Platinum-group-element distribution in subcontinental mantle: Evidence from the Ivrea zone (Italy) and the Betic-Rifean cordillera (Spain and Morocco)

The six platinum-group elements (PGE's) Os, Ir, Ru, Ph, Pt, and Pd and Au were analyzed by instrumental neutron-activation analysis after nickel sulfide fire assay, in peridotites and dyke rocks from the orogenic ultramafic massifs of the Ivrea Zone in the Italian western Alps (Baldissero, Balmuccia, Finero) and the Betico-Rifean cordillera in southern Spain and northern Morocco (Ronda, Beni Bousera). The peridotites are considered as variably depleted and reenriched low lithosphere, whereas the dyke rocks represent polybaric derivatives of basaltic melts !pyroxenites and gabbros), most coming from the underlying asthenosphere. The peridotites have total PGE content in the range 8.6-54.7 ppb, while mantle-normalized patterns generally grade from nearly flat and PGE rich, in less depleted lherzolites, to negative and PGE poor, in residual harzburgites and dunites. Dyke rocks have total PGE's in the range 5.4-250 ppb and positive mantle-normalized patterns. Negative anomalies of Ir-Pt are frequently observed in dykes, indicating that both metals were probably retained in the mantle source of these melts. Most of the peridotites display positive anomaly of Au, and in some case are enriched in Ru, Rh, and Pd, but exhibit the same negative anomalies in Ir and Pt as the dykes. These features are ascribed to reintroduction of noble metals into the residual mantle by reaction with the basaltic melts that generated the dykes, or alternatively by recycling of ''dyke material'' during further partial melting of the host mantle. The role of the sulfide phase as carrier of the recycled PGE is stressed by clear interelemental correlation in peridotites from the Ivrea Zone. Present data provide evidence that zones of PGE enrichment can originate this way in the subcontinental mantle, and may constitute a potential reservoir for noble metal fertile volcanism in continental rift systems.