Water strongly influences the physical properties of the mantle and enhances its ability to melt or convect. Its presence can also be used to trace recycling of surface reservoirs down to the deep mantle1, which makes knowledge of the water content in the Earth's interior and its evolution crucial for understanding global geodynamics. Komatiites (MgO-rich ultramafic magmas) result from a high degree of mantle melting at high pressures2 and thus are excellent probes of the chemical composition and water contents of the deep mantle. An excess of water over elements that show similar geochemical behaviour during mantle melting (for example, cerium) was recently found in melt inclusions in the most magnesium-rich olivine in 2.7-billion-year-old komatiites from Canada3 and Zimbabwe4. These data were taken as evidence for a deep hydrated mantle reservoir, probably the transition zone, in the Neoarchaean era (2.8 to 2.5 billion years ago). Here we confirm the mantle source of this water by measuring deuterium-to-hydrogen ratios in these melt inclusions and present similar data for 3.3-billion-year-old komatiites from the Barberton greenstone belt. From the hydrogen isotope ratios, we show that the mantle sources of these melts contained excess water, which implies that a deep hydrous mantle reservoir has been present in the Earth's interior since at least the Palaeoarchaean era (3.6 to 3.2 billion years ago). The reconstructed initial hydrogen isotope composition of komatiites is more depleted in deuterium than surface reservoirs or typical mantle but resembles that of oceanic crust that was initially altered by seawater and then dehydrated during subduction. Together with an excess of chlorine and depletion of lead in the mantle sources of komatiites, these results indicate that seawater-altered lithosphere recycling into the deep mantle, arguably by subduction, started before 3.3 billion years ago.
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All data used in this paper are included in the published article and its Supplementary Information and will be submitted to the EarthChem (https://doi.org/10.1594/IEDA/111310) and ResearchGate (https://www.researchgate.net/profile/Alexander_Sobolev) databases. Readers are welcome to comment on the online version of the paper.
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We thank S. Krasheninnikov for help in the heating of melt inclusions and V. Magnin for help with the EPMA facility at ISTerre, E. Füri for sharing with us reference synthetic forsterite and potentially H2O-free Suprasil 3002 quartz glass, and C. Bucholz and V. Polyakov for consultations on isotopic effects of hydrogen diffusion in olivine. The field work, business trips, participation in conferences, sample preparation and experimental study were funded by the Russian Science Foundation grant no. 14-17-00491 (to A.V.S.). Analytical work on EPMA facility in ISTerre was covered by Labex OSUG@2020 (Investissements d’avenir—ANR10 LABX56) and Institut Universitaire de France. The costs of SIMS analyses at CRPG, (Nancy, France) were covered by an INSU-CNRS grant to A.V.S. Maintenance of heating equipment in Vernadsky Institute was partially covered by Russian Foundation for Basic Research grant number 17-05-00856a (to A.V.S.). The additional study during revision of paper and publishing costs were covered by a grant from the Richard Lounsbery Foundation to A.V.S and E. Gazel. This is CRPG contribution no. 2702.
Nature thanks Claude Herzberg and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
The authors declare no competing interests.
Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Extended data figures and tables
Olivine-hosted melt inclusions (sample 1521) from the 3.3-Gyr-old Weltevreden komatiites. a, Untreated, partially crystallized melt inclusion containing clinopyroxene, glass and the shrinkage bubble. b–f, Glassy melt inclusions after the quenching experiments (see Methods): b, c and d contain shrinkage bubble; d contains fine olivine spinifex textures due to very high MgO contents of the melt (>26 wt% MgO); e and f, homogeneous melt inclusions; f, the 1521-9h melt inclusion reported in this study.
a, H2O calibration line obtained for the series of reference glass standards (see Methods and Supplementary Information Table 4a, b) used to calculate H2O contents of the samples. b, c, The correlation lines between IMF and Al2O3 and H2O contents, respectively. A multivariant correlation between IMF and both H2O and Al2O3 contents (p < 0.002, R2 ≈ 0.75; equation (2), Methods) was used to correct the matrix effect of the analysed materials. R, correlation coefficient.
a, Deviation of IMF of reference glasses from calculated values over the time of the analytical session. Lines with numbers represent linear correlations with correlation coefficient (R2). b, Deviation of IMF of reference glasses from calculated values over the analytical session time corrected for the apparent drift seen in a. c, Deviation of δD of reference glasses from accepted values over the analytical session time, corrected for apparent drift. Panels b and c are very similar but not the same. Grey shaded region, two standard errors; green shaded region, time interval of inclusion analyses. Error bars, two standard errors.
Extended Data Fig. 5 Significant correlation between isotope composition of H and size of melt inclusion in olivine of Weltevreden komatiites.
Inclusion 1521-ol12 (not shown) was excluded as a size outlier (120 μm).
a, Al2O3 versus MgO of melt inclusions in olivine of Weltevreden komatiites. Alumina contents apparently follow olivine crystallization path of the initial komatiite melt calculated for 1521 sample32. b, Uncorrected (open circles) and corrected (filled circles) initial H2O contents of melt inclusions. Corrected initial H2O contents of melt inclusions follow olivine crystallization trajectory within each sample. All error bars reported as 1σ. ol, olivine.
Extended Data Fig. 7 The correlation of calculated initial δD of melt inclusions and measured Fo of host olivines for Weltevreden komatiites.
Obtained using the Isoplot 3.75 software44. The dashed line corresponds to best fit, and red lines delineate the error envelope of regression line at 95% confidence level. R, Pearson correlation coefficient. Error bars correspond to 2σ. a, Correlation for all 23 inclusions. Symbols marked red indicate inclusions that did not suffer significant H loss by diffusion. These inclusions fit the correlation within the error. Inclusions marked blue are apparent outliers and do not fit the correlation. b, Correlation for 20 inclusions, with three outliers (marked blue in a) filtered out. Large circle with dashed outline indicates estimate of initial δD of melt associated with the most Mg-rich olivine (Fo 96 mol%) of Weltevreden komatiites. The other large circles correspond to average initial δD for melt inclusions and host olivines of sample 1523 (orange outline), sample 1522 (green outline) and melt inclusions in measured most Mg-rich olivines (Fo > 95 ± 0.1, mol%) (red outline).
This file contains Supplementary Tables 1-4.