Over the course of Earth’s history, the mantle has changed, with slow cooling resulting in an evolving mineral composition. Understanding how Earth’s mantle changed following planetary accretion is challenging, and relies on finding ancient samples of erupted lava to study.
Komatiites represent such ancient samples, and scientists have studied specimens from Canada, South Africa, and Australia for decades. Komatiites are ultramafic, and formed from lavas that were almost water-like in consistency. Because of this low viscosity, komatiite lava flows would have left only millimeter-thick deposits. The samples present on Earth today likely represent ancient magma chambers formed in the Archaean over 2.5 billion years ago, and are very rare.
Two hypotheses for komatiites dominate the literature, and are vigorously contested. Based on their chemical composition, komatiite magmas must have formed in the mantle, but at higher temperatures than exist in Earth today. The first hypothesis suggests that lavas depleted in water formed with mantle temperatures more than 500 °C higher than today. On the other hand, if komatiite magmas contained several weight % water, an increase in mantle temperature of only 100 °C would explain their chemical and physical characteristics.
In a new paper published in the journal Nature, a team of scientists including DCO collaborator Alexander Sobolev (Université Grenoble Alpes, France) performed new analyses on Canadian komatiite samples . The data suggest that these komatiites are not enriched in water, containing only 0.6% water by weight.
This relatively low water content has several consequences. Firstly, it points to a plume origin for komatiite melts deep in Earth’s mantle, since higher water contents would link komatiite eruptions to plate tectonics. However, that the melts contain 0.6% water is intriguing because it is 20 times higher than expected from the concentration of similarly incompatible elements.
Sobolev et al. suggest that this water came from the mantle transition zone, between 660-410 km deep, as the partially melted komatiite source plume passed through it. Pearson et al. showed recently that the transition zone contains an ocean of water . The age of the komatiites means the transition zone water has been there for at least 2.7 billion years. Consequently, either subduction started very early in Earth’s history, transporting water from the surface into the transition zone, or water was trapped in the planet’s interior during accretion.