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 diffe rentiation, derived fi om a heterogeneous mantle source. Platinum-group ele ments (PGE) are used to provide insights into the petrogenetic evolution of the SDRS, and to characterize the magma sources. Noble metal concentration s correlate well with indicators for magmatic differentiation (mg-number, M gO), exhibiting two distinct trends. Concentrations of Ir, Ru and Ph tend t o decrease with progressive differentiation, indicating compatible behaviou r of these elements during fractional crystallization processes. The variat ion of Pt and Pd shows segmented trends. lit primitive magmas, Pt and Pd ar e incompatible and become enriched in the melt. The primitive: magma is S u ndersaturated, despite derivation from a depleted mid-ocean ridge basalt so urce at a moderate degree of melting, reflecting enhanced S solubility in t he melt caused by high Fe content and elevated temperature. In the more evo lved lavas, Pt and Pd decrease with decreasing MgO and mg-number. This indi cates that S saturation had occurred with Pt and Pd being incorporated in s ulphides, which probably segregated during ascent. Bulk partition coefficie nts 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-pr imary 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 melti ng, whereas Pt and Pd are incompatible.