COPPER AND NOBLE-METAL ENRICHMENTS ACROSS THE LITHOSPHERE ASTHENOSPHERE BOUNDARY OF MANTLE DIAPIRS - EVIDENCE FROM THE LANZO LHERZOLITE MASSIF

The Lanzo lherzolite massif shows that the top of asthenospheric diapi rs is a zone of preferential S, Cu, Au, and platinum-group element (PG E) enrichment. Residual plagioclase Iherzolites which underwent a limi ted extraction of mid-ocean ridge basalt (MORB) melt during diapiric u prise are enriched in Ru, PPGE (Pt and Pd), and Au (3-10 times the pri stine 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 cont ents ranging between 95 and 215 ppm. The behaviour of chalcophile elem ents in the Iherzolites cannot be modelled by equilibrium batch meltin g. The precious metals vary independently of lithophile element conten t 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-cu t the Lanzo massif before rifting or by downward percolation of sulphi des segregated from the MORB magmas extracted from the asthenospheric diapir. Calculated chalcophile element compositions of the extracted m elts show high Cu/Pd ratios (2.7-14.5) typical of melts that have expe rienced early S saturation. This adds straightforward evidence to theo retical modelling and indirect assumptions based on extruded MORB, i.e ., it is possible to produce high D-element depleted magma composition s simply by low degrees of mantle melting under S-saturated conditions without fractionation in magma chambers. The MORB magma has circulate d 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 betwe en 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 all oys from the MORB melt before it cross-cut the peridotites. The duniti c bands are distinguished from the harzburgites and dunites analysed s o far by a large range of total precious metal content (17-77 ppb), po sitive 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 d unitic bands have formed from reaction with percolating S-saturated me lts. The progressive enrichments in Cu and PPGEs observed in the lates t percolation stages (Pd/Ir(mn) = 16-28; Cu/S = 0.4) may be modelled b y 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.