed 600 million years ago would have started to fracture in the Jurassic (160 million years ago).

The Pyrenees, a young mountain, only date back around 40 million years.

How do we apprehend these durations, we poor humans, for whom spending 100 years on the surface of the Earth already represents a very long stay?

It is science and its observational, analytical and conceptual tools that have made it possible to go beyond the myths and that lead to these numbers that make you dizzy.

We are therefore a bit like ephemera, these insects which only live for a day or two, which try to understand the seasons, the years… But science is also experimentation.

But here again, how can we reproduce phenomena that take place over periods much longer than those of our lives?

Take the dynamics of our planet.

Plate tectonics is the conceptual framework that, since the 1960s, brings together and unifies descriptions of the major geological events that have shaped the Earth's surface.

But its origin is deeper.

The movement of the plates results from vast movements of convection which animate the terrestrial mantle and allow the Earth to evacuate its internal heat.

Because the Earth is still a hot planet, hence its activity.

More than 98% of the volume of our planet is at temperatures above 1000°C and the core is as hot as the surface of the sun.

This internal heat has several origins: a part of primordial heat (remains of the formation of our planet by accretion), another which is extracted from the core in part due to the crystallization of the seed (solid, central part of the core), and finally another which comes from the radioactive disintegration of elements (uranium, potassium, thorium) present in small quantities in the rocks of the mantle.

It is therefore by transporting matter from the bottom (where the temperatures are the highest) to the top (where they are the coldest) that this heat is transported.

The particularity of this convective phenomenon (well known in liquids, we often make the analogy with the pan of water on fire) is that in the Earth's mantle made up of solid rocks, it is carried by the deformation of these rocks and the minerals that constitute them.

It is therefore these deformations that we must study if we want to understand and model the dynamics of our planet.

But this quest is fraught with difficulties.

Let's list a few.

​Extreme pressure and temperature conditions in the mantle

The Earth's mantle is this envelope of rocks, as we have said, which extends up to almost 2900 km under our feet.

The pressure and temperature conditions prevailing there are extreme.

In particular, under the weight of the rocks, the pressure increases sharply with depth to reach some 135 GPa (1.35 billion times atmospheric pressure) as it approaches the core.

The rocks that are present at these depths are not those found on the Earth's surface.

Under the influence of pressure, they are made up of more compact, denser minerals.

During the second half of the 20th century, great efforts were made to develop experiments capable of reproducing the pressure and temperature conditions of the Earth's interior.

They have made it possible to study the way in which minerals densify under pressure and to propose a mineralogical model of the Earth's mantle corresponding to what is thought to be its chemical composition (in particular by comparison with meteorites considered as the bricks of the solar system ).

Several stages of compression of the minerals are identified to end up, from 670 km deep and almost to the core, by forming a fairly simple assembly made up of three main minerals.

The most important (by volume: almost 80%) of them is a magnesium iron silicate (also containing some aluminum) of perovskite structure called bridgmanite (in honor of Percy Bridgman, American physicist laureate of the Nobel Prize in Physics in 1946 for his work on high pressures).

Calcium, present in the upper mantle in garnets and pyroxenes, would be hosted by another silicate, presenting the same perovskite structure: davemaoite.

Finally, the excess magnesium is expressed in the form of an oxide (also containing a little iron): ferropericlase.

​Conditions difficult to reproduce in the laboratory

It is therefore the way in which this mineralogical assembly is deformed that constitutes the key to the dynamics of the mantle.

To study this phenomenon in the laboratory, it is necessary to carry out deformation experiments while applying the very high pressures which make it possible to stabilize these minerals.

New technological developments were therefore necessary and in 2016, Girard and his colleagues from Yale University (USA) succeeded in the first deformation experiment of an assembly of bridgmanite and ferropericlase under the corresponding pressure and temperature conditions. approximately 700 km deep.

These experiments showed what we foresaw: the silicate (the bridgmanite) is much harder than the oxide (the ferropericlase).

They indeed observe that the ferropericlase absorbs almost all the deformation and finds itself strongly stretched in a matrix of quasi-rigid bridgmanite.

Such behavior can have important consequences on how the mantle can deform.

Thielmann and his colleagues at the University of Bayreuth in Germany used numerical models to further deform such an assembly.

They show that depending on the way ferropericlase is distributed in the rock, the mechanical properties (and therefore the ability of the mantle to deform and evacuate heat) are not the same.

If the “soft” phase, the ferropericlase forms continuous layers, it can “lubricate” the deformation and make the rock much less viscous.

But these experiments and the conclusions that can be drawn from them come up against other difficulties.

Reproducing the pressures and temperatures of the Earth's interior is already a challenge, but overcoming it is not enough.

It must indeed be remembered that the deformations of the mantle are slow, very slow and spread over hundreds of millions of years.

Studying these phenomena in the laboratory requires accelerating them considerably: more than 100 million times!

Are the mechanisms activated during these experiments the same as those that operate in nature?

Can the results of laboratory experiments be simply extrapolated to natural conditions?

A new model

It is to answer this question that the TimeMan project, funded by the European Research Council (ERC) and which I lead at the University of Lille in collaboration with the universities of Antwerp and Louvain-la-Neuve, is dedicated to. in Belgium.

Its originality?

Do not try to simply extrapolate, but rely on the most detailed understanding possible of the physics, of the mechanisms of deformation of these minerals under the pressure and temperature conditions of the mantle.

Let us return to the experiments of Girard and his collaborators.

Their measurements show that very high stresses are required to deform their samples at laboratory speeds.

Our models make it possible to reproduce the results of these experiments.

They show that they result from the activation of the sliding of crystalline defects, the dislocations which, under the influence of these strong stresses, shear the crystals.

But in the mantle, the stresses are much weaker and our models show that other mechanisms must take over.

At high temperature and under low stresses, the mechanisms of deformation of solid matter involve the migration of ions which slowly diffuse towards the dislocations to give them an additional degree of mobility which is called ascent.

It is therefore this diffusion step which controls the kinetics of the deformation.

But it is slow, very slow.

Particularly in magnesium oxide where it is the large oxygen ion that has the most difficulty in moving, especially when the pressure makes the structure more and more compact.

Ferropericlase therefore deforms more slowly than bridgmanite in this deformation regime which is impossible to reproduce on laboratory time scales.

It is this counterintuitive result that we describe in the article published on Wednesday, January 11, 2023 in the journal

.

It challenges the debates on the influence of the distribution of ferropericlase in the matrix.

OUR “GEOLOGY” FILE

If we make the analogy with peanut butter, we can say that the classic models made ferropericlase play the role of the oily phase which made the dough more fluid.

Our results see it rather as the rigid particles of “crunchy” peanut butter, without any noticeable influence on the rheology of the whole.

We therefore conclude that bridgmanite is the only mineral phase to be considered to model the deformation of the mantle under natural conditions, so slow that they escape our sensitive perception, but not our models!

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