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  • Accepted Manuscript: Hydrogen isotopes in high 3He/4He submarine basalts: Primordial vs. recycled water and the veil of mantle enrichment
Title: Hydrogen isotopes in high 3He/4He submarine basalts: Primordial vs. recycled water and the veil of mantle enrichment
The hydrogen isotope value (δD) of water indigenous to the mantle is masked by the early degassing and recycling of surface water through Earth’s history. High 3He/4He ratios in some 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 upper mantle. Hydrogen isotopes were measured in high 3He/4He submarine basalt glasses from the Southeast Indian Ridge (SEIR) at the Amsterdam–St. Paul (ASP) Plateau (δD =−51 to −90, 3He/4He =7.6 to 14.1 RA) and in submarine glasses from Loihi seamount south of the island of Hawaii (δD =−70 to −90, 3He/4He =22.5 to 27.8 RA). These results highlight two contrasting patterns of δD for high 3He/4He lavas: one trend toward high δD of approximately −50, and another converging at δD =−75. These same patterns are evident in a global compilation of previously reported δD and 3He/4He results. We suggest that the high δD values result from water recycled during subduction that is carried into the source region of mantle plumes at the core–mantle boundary where it is mixed with primordial mantle, resulting in high δD and moderately high 3He/4He. Conversely, lower more » δD values of −75, in basalts from Loihi seamount and also trace element depleted mid-ocean ridge basalts, imply a primordial Earth hydrogen isotopic value of −75or lower. δD values down to −100also occur in the most trace element-depleted mid-ocean ridge basalts, typically in association with 87Sr/86Sr ratios near 0.703. These lower δD values may be a result of multi-stage melting history of the upper mantle where minor D/H fractionation could be associated with hydrogen retention in nominally anhydrous residual minerals. Collectively, the predominance of δD around −75in the majority of mid-ocean ridge basalts and in high 3He/4He Loihi basalts is consistent with an origin of water on Earth that was dominated by accretion of chondritic material. « less
Authors:
Award ID(s):
1834758
Publication Date:
NSF-PAR ID:
10111995
Journal Name:
Earth and planetary science letters
Volume:
508
Page Range or eLocation-ID:
62-73
ISSN:
1385-013X
Sponsoring Org:
National Science Foundation
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    The hydrogen isotope value (δD) of water indigenous to the mantle is masked by the early degassing and recycling of surface water through Earth’s history. High 3He/4He ratios in some 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 upper mantle. Hydrogen isotopes were measured in high 3He/4He submarine basalt glasses from the Southeast Indian Ridge (SEIR) at the Amsterdam–St. Paul (ASP) Plateau (δD = −51 to −90 per mil, 3He/4He = 7.6 to 14.1 RA) and in submarine glasses from Loihi seamount south of the island of Hawaii (δD = −70 to −90 per mil, 3He/4He = 22.5 to 27.8 RA). These results highlight two contrasting patterns of δD for high 3He/4He lavas: one trend toward high δD of approximately −50 per mil, and another converging at δD = −75 per mil. These same patterns are evident in a global compilation of previously reported δD and 3He/4He results. We suggest that the high δD values result from water recycled during subduction that is carried into the source region of mantle plumes at the core–mantle boundary where it is mixedmore »with primordial mantle, resulting in high δD and moderately high 3He/4He. Conversely, lower δD values of −75 per mil, in basalts from Loihi seamount and also trace element depleted mid-ocean ridge basalts, imply a primordial Earth hydrogen isotopic value of −75 per mil or lower. δD values down to −100per mil also occur in the most trace element depleted mid-ocean ridge basalts, typically in association with 87Sr/86Sr ratios near 0.703. These lower δD values may be a result of multi-stage melting history of the upper mantle where minor D/H fractionation could be associated with hydrogen retention in nominally anhydrous residual minerals. Collectively, the predominance of δD around −75 per mil in the majority of mid-ocean ridge basalts and in high 3He/4He Loihi basalts is consistent with an origin of water on Earth that was dominated by accretion of chondritic material.« less
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