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Abstract The Icelandic hotspot has erupted basaltic magma with the highest mantle‐derived3He/4He over a period spanning much of the Cenozoic, from the early‐Cenozoic Baffin Island‐West Greenland flood basalt province (49.8RA), to mid‐Miocene lavas in northwest Iceland (40.2 to 47.5RA), to Pleistocene lavas in Iceland's neovolcanic zone (34.3RA). 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 have87Sr/86Sr = 0.703008–0.703021,143Nd/144Nd = 0.513094–0.513128,176Hf/177Hf = 0.283265–0.283284,206Pb/204Pb = 17.7560–17.9375,3He/4He up to 39.9RA, and mantle‐like δ18O of 5.03–5.21‰. The radiogenic isotopic compositions of the least crustally contaminated lavas are more geochemically depleted than Iceland high‐3He/4He lavas, a shift that cannot be explained by continental crust assimilation in the Baffin suite. Thus, we argue for the presence oftwogeochemically distinct high‐3He/4He 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 mid‐ocean ridge basalts (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‐3He/4He domain is dense, located in the deep mantle, and sampled by only the hottest plumes.
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Remnants of early Earth differentiation in the deepest mantle-derived lavas
The noble gas isotope systematics of ocean island basalts suggest the existence of primordial mantle signatures in the deep mantle. Yet, the isotopic compositions of lithophile elements (Sr, Nd, Hf) in these lavas require derivation from a mantle source that is geochemically depleted by melt extraction rather than primitive. Here, this apparent contradiction is resolved by employing a compilation of the Sr, Nd, and Hf isotope composition of kimberlites—volcanic rocks that originate at great depth beneath continents. This compilation includes kimberlites as old as 2.06 billion years and shows that kimberlites do not derive from a primitive mantle source but sample the same geochemically depleted component (where geochemical depletion refers to ancient melt extraction) common to most oceanic island basalts, previously called PREMA (prevalent mantle) or FOZO (focal zone). Extrapolation of the Nd and Hf isotopic compositions of the kimberlite source to the age of Earth formation yields a143Nd/144Nd-176Hf/177Hf composition within error of chondrite meteorites, which include the likely parent bodies of Earth. This supports a hypothesis where the source of kimberlites and ocean island basalts contains a long-lived component that formed by melt extraction from a domain with chondritic143Nd/144Nd and176Hf/177Hf shortly after Earth accretion. The geographic distribution of kimberlites containing the PREMA component suggests that these remnants of early Earth differentiation are located in large seismically anomalous regions corresponding to thermochemical piles above the core–mantle boundary. PREMA could have been stored in these structures for most of Earth’s history, partially shielded from convective homogenization.
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- Award ID(s):
- 1900652
- PAR ID:
- 10207135
- Publisher / Repository:
- Proceedings of the National Academy of Sciences
- Date Published:
- Journal Name:
- Proceedings of the National Academy of Sciences
- Volume:
- 118
- Issue:
- 1
- ISSN:
- 0027-8424
- Page Range / eLocation ID:
- Article No. e2015211118
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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