2023-07-07 17:19:45
While it was thought that granite was a very terrestrial specificity linked to the presence of water and plate tectonics, the discovery of an imposing granite body on the hidden side of the Moon raises many questions, in particular that of the hydration state of lunar rocks at the beginning of the history of our satellite.
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On Earth, granite is an exceedingly common rock. It is found almost everywhere and for good reason, the continental crust is mainly formed of this rock of magmatic origin rich in silica. However, this would be a very terrestrial specificity, granites being almost absent from the other telluric planets of the Solar System. However, the absence of this rock outside the Earth is quite easily explained. The production of granite indeed requires very specific conditions.
Granite, a typically terrestrial igneous rock
A magma of granitic composition can thus form either by fusion of the rocks of the continental crust during a tectonic collision, or by fusion of the rocks of the mantle. If in the first case we will directly obtain a magmatic liquid of granitic composition (the fusion of granite gives a granitic magma), in the second case the liquid produced will first of all be of basaltic composition. The granitic composition will only be obtained at the end of a long process of differentiation of the magma, in several stages. The chemical composition of the magmatic liquid will thus gradually evolve during its cooling and the gradual crystallization of the different minerals. Several rocks of different mineralogical composition will thus be able to form from the same primary magma. This is called a magmatic series. Granite is at the end of this series. In all cases, the process leads to a concentration of radioactive elements in the granite magmas. The crystallization of granite then requires very slow cooling of the magma deep within the crust. Granite is therefore a plutonic rock and not an effusive one.
Water and plate tectonics: the ingredients necessary for the formation of granite
On Earth, the formation of a liquid of granitic composition will be favored by the presence of large quantities of water, which favors the process of melting the rocks. However, water is mainly brought to depth by tectonic processes, and in particular by subduction zones. We understand better, in fact, why granites are so frequent on Earth and almost absent elsewhere.
However, we must say “almost” absent, because small samples of granite have indeed been found outside our planet, and in particular on the Moon, without our being able to clearly explain their origin so far. The hypothesis was that this was an anecdotal phenomenon, the geology and lunar geodynamics not allowing the establishment of large granitic plutons.
A massive granite body discovered on the far side of the Moon
An idea undermined by the recent discovery of an imposing granite body on the far side of the Moon. It was microwave frequency data obtained by China’s orbiting Chang’e-1 and Chang’e-2 probes that helped identify an abnormally hot area below the surface of what scientists believe is an ancient volcano known as Compton-Belkovich.
This anomaly of heat flow would be linked to a high concentration of radioactive elements. For scientists, this can only be the sign of the presence of an imposing granite body measuring 50 kilometers in diameter, resembling a batholith. On Earth, batholiths usually testify to the past existence of very large volcanic systems, like the Half-Dome of Yosemite Park.
A “hydrated” or simply very hot Moon?
The astonishing discovery of such a granite unit on the Moon necessarily raises questions and requires either reviewing the processes that can lead to the formation of granite in the absence of water (which involves very high temperatures), or considering the presence of water within the lunar crust at the time of its formation. The batholith would indeed have formed very early in the history of the Moon, 3.5 billion years ago. The results of the study have been published in the journal Nature.
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