Our planet is a heat engine that cools for several billion years. This massive cooling is due to thermal convection that develops vigorously in the earth’s mantle. This envelope composed of silicate rocks ranges from a few tens of kilometers below the surface of the Earth to nearly 2900 kilometers deep. The rocks of the base of the mantle are hotter and therefore less dense, they deform very slowly and cause upward strokes, while colder, denser regions produce downdrafts motions. A direct consequence of this convective cooling is the movement of tectonic plates at current speeds ranging from a few millimeters to a few centimeters per year. These tectonic movements are responsible for the majority of natural events well known on the surface (earthquakes, development of mountain chains, continental drift, volcanism), but also for the mixing of mantle rocks in the geological time scale.
The chemical composition of lavas originating from the continental or oceanic volcanism can tell us about the degree of heterogeneity of the mantle related to the very long history of our planet. Lavas sampled at the surface of the earth exhibit significant chemical variability. So far, this variability was partly considered as representative of the existence of “sources” regions of the earth’s mantle of distinct compositions. The survival of these chemical inhomogeneities within the mantle over time depends on the efficiency of the mixing generated by convection.
It is generally accepted that the movement of tectonic plates on a large scale is the main mechanism of homogenization of the mantle, yet the geochemical composition of lavas coming from different mid-ocean ridges appears considerably variable. This variability is often low where the speed of the plates are high (10 to 20 centimeters per year), which is explained by an effective mixing due to plate movements. Paradoxically, slow ocean ridges (spreading rate of the order of a few millimeters per year) like the Southwest Indian Ridge, are also associated with a homogenous geochemistry, which is incompatible with a mixing due to the action of the tectonic plates.
To clarify this paradox, the authors studied, by means of numerical simulations, the influence of the mantle flow depending on the size and the speed of the tectonic plates on the effectiveness of the geochemical mixing near the mid-ocean ridges. These simulations confirmed the already known progressive development of a secondary convection at smaller scales (from a few kilometers to tens of kilometers) beneath the tectonic plates in motion, which combines with the convection at the tectonic plates scale (several thousand kilometers). Furthermore, these numerical experiments confirmed that the development of this small scale convection depends upon the speed of the plates and upon their size. Thus, the faster speeds’ plates and smaller plates inhibit the development of convection at small scale. Hitherto ignored in the interpretation of geochemical data, the authors showed that this small-scale secondary convection significantly contributes to the mixing of the mantle rocks for slow-spreading ridges.
Thus, if the sources of the lavas coming from fast ocean ridges are well mixed by the mixing due to the movement of the plates on a large scale, the material issued from the slow oceanic ridges is well mixed by small-scale convection. These results have important significance for the interpretation of geochemical data at mid-ocean ridges and hot spots whose differences in isotopic variability can be explained without using insulated geochemical tanks, but only through a variable efficiency of the mixing process within the Earth’s mantle.
- Article : Mixing at mid-ocean ridges controlled by small-scale convection and plate motion. Henri Samuel1,2,3* and Scott D. King3,4NATURE GEOSCIENCE |PUBLISHED ONLINE: 20 JULY 2014 | DOI: 10.1038/NGEO2208
- 1-2 CNRS, IRAP, Université de Toulouse, UPS-OMP,
- 3-Bayerisches Geoinstitut, Universität Bayreuth, Germany,
- 4-Virginia Tech, Blacksburg, Virginia USA.
- IRAP News : https://www.irap.omp.eu/en/actualites/actu-lavesoceaniques
- INSU-CNRS Press Release : http://www.insu.cnrs.fr/node/4999
IRAP Contact :
- Henri SAMUEL, Institut de Recherche en Astrophysique et Planétologie (IRAP). CNRS,UPS /OMP, firstname.lastname@example.org, 05.61.33.30.04