Pb isotope evidence for contributions from different Iceland mantle components to Palaeogene East Greenland flood basalts

Abstract

We present new Pb isotope data on 21 samples of break-up-related flood basalts (56–54 Ma) from the Blosseville Kyst region of East Greenland. These samples show a considerable range in isotopic composition ( 206Pb/ 204Pb 17.6 to 19.3) that broadly correlates with compositional type. The ‘low-Ti’ type is characterised by the lowest 206Pb/ 204Pb (17.6 to 18.2) and highest εNd 55Ma (+8 to +11), whereas the ‘very high-Ti’ type has the opposite characteristics ( 206Pb/ 204Pb 18.9 to 19.3: εNd 55Ma +5 to +6), with the ‘high-Ti’ type lying in between ( 206Pb/ 204Pb 18.3 to 19.0: εNd 55Ma +6 to +8). Crustal contamination in this region produces unradiogenic 206Pb/ 204Pb and εNd values, as is clearly seen in one sample ( 206Pb/ 204Pb 16.2; εNd 55Ma +2). Thus, the negative correlation between 206Pb/ 204Pb and εNd shown by most of the samples confirms inferences from trace element data that crustal interaction has been minimal in the selected samples. Uncontaminated Palaeogene East Greenland flood basalts appear to have sampled the same broad range in mantle compositions seen in Recent Iceland basalts. In contrast to the peripheral lava suites from the British Isles and Southeast Greenland, where the inferred uncontaminated magmas have a restricted and relatively unradiogenic 206Pb/ 204Pb composition (‘North Atlantic End-Member’—NAEM: [J. Petrol. 41 (2000) 919]), the uncontaminated lavas and dykes of East Greenland and the Faroes from close to the plume track show a wide compositional range and include magmas with high 206Pb/ 204Pb, similar to the most radiogenic values found in recent Icelandic basalts. Furthermore, the main volume of lavas in East Greenland is displaced away from the NAEM towards this radiogenic Pb component. Thus, this ‘Iceland radiogenic Pb end-member’ component was a significant contributor to the break-up-related magmatism, at least close to the inferred plume track, suggesting that it has been an inherent component of the Iceland plume upwelling for the last 56 Myr or so, although perhaps absent earlier.