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

The Icelandic hotspot has erupted basaltic magma with the highest mantle-derived He-3/He-4 over a period spanning much of the Cenozoic, from the early-Cenozoic Baffin Island-West Greenland flood basalt province (49.8 R-A), to mid-Miocene lavas in northwest Iceland (40.2 to 47.5 R-A), to Pleistocene lavas in Iceland's neovolcanic zone (34.3 R-A). 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 and Ce/Pb that fall within the mantle range (Nb/Th = 15.6 +/- 2.6, Ce/Pb = 24.3 +/- 4.3). These lavas have Sr-87/Sr-86 = 0.703008-0.703021, Nd-143/Nd-144 = 0.513094-0.513128, Hf-176/Hf-177 = 0.283265-0.283284, Pb-206/Pb-204 = 17.7560-17.9375, He-3/He-4 up to 39.9 R-A, and mantle-like delta O-18 of 5.03-5.21%. The radiogenic isotopic compositions of the least crustally contaminated lavas are more geochemically depleted than Iceland high-He-3/He-4 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-He-3/He-4 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 degrees C) than Siqueiros mid-ocean ridge basalts (1300 to 1410 degrees C), which attests to a hot, buoyant plume origin for early Iceland plume lavas. These observations support the contention that the geochemically heterogeneous high-He-3/He-4 domain is dense, located in the deep mantle, and sampled by only the hottest plumes.