Despite a relatively 'uniform' fertile composition (Al.2O. 3 = 2·19-4·47 wt %; Fo% = 89·2 ± 0·3%; Cr#. Spl = 8·9 ± 1·5%), the Montferrier peridotite xenoliths show a wide range of S contents (22-590 ppm). Most sulphides are interstitial and show peculiar pyrrhotite-pentlandite intergrowths and low abundances of Cu-rich phases. Sulphide-rich samples are characterized by strong enrichment in the light rare earth elements and large ion lithophile elements without concomitant enrichment of the high field strength elements. Such trace-element fractionation is commonly ascribed to metasomatism by volatile-rich melts and/or carbonatitic melts. S and Se (11-67 ppb), as well as S/Se (up to ≈12 000), are correlated with La/Sm. Cu, however, remains broadly constant (30 ± 5 ppm). These features strongly suggest that the percolation-reaction of such volatile-rich fluids has led to sulphide enrichment with an atypical signature marked by strong fractionation of the chalcophile elements (i.e. S vs Se and Cu). S-rich xenoliths are also characterized by high (Pd/Ir). N (1·2-1·9; where subscript N indicates normalized to chondrite), (Pd/Pt). N between 1·5 and 2·2, and (Os/Ir). N up to 1·85. Despite the relative uniform fertile composition of the xenoliths, Re/Os ranges between 0·02 and 0·18. 187Os/. 188Os is extremely variable even within a single sample and can be as high as 0·1756 for the most S-rich samples. Sulphides show highly fractionated and variable abundances of the highly siderophile elements (HSE) [0·03 ≤ (Pd/Ir). N < 1283] and Re-Os isotopic composition (0·115 <. 187Os/. 188Os < 0·172). Such variation can be observed at the thin-section scale. Whole-rock and in situ sulphide data demonstrate that chalcophile and HSE systematics and the Os isotopic composition of the upper mantle could be significantly modified through metasomatism, even with volatile-rich fluids. These features highlight the complex behaviour of the HSE in fluid-rock percolation-reaction models and suggest a complex interplay between sulphide addition (crystallization or sulphidation) and partial equilibration of pre-existing sulphide. The specific fractionations observed in chemical proxies such as S and Se, Os and Ir, and Pd and Pt, as well as the low abundance of Cu-rich sulphides, suggest that sulphide addition may not have occurred via sulphide melts. Rather, we infer that S was present as a dissolved species in a (supercritical) oxidizing, volatile-rich fluid (C-O-H-S ± Cl) along with other chalcophile and siderophile elements such as Os, Pd, Re and Au. The highly radiogenic Os composition of this fluid (187Os/188Os > 0·17) would imply that such fluids are derived from an uncommon type of mantle source possibly related to carbonatite melts.