The Lanzo lherzolite massif shows that the top of asthenospheric diapirs is a zone of preferential S, Cu, Au, and platinum-group element (PGE) enrichment. Residual plagioclase Iherzolites which underwent a limited extraction of mid-ocean ridge basalt (MORB) melt during diapiric uprise are enriched in Ru, PPGE (Pt and Pd), and Au (3-10 times the pristine asthenospheric mantle), whereas they are moderately enriched in Cu (up to 38 ppm), depleted in Ir (Ir/Ni(mn) = 1-0.1), and have S contents ranging between 95 and 215 ppm. The behaviour of chalcophile elements in the Iherzolites cannot be modelled by equilibrium batch melting. The precious metals vary independently of lithophile element content and modal rock composition. It is suggested that the excess of PPGE, Au, and Cu was introduced either by a plume-type magma which cross-cut the Lanzo massif before rifting or by downward percolation of sulphides segregated from the MORB magmas extracted from the asthenospheric diapir. Calculated chalcophile element compositions of the extracted melts show high Cu/Pd ratios (2.7-14.5) typical of melts that have experienced early S saturation. This adds straightforward evidence to theoretical modelling and indirect assumptions based on extruded MORB, i.e., it is possible to produce high D-element depleted magma compositions simply by low degrees of mantle melting under S-saturated conditions without fractionation in magma chambers. The MORB magma has circulated via ariegite and gabbro dykes as well as dunitic bands. All of these rocks are strongly depleted in Ir. Their Ir/Ni(mn) ratios range between 0.192 in the gabbros and 0.068-0.168 in the dunites. The depletion in Ir (and probably Os) is attributed to segregation of Ir-bearing alloys from the MORB melt before it cross-cut the peridotites. The dunitic bands are distinguished from the harzburgites and dunites analysed so far by a large range of total precious metal content (17-77 ppb), positive Pd/Ir(mn) ratios, and an excess of S (up to 210 ppm) and/or Cu (up to 87 ppm). Chalcophile element data support a model whereby the dunitic bands have formed from reaction with percolating S-saturated melts. The progressive enrichments in Cu and PPGEs observed in the latest percolation stages (Pd/Ir(mn) = 16-28; Cu/S = 0.4) may be modelled by increasing the degree of melting that produced the percolating melt(s). A similar model may account for strong Cu and PPGE enrichments in the dunitic transition zone of some ophiolitic complexes.
