* Astronomy

Researchers of the INSU-CNRS, in collaboration with chemists at a CNRS research unit, have explained  the strong conductivity of the Earth's upper mantle is due to the presence of molten carbonates. Thanks to a ultra-powerful device, they highlighted the extremely high conductivity of this carbon form. Published in the 28 November issue of Science, their work revealed the percentage of carbon inside the upper mantle. This composition can itself be directly related to the quantity of carbon dioxide emitted by 80% of the volcanos. The  result is important to quantify the carbon cycle, which plays a major role in the greenhouse effect.

Geologists have long claimed that significant amounts of carbon have been present in the Earth's mantle for thousands of years. Until now, there was very little direct proof of this hypothesis, and samples from the surface of the mantle contained only very small quantities of carbon. Also, for the last thirty years, scientists have been unable to explain the conductivity of the mantle, which is crossed by natural electrical currents at depths of 70 to 350 kms, even though olivine, one of the main mineral components of the upper mantle, is completely isolating.
To explain these phenomena, researchers from the Institut des Sciences de la Terre d'Orléans  looked into liquid carbonates, one of the most stable forms of carbon within the mantle, along with graphite and diamond. The Masai volcano is Tanzania is the only place in the world where these carbonates can be observed. Elsewhere, the carbonates are dissolved in basalts and emitted into the atmosphere in gaseous form, as CO2.
Based on lab measurements at CNRS's CEMHTI, the researchers established the high conductivity of molten carbonates. Their conductivity is 1000 times higher than that of basalt, which was previously thought to be the only potential conductor in the mantle. Fabrice Gaillard and his team have shown that the conductivity of the Earth's mantle is a result of the presence of small amounts of molten carbonates between chunks of solid rock.
This work shows that the electrical characteristics of the asthenosphere, the conductive part of the upper mantle, are directly connected to the amount of carbonate in the layer. The work also points to varying carbon distribution according to the regions and depth of the mantle. The researchers calculated that the amount of carbon present as liquid carbonate directly within the asthenosphere is between 0.003 and 0.025%, which seems low but makes it possible to explain the amounts of CO2 emitted into the atmosphere by 80% of volcanoes .

Source (French)

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