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[EN VIDÉO] Seismology, the stethoscope that listens to the heart of the Earth “The Earth is opaque but transparent to seismic waves,” explains us…
Man dreams of the inaccessible and will never stop dreaming. It is this urge to continually push the boundaries of what is known that led Christopher Columbus to America and Neil Armstrong to the moon. Today, this impulse is often directed towards space. But the earth still holds many secrets, gray areas that humans have not yet been able to explore.
If we can confirm that the earth’s surface is now well mapped, if man has managed to climb the highest peaks and plunge into the deepest abysses, the interior of the earth remains a pristine territory. However, it is not completely unknown. A few decades ago, the development of seismology and other geophysical methods made it possible to create models of the structure and composition of the Earth. But these remain just models. And since man is what he is, he needs evidence. Tangible, real evidence he can touch.
Objective The Earth’s mantle: a scientific and technical challenge
This motivated Russian scientists to drill in Kola in 1970. The goal: to cross the earth’s crust and reach the famous Moho boundary, which marks the mineralogical separation from the most important layer of the earth, the mantle. 19 years later, researchers will realize the difficulty of their mission and its technical impossibility given the enormous pressures and temperatures at depth.
China will dig one of the deepest holes on Earth!
But the dream remained. For a spirit of conquest, but above all to answer numerous scientific questions that affect many areas. Understand how the mantle interacts with the crust, what the transition between these two shells is like, how magma and igneous rocks are formed, how the Earth was formed and how it evolves from a geochemical and thermal perspective…
However, these questions face the challenge of in situ mantle sampling. The Kola drilling has shown that drilling through the continental crust is far too complicated. In fact, it is on average 30 kilometers thick. The technical means to achieve this depth would be simply breathtaking, if not completely impossible. Therefore, it was necessary to find places where the earth’s crust is much thinner… or even non-existent.
Crust or no crust?
Oh yes, you read it well. There are many places on Earth where the crust, as we normally define it, does not exist. But let’s put things in order. One option that Japanese teams from Jamstec (Japan Agency for Marine-Earth Science and Technology) in particular are considering is to reach the ceiling of the Earth’s mantle by drilling through the oceanic crust, the thickness of which is much thinner than that of the Earth’s mantle continental crust. “Only” 6 kilometers to cross without forgetting the 4 kilometers of water layer. That’s still a lot, but with modern means the operation doesn’t seem impossible. However, it is not yet clearly planned.
The option “without crust” remains. This is the one selected by the scientific ocean drilling program IODP (International Ocean Discovery Program) for Expedition 399, which took place in spring 2023. To be clear, this wasn’t about finding a hole that gapes directly into the Earth’s mantle. There is no such thing. It’s physically impossible. Nevertheless, in the oceanic region there are certain areas that do not have the characteristics of a “normal” oceanic crust, that is, consisting of three levels of mafic rocks: a thin upper basalt layer (approximately 500 meters thick), an intruded intermediate level known as a vein complex is called, and a thick layer of gabbro. At the base of this gabbro is the Moho, which marks the mineralogical transition to the mantle, which itself consists of ultramafic rocks, mainly peridotites. Mafic crustal rocks are formed by partial melting of mantle peridotites.
The exhumed coat, a window into the depths?
However, in certain places this lithological sequence is replaced by a mixture of varying proportions of gabbros and serpentinized peridotites. The seabed therefore consists locally of rocks that come directly from the Earth’s mantle! So why bother traversing 6 kilometers of crust when all you have to do is dive and collect what you find on the ocean floor? Yes, but… everything is not that simple. Because this mantle that we find on the surface at the level of the oceanic crust is not a “fresh” mantle like that beneath the Moho. In fact, it has undergone a whole series of mineralogical transformations in contact with seawater. This is called chemical change and in this case more precisely serpentinization. But it hasn’t melted, and that’s the big difference from the rocks of the adjacent crust. In this case, instead of relying on partial melting of the mantle, oceanic expansion took advantage of the tectonic exhumation of mantle rocks pulled to the surface by large faults.
But whoever says “mistake” is saying “sea water intrusion”. In this way, the rocks of the Earth’s mantle chemically react with water as they rise. The olivine becomes hydrated and turns into serpentine. We therefore no longer observe fresh peridotites on the seafloor, but rather serpentinites. It’s a shame, because scientists hope to observe mantle rocks that have not undergone any changes. Nevertheless, these areas are of great interest: beneath this serpentinite crust lies the fresh mantle, theoretically at a depth of significantly less than 6 kilometers.
Advancing into the heart of mighty Atlantis: a geological and biological quest
The IODP 399 expedition therefore began with drilling in the heart of the giant Atlantis, an exhumed mantle dome on the Mid-Atlantic Ridge. The site was already in the spotlight when “Lost City,” a hydrothermal field extraordinarily rich in life, was discovered in 2000. Drilling therefore begins just 800 meters from this iconic location with the aim of crossing the serpentine mantle and reaching the fresh mantle. In May 2023, after a month of work, the well reached a depth of 1,267 meters. This is the first time we have penetrated so deeply into Earth’s serpentine mantle.
Small disappointment: The fresh coat of paint was obviously not achieved. It’s likely we’ll have to drill even deeper to find it. But the scientific results are still extremely extensive. In particular, they will make it possible to observe the evolution of the degree of serpentinization with depth, the mechanisms of this chemical change, the exhumation processes, etc.
For their part, microbiologists also provided some surprises: they provided data that should allow us to better understand the composition and origin of hydrothermal currents, which are supposed to be the source of the development of life on Earth. The serpentinization reaction produces hydrogen, which in turn reacts with CO2, potentially forming complex molecules that are precursors to life.
The search for the Earth’s mantle therefore not only serves to investigate Earth’s geology and its mechanisms, but also allows us to better understand how life arose on our planet.