Platinum-Group Elements (PGE) in Basalts of the Seaward-Dipping Reflector Sequence, SE Greenland Coast

Abstract

The rift-related, seaward-dipping reflector sequence (SDRS) SE of INTRODUCTION Greenland consists of basaltic lavas that exhibit variable degrees of Seaward-dipping reflector sequences (SDRSs) are charmagmatic differentiation, derived from a heterogeneous mantle source. acteristic features of volcanic rifted margins. The SDRS Platinum-group elements (PGE) are used to provide insights into SE of Greenland formed during the initial break-up of the petrogenetic evolution of the SDRS, and to characterize the the North Atlantic in the late Paleocene–early Eocene magma sources. Noble metal concentrations correlate well with (Duncan et al., 1996) and spans the whole period from indicators for magmatic differentiation (mg-number, MgO), excontinental break-up to seafloor spreading (Brooks & hibiting two distinct trends. Concentrations of Ir, Ru and Rh tend Nielsen, 1982; Saunders et al., 1997). Large volumes of to decrease with progressive differentiation, indicating compatible lava erupted in a short period of time under subaerial behaviour of these elements during fractional crystallization processes. or shallow water conditions along the continental margins The variation of Pt and Pd shows segmented trends. In primitive of Norway and Greenland. The high magma production magmas, Pt and Pd are incompatible and become enriched in the rates, which led to the formation of magmatic sequences melt. The primitive magma is S undersaturated, despite derivation up to 6 km thick, are inferred to require the presence of from a depleted mid-ocean ridge basalt source at a moderate degree a mantle plume (White & McKenzie, 1989). of melting, reflecting enhanced S solubility in the melt caused by The aims of Ocean Drilling Program (ODP) Legs 152 high Fe content and elevated temperature. In the more evolved lavas, and 163 were to investigate the origin and nature of the Pt and Pd decrease with decreasing MgO and mg-number. This SDRS with regard to the influence of a mantle plume indicates that S saturation had occurred with Pt and Pd being (Larsen et al., 1994a; Duncan et al., 1996). Nine sites were incorporated in sulphides, which probably segregated during ascent. drilled in two transects distal to the ancient plume track Bulk partition coefficients for the PGE during partial melting are at 63°N and 66°N (Fig. 1). calculated based on data from a primitive basaltic unit with MgO It has been shown that platinum-group element (PGE) >20 wt %, representing a near-primary magma composition. The systematics can provide important information about determined bulk partition coefficients for an S-undersaturated melt the petrogenesis of basaltic magmas, such as magmatic are about 2 (Ir), 4 (Ru), 1·2 (Rh), 0·5 (Pt) and 0·4 (Pd). This differentiation, sulphide segregation and mantle source indicates that Ir, Ru and Rh are compatible during partial melting, characteristics (e.g. Brugmann et al., 1987; Greenough & whereas Pt and Pd are incompatible. Owen, 1992; Barnes & Picard, 1993; Seitz & Keays,