Komatiites reveal a hydrous Archaean deep-mantle reservoir


Archaean komatiites (ultramafic lavas) result from melting under extreme conditions of the Earth’s mantle. Their chemical compositions evoke very high eruption temperatures, up to 1,600 degrees Celsius, which suggests even higher temperatures in their mantle source1,2. This message is clouded, however, by uncertainty about the water content in komatiite magmas. One school of thought holds that komatiites were essentially dry and originated in mantle plumes3,4,5,6 while another argues that these magmas contained several per cent water, which drastically reduced their eruption temperature and links them to subduction processes7,8,9. Here we report measurements of the content of water and other volatile components, and of major and trace elements in melt inclusions in exceptionally magnesian olivine (up to 94.5 mole per cent forsterite). This information provides direct estimates of the composition and crystallization temperature of the parental melts of Archaean komatiites. We show that the parental melt for 2.7-billion-year-old komatiites from the Abitibi greenstone belt in Canada contained 30 per cent magnesium oxide and 0.6 per cent water by weight, and was depleted in highly incompatible elements. This melt began to crystallize at around 1,530 degrees Celsius at shallow depth and under reducing conditions, and it evolved via fractional crystallization of olivine, accompanied by minor crustal assimilation. As its major- and trace-element composition and low oxygen fugacities are inconsistent with a subduction setting, we propose that its high H2O/Ce ratio (over 6,000) resulted from entrainment into the komatiite source of hydrous material from the mantle transition zone10. These results confirm a plume origin for komatiites and high Archaean mantle temperatures, and evoke a hydrous reservoir in the deep mantle early in Earth’s history.

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Figure 1: Crystallization temperatures and H2O contents in melt versus olivine composition of Abitibi komatiites.
Figure 2: Compositions and oxygen fugacity of komatiite melt and coexisting olivine.
Figure 3: Incompatible element compositions of primary and evolved melts.
Figure 4: Cartoon illustrating a hot Archaean plume passing through the mantle transition zone at 410–660 km depth.


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We thank A. Kadik, A. Borisov and A. Kargal’tsev for their assistance in high-temperature experiments, V. Magnin for assistance in maintenance of the EPMA laboratory, U. Westernströer for help with laser-ablation ICP-MS measurements, and V. Kamenetsky for providing sample 41F of the Cape Vogel boninites. The paper benefited greatly from the constructive reviews of C. Herzberg and I. Puchtel and the comments of S. Sobolev. This study was funded by the Russian Science Foundation grant number 14-17-00491 (to A.V.S.). The EPMA facility in ISTerre was established and maintained by funds of the Agence Nationale de la Recherche, France, the Chair of Excellence grant ANR-09-CEXC-003-01 and partly by CNRS and Labex OSUG@2020 (Investissements d’avenir—ANR10 LABX56). A.V.S. acknowledges the support of Institut Universitaire de France and the Deep Carbon Observatory. The costs of SIMS analyses were covered by CRPG (A.A.G.’s internal funds). This is CRPG contribution number 2430.

Author information

A.V.S. designed the study, participated in sample collection, data processing and interpretation, and wrote the paper. E.V.A. participated in sample collection, found and prepared melt inclusions in olivines, conducted EPMA analyses and participated in the data processing and interpretation. A.A.G. performed SIMS analyses and participated in data interpretation and writing the paper. N.T.A. led the field work and sample collection, participated in data interpretation and co-authored the paper. V.G.B. managed the EPMA analyses. M.V.P. performed the laser-ablation ICP-MS analyses and participated in data interpretation and writing the paper. D.G.-S. managed the laser-ablation ICP-MS analyses. S.P.K. conducted the heating experiments. All authors discussed the results, problems or methods and participated in preparation of the paper.

Correspondence to Alexander V. Sobolev.

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A copy of the Supplementary Information has been submitted to Researchgate (https://www.researchgate.net/profile/Alexander_Sobolev) and GEOROC (http://georoc.mpch-mainz.gwdg.de/georoc/) databases.

Extended data figures and tables

Extended Data Figure 1 Melt inclusions in olivine from Abitibi belt komatiites.

a, Back-scattered electron image of partly crystallized (unheated) melt inclusion 823-th-ol8 in olivine (ol) of Alexo flow sample M823. The inclusion is composed of glass, quenched clinopyroxene (cpx), spinel (spl) and gas bubble. b, Heated and quenched melt inclusion (810-7-ol1) in olivine from Pyke Hill komatiite sample M810. The inclusion contains glass, gas bubble and spinel. c, Heated and quenched melt inclusion (810-9-ol16) in olivine from Pyke Hill komatiite sample M810. The inclusion contains glass, gas bubble and spinel. d, Back-scattered electron image of the inclusion in c.

Extended Data Table 1 Representative and average compositions of olivine-hosted melt inclusions and primary melt of the Abitibi belt komatiites

Supplementary information

Supplementary Tables

This file contains Supplementary Tables 1–5. This file was replaced on 31 March 2016 to correct a formatting error. Supplementary Table 4 was replaced on 6 June 2016 to correct typos in sample 823. (XLSX 203 kb)

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Sobolev, A., Asafov, E., Gurenko, A. et al. Komatiites reveal a hydrous Archaean deep-mantle reservoir. Nature 531, 628–632 (2016) doi:10.1038/nature17152

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