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Research Departments

Stable isotope geochemistry

  Thèse de Emilie Thomassot

Origine et formation des diamants et de leurs inclusions de sulfure : contribution à l’étude des processus géochimiques, des cycles (C, N et S) et de leur évolution dans le manteau supérieur terrestre.
Encadrant (et co-encadrant) : 

The aim of this thesis was to better understand the origin and speciation of carbon in metasomatic fluids that give rise to diamonds. It included an analysis of diamonds from a same xenolith , polycrystalline diamonds and three suites of sulfide-bearing inclusion diamonds. This work was conducted in close collaboration with, among others , Dr. Jeff Harris ( University of Glasgow ) for the selection and characterization of samples and Dr. Marc Chaussidon and Dr Claire Rollion Bard ( CRPG -Nancy ) for implementation of the first analyses of    33S/32S  and  34S/32S isotopic compo sitions on the 1270-irms at CRPG. 



This work has led to several conclusions. First, the existence of reduced mantle fluids (ie methane-bearing) has been demonstrated. Finally, several opposite conclusions derived from previous studies have been clarified by the systematic analyses of C-, N, S- isotopic compositions of diamonds and their sulfide inclusion.

This work has confirmed the superficial origin of sulfur (Δ33S ≠ 0 per mille) of sulfide inclusions yet with diamonds mantle signatures ( δ13C and δ15N ~ -5 per mille). The data support a model of metasomatic formation of diamond by precipitation of mantle fluids including a pre- existing sulphide.



The main implications of this work relates to our understanding of diamond crystallisation ages. Metasomatic diamond formation encapsulating a pre-existing mineral implies
that diamond genesis ages inferred from the the study of diamond inclusions may not necessarily reflect the age of diamond precipitation.

Furthermore , this study shows significant differences between diamonds with an inclusion of sulfide compared to those with a silicate inclusion , suggesting that the “ages” of diamond formation inferred from either silicate or sulfide inclusion can not be generalized to diamonds from this locality.


Date de soutenance: 
Thursday 07 December 2006 - 10:30
Prof. Claude Jaupart – Prof. P7(Président du Jury), Dr Marc Chaussidon – DR-CNRS CRPG-Nancy (Rapporteur), Dr Olivier Alard – CR-CNRS Université de Montpellier (Rapporteur), Dr Jean-Pierre Lorand DR-CNRS MNHN (Examinateur), Pr Marc Javoy – Prof. P7 (Co-directeur de thèse), Dr Pierre Cartigny - CR-CNRS IPGP (Co-directeur de thèse)