An ‘anomaly’ in the Earth’s mantle

The chemical composition
of the basalts underlying the Indian Ocean indicates that the Earth’s
mantle is quite different here than it is beneath the Atlantic and Pacific
Oceans. In particular, chemical analyses reveal an unusual lack of titanium.
Five CNRS researchers from the Sciences of the Universe Department [Laboratoire
de géosciences marines (1) and Centre de recherches
pétrographiques et géochimiques (2)]
have reported these findings in the scientific journal Nature.

In order to gain information about the Earth’s interior, which may be
studied from the surface by observing volcanic activity, these scientists
concentrated especially on oceanic ridges—underwater volcanic chains
that are 60,000 km long. Inside the Earth, the high temperatures of the
mantle and the variations in temperature cause convective currents. Understanding
all their characteristics makes it possible to explain the history of
the planet, its thermal state, and its evolution. Taking rock samples
at 5,000 meters below the Earth’s surface in order to determine the temperature
of the mantle, however, would mean attempting the impossible!

In 1997, researchers Christine Meyzen, Mike Toplis and John Ludden, along
with Catherine Mevel and Eric Humler, organized the EDUL oceanographic
mission for the study of an ultra-slow spreading ridge (3)
from aboard the oceanographic research vessel Marion Dufresne. This campaign
enabled them to obtain very fine images of the bottom of the Indian Ocean,
as well as over 5 tons of samples, thanks to a new dredging method. Several
years were required for the analysis of the chemical composition of the
basalt samples they gathered. The analyses enabled them to determine the
temperature at the interior of the Earth under this portion of the ridge.
It is the coldest region of the global mid-ocean ridge system: the temperature
is around 1,200°C (the warmest point being around 1,460°C) and
is located at a depth of 5,000 meters. In addition to providing information
about the Earth’s temperature, this analysis also reveals a substantial
lack of titanium. The researchers have published their hypotheses concerning
the geochemical mechanisms at play in the February 13, 2003, issue of
the British scientific journal Nature.

(1)
The "Centre de recherches pétrographiques et géochimiques"
(CRPG, Center for petrographic and geochemical research) is a CNRS laboratory
which focuses on the following research themes: cosmochemistry and planetology;
kinetics and mass balance of magmatic processes; space-time modelling
of the lithosphere; environments and paleo-environments.
http://www.crpg.cnrs-nancy.fr

(2) The "Laboratoire Géosciences marines" (LGM,
Marine geosciences laboratory) is one of the 22 teams of the "Institut
de Physique du Globe de Paris" (IPGP). This research unit, which
is run jointly by the CNRS, IPGP and the Universities of Paris 6 and Paris
7, follows a multidisciplinary approach to petrology, geochemistry, marine
geology, marine geophysics, and theoretical seismology.
http://www.ipgp.jussieu.fr/francais/rub-recherche/eq11geosciences-marines/acc11.html

(3)
The Indian ridge offers an example of an ultra-slow spreading and cold
ridge (spreading rate = 0.7 cm/year).