An exciting trip to the center of the Earth

Paris, February 25, 1999

An article published this week in the magazine Science (February 26, 1999) by three researchers working in joint units of the CNRS, the Ecole normale supérieure in Lyon and the Observatoire de physique du globe in Clermont-Ferrand describes the complex chemical interactions occurring deep below the surface of the Earth, between the mantle and pieces of plates dragged down by subduction of the ocean lithosphere (1). The results of their research confirm the "marble cake" model presented in 1986 by Claude Allègre and Don Turcotte, researchers of the Institut de physique du globe in Paris and Cornell University in the United States.

Acording to geophysicists, when the earth was divided into lithosphere, mantle and core, the ocean lithosphere was formed by the rise of basalt lava on the axis of ocean ridges, resulting from the fusion of the upper mantle. It was then dragged back down into the Earth's depths, into the subduction zones, and covered by sediments. What happens to the ocean lithosphere down in the Earth's hot mantle? Does it dissolve? Does it remain intact despite the circulation caused by convection? Does the mantle's composition change over time? Did some of the mantle remain intact? What happens when magma forms due to the fusion of the mantle and reaches the ocean ridges and volcanoes in hot points of the Earth?
According to the dominant theory, the pieces of ocean lithosphere dragged by subduction into the mantle were slowly stretched, fragmented and mixed by convection during hundreds of millions of years. Under the combined influence of pressure, temperature and dehydration, these rocks, mainly of basaltic origin, melt into a residue called pyroxenite (a crystalline rock rich in pyroxenes and basalts) which rise back to the surface. Until now, it was believed that these pieces of pyroxenite were passive tracers, mechanically stretched and refolded. The mantle was generally seen as a matrix of peridotites (rocks rich in olivine) and pyroxenites. In the "marble cake" model proposed in 1986 by Claude Allègre and Don Turcotte, from the Institut de physique du globe in Paris and Cornell University (United States), each of the "cake's" ingredients preserves its original composition.
In order to analyze this mantle, hidden under the thick crust of ocean and earth, geochemists study basalt lava, fragments of rocks which have come from the depths, carried up by the lava as it rises to the surface, or peridotite rock masses — more or less altered pieces of the mantle brought close to the surface by plate tectonics and thus accessible to geologists.
Janne Blichert-Toft, Francis Albarède and Jacques Kornprobst studied a small rock mass of peridotites in Beni Bousera, Morocco, formed in the Tertiary era. This mass is extremely well preserved and comes fr om very deep down, probably more than 150 km, as shown by the presence of diamond ghosts on the site. This mass is an archetype of the mantle as could be seen if we travelled to the center of the Earth. According to Francis Albarède, "The layers of pyroxenite in the peridotite are spectacular (see photo). The isotopic composition of the oxygen in the pyroxenites, previously determined by a British research team, clearly indicates that these rocks formed next to the surface at some point in their history, whereas the peridotite matrix has preserved its initial characteristics. What we have here is a piece of the mantle in which different ingredients of various origins have been blended together". What remains to be dsicovered is whether these elements also preserved their original composition.
Thanks to the acquisition, in 1994, of the first plasma source mass spectrometer, which increases by 1,000 the level of detection of chemical components, researchers from the Ecole Normale Supérieure in Lyon were able, for the first time, to date garnets contained in the pyroxenites of the Beni Bousera rock mass (25 million years) and begin the analysis of the lutetium-hafnium isotopic ratio.
The isotope 176 of hafnium is radiogenic: it is produced by the radioactive desintegration of the isotope 176 of lutetium. The proportions of radiogenic isotope in the hafnium of basalts are well known, in particular since the invention of laser mass spectrometry. The proportion of radiogenic hafnium measured in the Beni Bousera pyroxenites shows a much larger variation interval than that observed in the basalts. This is contradictory. The authors thus propose an integrative process to explain these results: the preferential fusion of pyroxenite beds is followed by a reaction of the liquids formed with the peridotites encountered during their ascent. This happens more than 100 km below the surface of the earth. The complex liquid which manage to escape the melted environment and are flushed out on ocean ridges or by volcanoes is the well-known basaltic magma.
Given the permanent nature of this process, we can see how difficult it is for geologists to identify its different stages. If the earth mantle is a marble cake, its ingredients are so well blended that it is quite a job to determine the original recipe.

Contacts

Researcher contact
Francis Albarède
CNRS- ENS de Lyon
Tel: 33 4 72 72 84 14
e-mail: albarede@ens-lyon.fr

INSU - CNRS contact
Christiane Grappin
Tel: 33 1 44 96 43 37
e-mail: cgrappin@mesiob.obspm.fr

(1) the Science article can be obtained at the INSU/ Photo available by contacting the researchers.

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