Enriched component of the proto-Icelandic mantle plume revealed in alkaline Tertiary lavas from East Greenland

Abstract

Alkaline ultramafic lavas in north East Greenland were contemporaneous with Tertiary tholeiitic flood basalts in East Greenland. The alkaline lavas are enriched in light rare earth elements (REEs) and Ti and Nb, but depleted in Rb, Ba, K, Th, and P. The flood basalts have trace element profiles similar to those of the alkaline lavas. These features are not caused by fractional crystallization or crustal contamination and are thus regarded as inherited from the mantle source. The trace element profiles and Sr and Nd isotopic compositions of the flood basalts are shown to be consistent with a simple model, in which 5%–10% alkaline melt is mixed into normal-mid-ocean ridge basalt. This alkaline melt has a composition reflecting low-degree melting at high pressures of incompatible element– enriched mantle. In areas where only deep melting occurred, this enriched melt component reached the surface in an essentially undiluted form. Elsewhere, under the rift axis, it mixed with asthenospheric melts, resulting in the typical enriched characteristics of East Greenland flood basalts. The presence of pronounced negative P anomalies in modern Iceland lavas suggests this to be a long-term feature of the Iceland mantle plume.