title: Komatiites reveal a hydrous Archaean deep-mantle reservoir.
abstract: 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 source. 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 plumes while another argues that these magmas contained several per cent water, which drastically reduced their eruption temperature and links them to subduction processes. 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 zone. 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.
authors.affiliation: Université Grenoble Alpes, Institute Science de la Terre (ISTerre), CNRS, F-38041 Grenoble, France.|Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, 19 ul. Kosygina, Moscow 119991, Russia.
authors.forename: Alexander V
authors.initials: AV
authors.lastname: Sobolev
authors.affiliation: Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, 19 ul. Kosygina, Moscow 119991, Russia.
authors.forename: Evgeny V
authors.initials: EV
authors.lastname: Asafov
authors.affiliation: Centre de Recherches Pétrographiques et Géochimiques (CRPG), UMR 7358, Université de Lorraine, 54501 Vandoeuvre-lès-Nancy, France.
authors.forename: Andrey A
authors.initials: AA
authors.lastname: Gurenko
authors.affiliation: Université Grenoble Alpes, Institute Science de la Terre (ISTerre), CNRS, F-38041 Grenoble, France.
authors.forename: Nicholas T
authors.initials: NT
authors.lastname: Arndt
authors.affiliation: Université Grenoble Alpes, Institute Science de la Terre (ISTerre), CNRS, F-38041 Grenoble, France.|Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, 19 ul. Kosygina, Moscow 119991, Russia.
authors.forename: Valentina G
authors.initials: VG
authors.lastname: Batanova
authors.affiliation: Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, 19 ul. Kosygina, Moscow 119991, Russia.|GEOMAR Helmholtz Centre for Ocean Research Kiel, Wischhofstrasse 1-3, 24148 Kiel, Germany.
authors.forename: Maxim V
authors.initials: MV
authors.lastname: Portnyagin
authors.affiliation: CAU Kiel University, Institute of Geosciences, Ludewig-Meyn-Strasse 10, 24118 Kiel, Germany.
authors.forename: Dieter
authors.initials: D
authors.lastname: Garbe-Schönberg
authors.affiliation: Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, 19 ul. Kosygina, Moscow 119991, Russia.
authors.forename: Stepan P
authors.initials: SP
authors.lastname: Krasheninnikov