Figure 4: Larger range in isotopic composition of the continental flood volcanism most likely reflects continental lithospheric contamination.
From: How and when plume zonation appeared during the 132 Myr evolution of the Tristan Hotspot
The range in isotopic composition of continental flood volcanism and Tristan-Gough oceanic hotspot track volcanism are shown on (a) 87Sr/86Sr versus 143Nd/144Nd and (b) 206Pb/204Pb versus 207Pb/204Pb isotope correlation diagrams. In (a) samples with MgO>11 wt.% (enclosed within dashed line) show a much more restricted range in isotopic composition, yet 87Sr/86Sr ratio in some mafic samples is still higher than in the oceanic part of the hotspot track, either reflecting contamination by crustal material or lithospheric mantle, both of which can have extremely radiogenic Sr27,28. In (b), if only Etendeka and Parana flood basalts with 87Sr/86Sr<0.7067 (highest value in the oceanic hotspot track) are considered on the uranogenic Pb isotope diagram (marked with a cross and enclosed within the field defined by the dashed line), they only show a slightly greater range than the Gough field, suggesting that 87Sr/86Sr ratio can be used to effectively filter for continental lithospheric contamination. Arrows denote directions for upper and lower crustal and/or lithospheric mantle contamination. In (a) the arrow labelled ‘Upper continental crust contamination’ extends into the field for Damara S-type granites and points to the field for Damara metasediments27. The arrow labelled ‘Lower continental crust contamination’ points toward the Kaokoland gneisses (Pre-Damara basement)27. In (b) the ‘Lower crustal contam. (contamination)’ arrow overlaps with and points to lower crustal granulites from the Namaqua-Natal Belt in South Africa28. The ‘Upper crustal/lithospheric mantle contamination’ arrows point to the following rock groups in Namibia: (1) upper arrow–Khan granodiorites (samples G12 and G13 (ref. 30) and 02/99 and 03/99 (ref. 31)) and (2) lower arrow–Kuiseb schists (sample Kh27 (ref. 30)) and lithospheric mantle, estimated to have a present-day composition of 206Pb/204Pb∼19.8 and 207Pb/204Pb ∼15.7 (based on sample VB32) beneath the Spitzkoppe region in Namibia27. See Supplementary Dataset 5 and additional data from GEOROC (http://georoc.mpch-mainz.gwdg.de/georoc/). Average radiogenic ingrowth correction and 1σ variation for Parana and Etendeka as defined in the Fig. 3 caption.
