A radiogenic isotopic (He-Sr-Nd-Pb-Os) study of lavas from the Pitcairn hotspot: Implications for the origin of EM-1 (enriched mantle 1)

We present new He-Sr-Nd-Pb-Os isotopic compositions and major and trace-element concentrations for ten subaerially-erupted lavas and one seamount lava associated with the Pitcairn hotspot. The most geochemically-enriched lavas at the Pitcairn hotspot have signatures that are consistent with recycled sediments derived from upper continental crust. Pitcairn lavas have elevated Ti, which also supports the presence of a mafic protolith in the Pitcairn mantle. A subset of Pitcairn seamount samples, including the seamount sample presented here, are tholeiitic. Tholeiitic lavas are uncommon at ocean hotspots located far from mid-ocean ridges. Like tholeiites that erupted in Hawaii, the presence of tholeiites in the Pitcairn magmatic suite can be explained by melting a silica-saturated recycled mafic component in the Pitcairn mantle source. We also present the highest He-3/He-4 ratio (12.6 Ra, ratio to atmosphere) from the Pitcairn hotspot. This sample anchors the high Pb-206/Pb-204 portion of the Pitcairn array and provides evidence for a plume component in the Pitcairn mantle. In contrast, Pitcairn lavas that have the lowest Pb-206/Pb-204 are the most geochemically enriched, and have the highest Sr-87/Sr-86 and lowest Nd-143/Nd-144 in the Pitcairn suite; these EM-1 end-member lavas have MORB-like He-3/He-4 (similar to 8 Ra, ratio to atmosphere). Recycled oceanic crust and sediment suggested to be in the Pitcairn EM-1 mantle are expected to have low He-3/He-4 (< 0.1 Ra). Therefore, the higher, MORB-like 3He/4He in Pitcairn EM-1 lavas is paradoxical, but might be explained by diffusive exchange of helium, but not the heavy radiogenic isotopes, with the ambient mantle over billion-year timescales. (C) 2015 Elsevier B.V. All rights reserved.