Grand Comore Island: A well-constrained "low ³He/⁴He" mantle plume

We report He isotope (³He/⁴He) variations in samples from alkali basaltic and basanitic lava flows from Grande Comore Island complemented by existing [1,2] [C. Class, S.L. Goldstein, Plume-lithosphere interactions in the ocean basins: constraints from the source mineralogy. Earth Planet. Sci. Lett., 150 (1997) 245-260, C. Class, S.L. Goldstein, R. Altherr, P. Bachèchlery, The process of plume-lithosphere interaction in the ocean basins - the case of Grande Comore. J. Petrol., 39 (5) (1998) 881-903] and new Sr-Nd-Pb isotope ratios and major and trace element abundances. He isotope data in samples from Tristan da Cunha and Gough islands and the Huri Hills in Kenya are reported also. Grande Comore ³He/⁴He ratios vary between 5.05 and 7.08 R_A (⁴He/ ³He ≈ 141,000-101,000). Chemical and Sr-Nd-Pb isotopic variations of Grande Comore lavas were previously shown to reflect melts derived from the deep mantle plume and the shallow lithospheric mantle [1-3] [C. Class, S.L. Goldstein, Plume-lithosphere interactions in the ocean basins: constraints from the source mineralogy. Earth Planet. Sci. Lett., 150 (1997) 245-260, C. Class, S.L. Goldstein, R. Altherr, P. Bachèchlery, The process of plume-lithosphere interaction in the ocean basins - the case of Grande Comore. J. Petrol., 39 (5) (1998) 881-903, C. Claude-Ivanaj, B. Bourdon, C.J. Allègre, Ra-Th-Sr isotope systematics in Grande Comore Island: a case study of plume-lithosphere interaction. Earth Planet. Sci. Lett. 164 (1998) 99-117]. The lithosphere-dominated end-member (La Grille volcano) shows uniform ³He/ ⁴He ratios within error of 6.75 - 7.08 R_A (⁴He/³He ≈ 106,000 - 101,000) over a range of [He] = 36-428 × 10^-9 ccSTP/g. The plume end-member (of the Karthala volcano suite), as constrained by Sr, Nd, Pb isotope ratios, shows uniformly lower ³He/⁴He ratios with 5.05 - 5.41 R_A (⁴He/³He -132,000) over a range of [He] = 11-136 × 10^-9 ccSTP/g. All samples show good correlations between Sr-Nd-He isotope ratios, indicating that the Grande Comore ³He/⁴He ratios are not significantly influenced by crustal contamination and reflect recent mixing between plume- and lithosphere-derived melts. The lithosphere beneath Grande Comore has retained its MORB-like helium isotopic composition, which suggests that the previously identified amphibole-forming metasomatism of the lithospheric mantle [1] [C. Class, S.L. Goldstein, Plume-lithosphere interactions in the ocean basins: constraints from the source mineralogy. Earth Planet. Sci. Lett., 150 (1997) 245-260] occurred prior to the arrival of the Comoro plume. The well-constrained ³He/⁴He = 5.2 ± 0.2 R_A (⁴He/³He ≈ 137,000 ± 5000) of the Comoro plume confirms the existence of "low ³He/⁴He" mantle plumes. A global compilation of OIB shows that OIB from "low ³He/⁴He" plumes form one "side" of the roughly triangular distribution of the global OIB data set in ¹⁴³Nd/¹⁴⁴Nd versus ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb, encompassing the lowest Nd to the highest Pb isotope ratios. It is also shown that "low ³He/⁴He" plumes are more enriched in Th and U relative to other plumes.