Platinum-group elements (PGE) in basalts of the seaward-dipping reflector sequence, SE Greenland coast

The rift-related, seaward-dipping reflector sequence (SDRS) SE of Greenland consists of basaltic lavas that exhibit variable degrees of magmatic differentiation, derived fi om a heterogeneous mantle source. Platinum-group elements (PGE) are used to provide insights into the petrogenetic evolution of the SDRS, and to characterize the magma sources. Noble metal concentrations correlate well with indicators for magmatic differentiation (mg-number, MgO), exhibiting two distinct trends. Concentrations of Ir, Ru and Ph tend to decrease with progressive differentiation, indicating compatible behaviour of these elements during fractional crystallization processes. The variation of Pt and Pd shows segmented trends. lit primitive magmas, Pt and Pd are incompatible and become enriched in the melt. The primitive: magma is S undersaturated, despite derivation from a depleted mid-ocean ridge basalt source at a moderate degree of melting, reflecting enhanced S solubility in the melt caused by high Fe content and elevated temperature. In the more evolved lavas, Pt and Pd decrease with decreasing MgO and mg-number. This indicates that S saturation had occurred with Pt and Pd being incorporated in sulphides, which probably segregated during ascent. Bulk partition coefficients for the PGE during partial melting are calculated based on data from a primitive basaltic unit with MgO similar to 20 wt %, representing a near-primary magma composition. The determined bulk partition coefficients for an S-undersaturated melt are about 2 (Ir), 4 (Ru), 1.2 (Rh), 0.5 (Pt) and 0.4 (Pd). This indicates that Ir, Ru and Ph are compatible during partial melting, whereas Pt and Pd are incompatible.