Article ID Journal Published Year Pages File Type
4715685 Lithos 2015 11 Pages PDF
Abstract

•New data from lavas associated with Pitcairn hotspot•Presence of recycled sediment in the source•Evidence of tholeiite volcanism, uncommon far from mid-ocean ridges•The highest 3He/4He ratio (12.6 Ra, ratio to atmosphere) from the Pitcairn hotspot•We propose a model that explains apparent high 3He/4He paradox in EM-1 mantle.

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 3He/4He ratio (12.6 Ra, ratio to atmosphere) from the Pitcairn hotspot. This sample anchors the high 206Pb/204Pb portion of the Pitcairn array and provides evidence for a plume component in the Pitcairn mantle. In contrast, Pitcairn lavas that have the lowest 206Pb/204Pb are the most geochemically enriched, and have the highest 87Sr/86Sr and lowest 143Nd/144Nd in the Pitcairn suite; these EM-1 end-member lavas have MORB-like 3He/4He (~  8 Ra, ratio to atmosphere). Recycled oceanic crust and sediment suggested to be in the Pitcairn EM-1 mantle are expected to have low 3He/4He (<  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.

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Physical Sciences and Engineering Earth and Planetary Sciences Geochemistry and Petrology
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