David W. Graham Ilya N. Bindeman John E. Lupton Michael O. Garcia Matthew W. Loewen 2019 <p><span>The&nbsp;hydrogen isotope&nbsp;value (</span><i>δ</i><span>D) of water indigenous to the mantle is masked by the early degassing and&nbsp;recycling&nbsp;of surface water through Earth's history. High&nbsp;</span>³<span>He/</span>⁴<span>He ratios in some&nbsp;ocean island basalts, however, provide a clear geochemical signature of deep, primordial mantle that has been isolated within the Earth's interior from melting, degassing, and convective mixing with the&nbsp;upper mantle. Hydrogen isotopes were measured in high&nbsp;</span>³<span>He/</span>⁴<span>He submarine&nbsp;basalt&nbsp;glasses from the Southeast Indian Ridge (SEIR) at the Amsterdam–St. Paul (ASP) Plateau (</span><i>δ</i><span>D = −51 to −90‰,&nbsp;</span>³<span>He/</span>⁴<span>He = 7.6 to 14.1 R</span>_A<span>) and in submarine glasses from Loihi&nbsp;seamount&nbsp;south of the island of Hawaii (</span><i>δ</i><span>D = −70 to −90‰,&nbsp;</span>³<span>He/</span>⁴<span>He = 22.5 to 27.8 R</span>_A<span>). These results highlight two contrasting patterns of&nbsp;</span><i>δ</i><span>D for high&nbsp;</span>³<span>He/</span>⁴<span>He lavas: one trend toward high&nbsp;</span><i>δ</i><span>D of approximately −50‰, and another converging at&nbsp;</span><i>δ</i><span>D = −75‰. These same patterns are evident in a global compilation of previously reported&nbsp;</span><i>δ</i><span>D and&nbsp;</span>³<span>He/</span>⁴<span>He results. We suggest that the high&nbsp;</span><i>δ</i><span>D values result from water recycled during&nbsp;subduction&nbsp;that is carried into the source region of&nbsp;mantle plumes&nbsp;at the core–mantle boundary where it is mixed with primordial mantle, resulting in high&nbsp;</span><i>δ</i><span>D and moderately high&nbsp;</span>³<span>He/</span>⁴<span>He. Conversely, lower&nbsp;</span><i>δ</i><span>D values of −75‰, in basalts from Loihi seamount and also&nbsp;trace element&nbsp;depleted mid-ocean ridge basalts, imply a primordial Earth hydrogen isotopic value of −75‰ or lower.&nbsp;</span><i>δ</i><span>D values down to −100‰ also occur in the most trace element-depleted mid-ocean ridge basalts, typically in association with&nbsp;</span>⁸⁷<span>Sr/</span>⁸⁶<span>Sr ratios near 0.703. These lower&nbsp;</span><i>δ</i><span>D values may be a result of multi-stage melting history of the upper mantle where minor D/H&nbsp;fractionation&nbsp;could be associated with hydrogen retention in nominally anhydrous residual minerals. Collectively, the predominance of&nbsp;</span><i>δ</i><span>D around −75‰ in the majority of mid-ocean ridge basalts and in high&nbsp;</span>³<span>He/</span>⁴<span>He Loihi basalts is consistent with an origin of water on Earth that was dominated by&nbsp;accretion&nbsp;of chondritic material.</span></p> application/pdf 10.1016/j.epsl.2018.12.012 en Elsevier Hydrogen isotopes in high 3He/4He submarine basalts: Primordial vs. recycled water and the veil of mantle enrichment article