Contrasting behavior of noble-metal elements during magmatic differentiation in basalts from the Cook Islands, Polynesia

Noble metals are highly siderophilic and chalcophilic, and knowledge of their geochemical behavior in basaltic magmas is the key to understanding mantle processes such as partial melting and core-mantle interaction. It has been demonstrated that the mantle sources from some ocean island basalts (OIBs) have elevated 187 Os/188 Os ratios compared with both chondrites and mid-ocenic ridge basalts (MORBs) (Martin, 1991 ; Hauri and Hart, 1993), which could be caused by addition of outer core materials with high Re/Os ratios to the base of the mantle (Walker et al.,1995). This is consistent with the idea that a possible location of the hot-spot source, from which a mantle plume rises to generate OIB magmas, is the D" layer at the base of the mantle, where a relative reduction of seismic velocity takes place (e.g., Stacey and Lauper, 1983). The following seismic observations further suggest that D" may be the source for at least some mantle plumes : (1) the presence of an ultra-low velocity zone near the core-mantle boundary, which is consistent with partial melting in that region (Wen and Helmberger, 1998), and (2) the presence of a mantle-scale low-velocity anomaly beneath the South Pacific, probably related to upwelling of a large plume (Fukao, 1992). Although the geochemistry of platinum-group elements (PGEs) in OIBs has the potential to provide key information on deep-mantle processes, few data on PGE abundances in OIBs have appeared since the work of Crocket and Skippen (1966). The reason for this may be the difficulty in analysing PGEs at such low concentration levels in basalts. This paper presents the first complete data set of PGE and Au abundance in Polynesian OIBs and examines the behavior of these elements during magmatic differentiation. (Résumé d'auteur)