Platinum-group elements in the Icelandic rift system: melting processes and mantle sources beneath Iceland

Abstract

Thirty new platinum-group element (PGE) analyses of various basalt types from Iceland are presented in this study. The analysed samples are divided into three groups based on their Mg-contents, high-Mg tholeiites (10–14 wt.% MgO), composed of primitive olivine tholeiites and picrites, evolved olivine tholeiites (7–10 wt.% MgO) and evolved basalts (4–7 wt.% MgO) consisting of FeTi basalts, quartz tholeiites and alkaline basalts. The high-Mg tholeiites have a range of compositions between a relatively Cu-rich and Pd-poor (120 ppm Cu and 6 ppb Pd) end-member and a relatively Cu-poor and Pd-rich (74 ppm Cu and 17 ppb Pd) end-member. There is a positive correlation between the highly siderophile elements, whereas Cu and Pd correlate negatively between the Cu-rich primitive olivine tholeiite and the Cu-poor picrite end-member. Negative correlation between Cu and the PGEs cannot be reconciled in a model where the two end-members form by varying degrees of partial melting of a common source. The sub-primitive mantle Cu/Pd ratio of the picrite end-member (∼4300) could indicate a strongly depleted mantle source, where Pd was efficiently retained in mantle sulphide, relative to Cu, during previous melt extraction episodes. In mantle melting models presented here, the picrite composition can be approximated by 25% melting of a mantle source that previously lost a normal MORB component ( F∼15%). The olivine tholeiite end-member (Cu/Pd∼19000) can be approximated in mantle melting models by assuming derivation from a separate source, one slightly Cu-enriched source undergoing melting beneath a rift. Based on Cu and Pd variations, the majority of high-Mg tholeiites are interpreted to be mixtures between melts derived from highly depleted to slightly enriched sources, which is consistent with previous studies of Sr–Nd isotopic variations. The similar range in Cu/Pd ratios of the high-Mg tholeiites and the evolved olivine tholeiites could infer that the latter evolved from parental liquids similar to the high-Mg tholeiites under S-undersaturated conditions. Relatively high Pd and low Ir concentrations in the evolved olivine tholeiite samples (∼7 wt.% MgO) indicate, too, that these magmas underwent S-undersaturated fractionation during which Pd accumulated in the melt whereas Ir was incorporated into the fractionating assemblage. This is in contrast to the group of evolved basalts with less than 7 wt.% MgO with low PGE contents and high Cu/Pd ratios that strongly suggest that these samples have experienced S-saturation. We propose a scenario for melt generation beneath the Icelandic rift zone, where picritic magma is generated from a highly depleted mantle, whereas primitive olivine tholeiite magma forms from the depleted as well as a Cu-enriched source. It is proposed that Cu-enriched mantle domains were exhausted at depth since their melting products are absent in the most depleted picrite derived from the uppermost part of the central melting region. The olivine tholeiites can be approximated by around 11–12% partial melting in a triangular melting regime beneath the Icelandic rift zone. The olivine tholeiites are interpreted to reflect efficient melt collection from the deepest and most distal parts of the melting triangle. In contrast, the picrite represent a high degree of melting ( F∼25%) without incorporation of melt batches from the Cu-enriched mantle source.