Résumé
Platinum-group elements (PGEs) have now been recognized to be highly sensitive geochemical tracers of Earth's mantle. However, compared to lithophile trace elements, PGEs occur as ultra-trace elements which may occasionally be present as major elements in discrete platinum-group minerals (PGMs) in addition to the base-metal sulfides (BMS). PGMs are not expected to be stable in the upper mantle because PGEs are highly soluble (at wt. % levels) in base metal sulfides (BMS), which control the largest part of the PGE budget in the uppermost mantle. However, micrometric PGMs (0.1 mu m-3mu m) have now been identified in a large panel of mantle-derived rocks (abyssal peridotites, orogenic peridotites, basalt-borne spinel peridotite xenoliths, cratonic peridotites) within both lherzolitic and harzburgitic compositions. These minerals may impact analytical precision (e.g. Os-Ir) and reproducibility of whole-rock analyses by generating nugget effects on some elements (e.g. Pt) and incomplete dissolution of some acid-resistant minerals (Os-Ir). The PGMs so far recovered from the study of peridotites are invariably associated with BMS phases or derived from a BMS precursor indicating the key role of the BMS in the PGMs genesis. Decreasing temperature, decreasing activity of S or increasing semi-metal concentrations (Te, Bi, As, Sb...) are major factors triggering crystallization of PGMs. Adiabatic partial melting processes generate refractory PGMs (Pt-Ir-(Os) alloys Os-Ir-Ru alloys/ sulfides) by consuming all of the available BMS in the mantle residues. Refractory PGMs are long-lived minerals that may survive recycling and rejuvenation of residual peridotites while preserving Os isotopic signatures of Archean partial melting events. Reactions at decreasing melt-rock ratios (( mantle metasomatism nearly equal ) as well as magmatic ( refertilization nearly equal concentrate volatile semi-metals (Te, As, Bi...) which precipitate as high-temperature Pt-Pd-Te-Bi or Pt-As-S microphases intimately associated with Cu-Ni-rich sulfides. Transient fluid - rock interactions, subsolidus cooling, crustal contamination and serpentinization are also PGM-maker (PdSb, PdCuNi, Pt-Bi-Sb phases ; Pt-Fe-Ru alloys; Pd sulfides, native gold...). These non cogenetic HSE-rich minerals have thus important implications for our interpretation of the mantle HSE signatures and their use to constrain planetary processes.