Hot and heterogenous high-3 He/ 4 He components: New constraints from proto- Iceland plume lavas from Baffin Island

Lori N Willhite (1) , Matthew G Jackson , Janne Blichert-Toft (2) , Ilya Bindeman , Mark D Kurz , Saemundur A Halldórsson , Sunna Harðardóttir , Esteban Gazel , Allison A Price , Benjamin L Byerly

1 UC Santa Barbara - University of California [Santa Barbara]
2 LGL-TPE - Laboratoire de Géologie de Lyon - Terre, Planètes, Environnement

Key Points: • Baffin Island-West Greenland high-3 He/ 4 He lavas are more geochemically depleted than any other high-3 He/ 4 He lavas globally. • The isotopic composition of the high-3 He/ 4 He mantle source in the Iceland plume has evolved through time. • Baffin Island and West Greenland primary melts record hotter temperatures than high-MgO MORB, consistent with a deep, dense plume source. Abstract The Icelandic hotspot has erupted basaltic magma with the highest mantle-derived 3 He/ 4 He over a period spanning much of the Cenozoic, from the early-Cenozoic Baffin Island-West Greenland flood basalt province (49.8 RA), to mid-Miocene lavas in northwest Iceland (40.2 to 47.5 RA), to Pleistocene lavas in Iceland's neovolcanic zone (34.3 RA). The Baffin Island lavas transited through and potentially assimilated variable amounts of Precambrian continental basement. We use geochemical indicators sensitive to continental crust assimilation (Nb/Th, Ce/Pb, MgO) to identify the least crustally contaminated lavas. Four lavas, identified as "least crustally contaminated", have high MgO (>15 wt.%), and Nb/Th, Ce/Pb that fall within the mantle range (Nb/Th=15.6±2.6, Ce/Pb=24.3±4.3). These lavas have 87 Sr/ 86 Sr = 0.703008-0.703021, 143 Nd/ 144 Nd = 0.513094-0.513128, 176 Hf/ 177 Hf = 0.283265-0.283284, 206 Pb/ 204 Pb = 17.7560-17.9375, 3 He/ 4 He up to 39.9 RA, and mantle-like δ 18 O of 5.03-5.21‰. The radiogenic isotopic compositions of the least crustally contaminated lavas are more geochemically depleted than Iceland high-3 He/ 4 He lavas, a shift that cannot be explained by continental crust assimilation in the Baffin suite. Thus, we argue for the presence of two geochemically distinct high-3 He/ 4 He components within the Iceland plume. Additionally, the least crustally contaminated primary melts from Baffin Island-West Greenland have higher mantle potential temperatures (1510 to 1630 °C) than Siqueiros MORB (1300 to 1410 °C), which attests to a hot, buoyant plume origin for early Iceland plume lavas. These observations support the contention that the geochemically heterogeneous high-3 He/ 4 He domain is dense, located in the deep mantle, and sampled by only the hottest plumes.

Domaines: Géochimie

Lori N Willhite, Matthew G Jackson, Janne Blichert-Toft, Ilya Bindeman, Mark D Kurz, et al.. Hot and heterogenous high-3 He/ 4 He components: New constraints from proto- Iceland plume lavas from Baffin Island. Geochemistry, Geophysics, Geosystems, 2019, ⟨10.1029/2019GC008654⟩. ⟨hal-02995379⟩