Komatiites reveal a deep, hydrous mantle reservoir 2.7 Ga ago

Earth scientists from the Institut des Sciences de la Terre de Grenoble, the Vernadsky Institute (Moscow), Centre de Recherches Pétrographiques et Géochimiques de Nancy (CNRS, Université de Lorraine), in collaboration with German (GEOMAR, Kiel) researchers, have studied komatiites (unusual volcanic rocks) from the 2.7 billion year old Abitibi Belt in Canada (Fig.1). They obtained the first analyses of the concentrations of water and mobile elements (Rb, Ba, Cl, Pb, Sr etc) in small (tens of microns) glass inclusions trapped in exceptionally MgO-rich olivine (Fig. 2). These results indicate the presence of elevated water contents deep in the mantle during the Archean. The results were published in Nature, 31 March, 2016.

Fig. 1 - Photo of komatiites in thin section (in polarised light).
The incomplete (skeletal) crystals are olivine. They lie in a matrix of pyroxene and devitrified glass. ©SOBOLEV et al. 2016
Fig. 2 - Melt inclusion in olivine from komatiite heated to 1500oC and quenched to glass.
This glass represents the composition of 2.7 billion years old komatiite melt and contains a significant amount of H2O from the deep mantle. ©SOBOLEV et al. 2016

For decades, geologists have debated the geodynamic processes that operated in the young Earth. In the Archean, 4 and 2.5 billion years ago, the interior of the planet was much hotter which led to more rapid convection and, according to some authors, to an absence of plate tectonics. Komatiites (Fig. 1) – volcanic rocks with abnormal, olivine-enriched compositions – are thought to result from high-degrees of partial melting of extremely hot parts of the Earth’s mantle. This interpretation is blurred, however, by uncertainty as to the water content of komatiitic magmas.

There are two schools of thought on this question: the first proposes that the magmas were dry (<0.1% water) and very hot (> 1600°C), and were produced in mantle plumes from the base of the mantle; the second suggests that the magmas were hydrated, with lower melting temperatures, and had formed in subduction settings.
The analysed komatiite melt contained 30% magnesium oxide and 0.6% water and began to crystallize at a relatively low temperature of 1530°C. The chemical composition of the magma and low oxygen fugacity are inconsistent with a subduction setting.

Fig. 3 - The mantle plume (orange) traverses the transition zone, which contains excess H2O, F and Cl in ringwoodite and/or wadsleyite (high pressure polymorphs of olivine).
The plume is hot enough to be partially molten near the top of the transition zone (small black dots) and entrains hydrous melt (blue shapes) either from the layer at the upper boundary of the transition zone or from the hot boundary between the plume and the transition zone. Alternatively or additionally, the plume may entrain solid wadsleyite from the transition zone (green shapes). All these hydrous materials introduce H2O and possibly F and Cl into the plume and accelerate its melting (larger black dots). Further ascent of the plume generates more melt during decompression (large black dots), which then separates from the source and ascends to the surface without reaction with peridotite (purple stripes). ©SOBOLEV et al. 2016

Instead, the authors suggest that the magmas were generated in a deep mantle plume and that the water and other volatile components, especially the halogens (F, Cl), were entrained into the komatiitic magma as it passed through the transition zone between the upper and lower mantle, at a depth below 410 km (Fig. 3). This implies the existence of a deep reservoir of water in the mantle: a portion of the mantle containing a few thausends of parts per million of water in high pressure polymorphs of olivine wadsleyite, ringwoodite. This water may have accumulated during the primordial accretion of the Earth or by the unexpectedly early subduction of hydrated slabs that became trapped in the transition zone. Finally, the authors propose that modern mantle plumes do not extract water from the transition zone because they are colder and therefore entirely solid when they crossed the transition zone.

Source :
Komatiites reveal an Archean hydrous deep-mantle reservoir. Sobolev, A.V., Asafov, E.V., Gurenko, A.E., Arndt, N.T., Batanova, V.G., Portnyagin, M.V., Garbe-Schönberg, D., and Krasheninnikov, S.P. - Nature, 31 mars 2016. DOI: 10.1038/nature17152.

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