Heterogeneity and evolution of the Miocene Iceland mantle plume
Martin-Roberts, Emma L.
The North Atlantic Igneous Province (NAIP) is unique in that it presents a complete magmatic history from ~62 Ma to today. This provides an unrivalled opportunity to study the geochemical and thermal evolution of a mantle plume through time. Despite the wealth of geochemical data available for Iceland, our knowledge of mantle heterogeneity beneath Iceland during the mid-Miocene remains sketchy. The oldest basalts (15-16 Ma) on Iceland are preserved in several ~1 km thick sequences in Vestfirdir in Northwest Iceland. Each sequence contains a hiatus in volcanism, marked by a laterite-lignite horizon, that is likely the result of the relocation of a spreading axis. The basalt geochemistry from above and below this horizon provide an insight into the heterogeneity of the Iceland mantle plume during the mid-Miocene. The Vestfirdir basalts are also characterised by the highest 3He/4He (~40 Ra) on Iceland requiring the involvement of deep mantle. Incompatible trace element and Nd-Sr-Pb isotope data indicate that there are two enriched components (NWE1 and NWE2) and one depleted component (NWD1) within the mantle beneath Iceland during the Miocene. Both enriched components have low 143Nd/144Nd and high 87Sr/86Sr, that are comparable with the most enriched modern Iceland basalts. The two components are differentiated by the relative enrichment in the more incompatible trace elements and their Pb-isotope compositions. Component NWE1 is similar to the modern Iceland mantle plume composition, while NWE2 is closer in composition to modern depleted Iceland rift-zone basalts. The depleted component identified in the Vestfirdir basalts (NWD1) is distinct from modern North Atlantic N-MORB and the depleted component in the neovolcanic rift zones by, for instance, higher incompatible trace element concentrations and a more negative ∆207Pb composition. These observations allow the identification of a depleted mantle component present at 15-16 Ma that is no longer sampled in the modern Iceland plume setting and argue for a distinct compositional change in the mantle beneath Iceland over the last 15-16 Myr. Existing and new 3He/4He and magmatic temperature measurements help understand how primordial helium and heat in mantle plumes vary with time and provide insight into the nature of the deep mantle and the source of primordial terrestrial volatiles. The highest 3He/4He (42 RA) recorded in Vestfirdir is midway between the starting plume (50 RA) and modern Iceland values (34 RA), demonstrating that 3He/4He appears to have steadily decreased with time. The high-3He/4He Vestfirdir basalts have a large range of incompatible trace element and Sr-Nd-Pb isotope ratios, as also seen in the PIP picrites, implying that there is no compositionally-unique mantle source for the high 3He/4He component in the Iceland plume. Additionally, the uncontaminated Vestfirdir basalts that haven’t been erupted through continental crust also have Pb-isotope values that rule out the need for a primordial, isolated mantle reservoir formed ~4.55-4.45 Ga. The new Al-in-olivine data from high-3He/4He Vestfirdir basalts yield crystallisation temperatures significantly less than that of proto-Iceland plume and modern Iceland picrites. The apparent ~40°C increase of the Iceland plume temperature in the last 15 million years contrasts strongly with the decrease of 3He/4He. This suggests that the heat and primordial 3He are decoupled in the Iceland plume. This may reflect the different sources (i.e. heat from the core, 3He from the deep mantle) or alternatively different diffusion rates of heat and helium across the core-mantle boundary.