La convection mantellique : contraintes apportées par les isotopes stables du carbone et de l’azote dans les diamants du manteau inférieur

Devant le jury composé de :

Bernard Marty ................................ Rapporteur.
Jane Blichert-Toft ........................... Rapportrice .
Vincent Courtillot ........................... Président du Jury.
Cinzia Farnetani ............................. Examinatrice.
Pierre Cartigny .............................. Directeur de thèse.


Abstract:

The dominant mode of convection in the mantle has been a hotly
debated topic for decades. Diamond provides a unique opportunity to
sample parts of the mantle that remain unaccessible by any other
means. Some mineral associations in diamond, such as majoritic garnet,
calcic and magnesian perovskite and manganoan ilmenite with
ferropericlase have been recognised as originated from the transition
zone down to the lower mantle. In addition, nitrogen is potentially a
good tracer for mantle geodynamics. Exchanges between an inner
reservoir (characterised by negative d^15 N) via degassing at oceanic
ridges with an outer reservoir (characterised by positive d^15 N) via
recycling at a subduction zones can lead to nitrogen isotopic contrast
in a stratified mantle.


Taking advantage of the rather common occurence super-deep mineral
inclusion assemblages in diamonds from Juina (Brazil) and Kankan
(Guinea), we carried out a detailed study of C and N-isotopes. Both
sets of results suggests extensive material-isotopic exchange through
the 660km discontinuity, this contrast with the existence of isolated
lower mantle.

Therefore the C- and N- independent isotopic tracers provide evidences
that a significant amounts of material are exchanged across the 660km
discontinuity.