Using platinum-group elements to investigate the origin of the Ontong Java Plateau, SW Pacific

Abstract

Fractionated basalts (4.5–8.9 wt.% MgO) from the islands of Malaita and Makira (San Cristobal) in the Solomon Islands represent outpourings of magma that formed the Ontong Java Plateau (OJP), SW Pacific. The origin of the OJP is explored by examining platinum-group element (PGE: Ru, Rh, Pd, Ir, Pt) abundances in these basalts. On Makira, mid-ocean ridge basalts (MORBs) and ocean island basalts (OIBs) are occasionally intercalated within the stratigraphic sequence of the OJP—the (OIB)-type basalts could be representative of the plume tail sequence and the MORB-type flows may be an indication of heterogeneity within the OJP source. Partial melting models indicate that the MORB-type basalts were generated from a source dominated by upper mantle material. However, they suggest that the OJP basalts and the OIB-type basalts were not generated from an exclusively upper mantle source. Our illustrative modeling shows the range of PGE compositions in these plume-generated basalts are best generated from a hybrid source that incorporates 0–0.5 wt.% of outer core material, 50% depleted upper mantle and 50–49.5% lower (primitive) mantle.