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 ni ckel sulfide fire assay, in peridotites and dyke rocks from the orogen ic ultramafic massifs of the Ivrea Zone in the Italian western Alps (B aldissero, Balmuccia, Finero) and the Betico-Rifean cordillera in sout hern Spain and northern Morocco (Ronda, Beni Bousera). The peridotites are considered as variably depleted and reenriched low lithosphere, w hereas the dyke rocks represent polybaric derivatives of basaltic melt s !pyroxenites and gabbros), most coming from the underlying asthenosp here. The peridotites have total PGE content in the range 8.6-54.7 ppb , while mantle-normalized patterns generally grade from nearly flat an d 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 a nomalies of Ir-Pt are frequently observed in dykes, indicating that bo th metals were probably retained in the mantle source of these melts. Most of the peridotites display positive anomaly of Au, and in some ca se are enriched in Ru, Rh, and Pd, but exhibit the same negative anoma lies in Ir and Pt as the dykes. These features are ascribed to reintro duction 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 mantl e. The role of the sulfide phase as carrier of the recycled PGE is str essed by clear interelemental correlation in peridotites from the Ivre a 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.