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

The Icelandic hotspot has erupted the highest terrestrial mantle-derived 3He/4He over a period spanning much of the Cenozoic, from the early-Cenozoic Baffin Island-West Greenland flood basalt province (49.8 RA), to the mid-Miocene lavas in northwest Iceland (40.2 to 47.5 RA), to Pleistocene lavas in Iceland’s neovolcanic zone (34.3 RA). This study provides a detailed geochemical data set—He-O-Sr-Nd-Hf-Pb isotopic compositions, as well as whole rock major and trace element concentrations—for a suite of 18 Baffin Island lavas. The Baffin Island lavas transited through and potentially assimilated variable degrees of Precambrian continental basement. We therefore use geochemical indicators sensitive to continental crust assimilation (whole rock Nb/Th, Ce/Pb, MgO) to identify the least crustally-contaminated lavas in the suite. Four lavas, identified as “least crustally-contaminated”, have high MgO (>15 wt.%) and Nb/Th and Ce/Pb ratios that fall within the mantle range (Nb/Th=15.6±2.6, Ce/Pb=24.3±4.3). These four lavas have 3He/4He up to 39.9 RA and mantle-like δ18O of 5.03 to 5.21‰, 87Sr/86Sr = 0.703008–0.703021, 143Nd/144Nd = 0.513094–0.513128, 176Hf/177Hf = 0.283265–0.283284, 206Pb/204Pb = 17.7560–17.9375, and are located on or near the 4.5 Ga Pb isotope geochron. The radiogenic isotopic compositions of the least crustally-contaminated Baffin Island lavas are offset to more geochemically depleted compositions compared to high-3He/4He lavas from Iceland, a shift that cannot be explained by continental crust assimilation in the Baffin suite. While Sr-Nd-Pb isotopic heterogeneity among high-3He/4He localities has been previously observed, this is an important observation of geochemically distinct high-3He/4He endmembers within a single hotspot. Additionally, the least crustally-contaminated primary melts from Baffin Island-West Greenland have higher mantle potential temperatures (1510 to 1630 °C) than global MORB primary magmas located far from hotspots (1320 to 1480 °C), which supports a hot, buoyant plume origin for these early Iceland plume lavas. These observations support the contention that the geochemically heterogeneous high-3He/4He domain is dense, located in the deep mantle, and sampled by only the hottest plumes