Why is the Earth's mantle conductive?

Dec 04, 2008
Above: Image of the Masai volcano, Oldoinyo Lengia (mountain of the gods) with, in white, a recent flow of liquid carbonate. Below: Two images of the lava lake, illustrating the very fluid character of this lava with extraordinary electrical properties. © Hannes Mattsson, ETH Zurich,

(PhysOrg.com) -- Researchers from INSU-CNRS (France), working with chemists at a CNRS research unit, have explained that the high conductivity of the Earth's upper mantle is due to molten carbonates. They demonstrated the very high conductivity of this form of carbon.

Appearing in the 28 November issue of Science, their work has revealed the high carbon content of the interior of the upper mantle. This composition can be directly linked to the quantity of carbon dioxide produced by 80% of volcanoes. This result is important for quantifying the carbon cycle, which contributes significantly to the greenhouse effect.

Geologists have long claimed that significant amounts of carbon have been present in the Earth's mantle for thousands of years. Up 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 (ISTO, CNRS / Université de Tours / Université 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(5). This nonetheless represents a reservoir of carbon integrated into the mantle which is higher than that present on the surface of the earth. These results are unmatched in helping to quantify the carbon cycle, which plays a major role in the greenhouse effect. Indeed, the CO2 emitted by volcanic activity had never before been evaluated at the source (at the level of the mantle).

The presence of molten carbonates in the asthenosphere certainly has major implications on the viscosity of this region of the mantle, which participates in the sliding of tectonic plates, a phenomenon we know little about. The behavior of liquid carbonates in solids and potential effects on viscosity remain to be studied. Everything seems to indicate that the asthenosphere contains only oxidated forms of carbon (carbonates), and not carbon in its reduced solid form (diamond). Diamond formation remains mysterious, but researchers are guessing that diamonds form from liquid carbonates at the base of the lithosphere, below the asthenosphere. Enfin, the electrical measurements of the team on liquid carbonates are of interest to the field of clean energy production, as they can be used as electrolytes in high temperature batteries (eg. lithium carbonate).

This work was funded through a Young Researcher ANR project led by Fabrice Gaillard. He hopes to continue the work on liquid electrolytes through another ANR project and to therefore clarify these new hypotheses.

Reference:
Carbonatite Melts and Electrical Conductivity of the Asthenosphere. F. Gaillard, M. Malki, G. Iacono-Marziano, M. Pichavant, B. Scaillet. Science. 28 November 2008.

Provided by CNRS

Explore further: Powerful undersea earthquake shakes southern Indonesia

add to favorites email to friend print save as pdf

Related Stories

A song of fire and ice in the ocean

Feb 10, 2015

Cyclic changes in the tilt of the Earth's axis and the eccentricity of its orbit have left their mark on hills deep under the ocean, a study published in Science has found.

Some of the best pictures of the planets in our solar system

Jan 19, 2015

Our Solar System is a pretty picturesque place. Between the Sun, the Moon, and the Inner and Outer Solar System, there is no shortage of wondrous things to behold. But arguably, it is the eight planets that make up our Solar ...

Professor researches rare rock with 30,000 diamonds

Jan 05, 2015

Diamonds are beautiful and enigmatic. Though chemical reactions that create the highly coveted sparkles still remain a mystery, a professor from the University of Tennessee, Knoxville, is studying a rare ...

Recommended for you

Antarctica's retreating ice may re-shape Earth

6 hours ago

(AP)—From the ground in this extreme northern part of Antarctica, spectacularly white and blinding ice seems to extend forever. What can't be seen is the battle raging underfoot to re-shape Earth.

The sun has more impact on the climate in cool periods

6 hours ago

The activity of the Sun is an important factor in the complex interaction that controls our climate. New research now shows that the impact of the Sun is not constant over time, but has greater significance ...

User comments : 6

Adjust slider to filter visible comments by rank

Display comments: newest first

CWFlink
3.3 / 5 (3) Dec 04, 2008
Hum... Can we get wires down that far? Spinning conductor in a magnetic field.... non polluting source of electricity?
GrayMouser
3 / 5 (4) Dec 04, 2008
Why did they have to tie this to GW?
Bob_Kob
1 / 5 (4) Dec 05, 2008
Why did they have to tie this to GW?


EVERYTHING is global warming.
KBK
2.3 / 5 (3) Dec 05, 2008
Cow Farts!
axemaster
5 / 5 (1) Dec 05, 2008
An interesting piece of work. However, I would hesitate to make the statement:

"Indeed, the CO2 emitted by volcanic activity had never before been evaluated at the source (at the level of the mantle)."

In fact, they still have not been evaluated "at the source", since that would imply that we actually measured it directly in the mantle. Nonetheless, very compelling evidence. I would imagine this has a significant effect on the Earth's magnetic field strength.
theophys
not rated yet Dec 05, 2008
I wonder if this might (in another century or two) lead to some modicum of control over the strength of Earth's magnetic field. That would be an interesting for of energy, climate control, and possibly global sun block.
Or it could just end up as an interesting footnote in the dustbin of scietific leaps that never were.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.