An anlytical methode allowing Ir, Ru, Rh, Pt, Pd and Au determination is presented in the first chapter. It consist in an hydrochemical concentration before flameless atomic absorption spectrophotometry detection. It allows subppb (ng/ g) range detection limits for these elements. The second chapter is devoted to subcontinental upper mantle platinum-group elements (PGE) distribution. The Lherz spinel peridotite body (Ariège, France) shows flat (PGE) mantle normalized patterns with a strong Pt depletion (<0.5 ppb). This caracteristicseem to be primitive and argue against fiat mantle normalized patterns for PGE in ail the upper mantle. The study of the Lherz harzburgites (cross-cutting the peridotite) and the pyroxenite sills (anhydrous pyroxenites) and dikes (amphibole pyroxenites and hornblendites) lead us to tackle the PGE behavior during partial melting events and fractionnai cristallisation in an "open system". The Filon Robert (Cape Smith belt, Quebec, Canada) is a komatiitic basait liquid fractionned in a "closed system" . It allows us to etablish the thermodynamic control on PGE differenciation. This study shows the great variety of the phenomeons wich control PGE segregation and argue against 100% sulphide normalisation. Further more, it demonstrate that in such a layered intrusion, it is not the sulphide richest rocks the PGE richest.
