Thèse de Médéric Palot | INSTITUT DE PHYSIQUE DU GLOBE DE PARIS

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Stable isotope geochemistry

  Thèse de Médéric Palot

Stable isotope constraints on mantle convection: carbon and nitrogen isotopes in super deep-diamonds
Encadrant (et co-encadrant) : 
Résumé: 

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 (Stachel et al., 2000; Kaminsky et al., 2001). In addition, nitrogen which is the main diamond impurity is also potentially a good tracer for mantle geodynamic. Exchanges between an inner reservoir (characterised by negative δ15N) via degassing at oceanic ridges with an outer reservoir (characterised by positive δ15N) via recycling at subduction zones could lead to nitrogen isotopic contrast in a stratified mantle.
 

Taking advantage of the rather common occurrence of super-deep mineral inclusion assemblages in diamonds from Juina (Brazil) and Kankan (Guinea), we carried out a detailed study of C and N-isotopes. There are broadly similar ranges of δ15N between upper (UM) and lower (LM) mantle diamonds from +3.8‰ down to -8.8‰ and from +9.6‰ down to -39.4‰ respectively for Juina and Kankan diamonds. Both sets of results suggest extensive materialisotopic exchange through the 660km discontinuity, this contrast with the existence of isolated lower mantle. Most of Juina and Kankan lower mantle diamond N isotopic values are compatible with a mantle origin (i.e negative δ15N) which contrasts with the OIB values (i.e positive δ15N see Marty and Dauphas, 2003). That implies a distinct source between these two types of samples. It has been recovered 3 Kankan diamond values down to -39.4‰. These lowest δ15N values ever recorded in terrestrial samples are compatible with primitive material and provide evidence that mantle has kept heterogeneities.

 

 

The rising mode of lower mantle diamonds could give constraints of the mantle geodynamic when there have passed trough zones with specific δ13C and have equilibrated.
Indeed, we found 5 zoned super-deep diamonds which demonstrate large δ13C variations with parts which are typical of their local transition zone δ13C value. Most likely these diamonds have initiated their growth in the LM and following slow uplift in a convective mantle have equilibrated in the TZ, and in doing so show an evolution in carbon isotopic composition.

Therefore the C- and N- independent isotopic tracers provide evidences that significant amounts of material are exchanged across the 660km discontinuity.
Finally we modelled the internal geodynamic of nitrogen using the N-K-Ar systematics. The evolution of mantle δ15N from the lowest value ever recorded (i.e. -39.4‰) to the actual mantle value of -5±4‰ for MORB source (convecting mantle), and to the value of +3±2‰ for the primordiale OIB source (deep mantle), suggest a more likely proterozoic age for diamonds. As results nitrogen behaves like an incompatible element with a mantle from 0.17 to 0.58ppm for a convecting mantle size from 750 to 1200km. This is additional evidence in favour of hybrid mantle convection with a deep reservoir in the mantle located beyond the 660km discontinuity compatible with rare gas and seismic tomography results.

Date de soutenance: 
Wednesday 22 September 2010 - 14:00
Jury: 
V. COURTILLOT Professeur (IPGP) Président du jury, P. CARTIGNY Directeur de recherche (Univ. Paris 7-IPGP) Directeur de thèse, B. MARTY Professeur (CRPG Nancy) Rapporteur, J. BLICHERT-TOFT Directeur de recherche (ENS Lyon) Rapporteur, J. BADRO Directeur de recherche (IMPMC) Examinateur, C. FARNETANI Maitre de conférences (Université Paris 7) Examinateur