New research reimagines Earth’s mantle as a relatively uniform reservoir

Lavas from hot spots, whether they erupt in Hawaii, Samoa or Iceland, likely originate from a uniform global reservoir in the Earth’s mantle, according to an assessment of volcanic hotspots published in Geosciences of nature.

The results indicate that the Earth’s mantle is much more chemically homogeneous than scientists previously thought, and that lavas only acquire their unique chemical “flavors” en route to the surface.

“This discovery literally changes our understanding of hot spot lavas and the Earth’s mantle,” said Dr. Matthijs Smit, associate professor and Canada Research Chair in the Department of Earth, Ocean and Atmospheric Sciences at the University of British Columbia. “In some ways, Earth’s lavas are a lot like humanity itself: a beautifully diverse population with a common ancestor, who developed differently depending on where they went.”

Studying the Earth’s mantle is complicated by the fact that it cannot be sampled directly. So scientists must engage in geoscientific detective work: they study this important part of our planet by analyzing the trace elements and isotopes of lava that originates in the mantle and is emitted by ocean volcanoes around the world.

The large differences in the composition of these lavas, together with the assumption that the isotopic composition of the magma does not change between its source and the surface, have led to the general idea that the mantle contains distinct reservoirs of different ages, located in different regions and formed by different processes.

Observations by Dr Smit and co-author Dr Kooijman from the Department of Geosciences at the Swedish Museum of Natural History indicate that the reality may be quite different.

“By studying a specific set of elements, we were able to discern the chemical effects of various processes that act on the melting of magma on its way to the surface and discover that all the lavas from the hotspots actually share the same starting composition,” Dr. Smit said. “The lavas come out differently only because the magmas interact with different types of rocks as they rise.”

The Earth’s mantle is a bubbling layer of molten and semi-molten material, comprising about 84% of the planet’s volume, that lies between the iron core and the outer crust. When magma from the mantle penetrates the crust and bursts to the surface, it is called lava.

Knowing the composition of the mantle is essential to understanding how our planet formed and how it changed over time. It can also provide clues about why the mantle behaves, how it governs plate tectonics, and its role in the global cycle of elements.

In addition to shedding entirely new light on hot-spot lavas in the world’s oceanic regions, the analysis also revealed an exciting new connection to basaltic lavas on continents. These melts, which contain diamond-bearing kimberlites, are fundamentally different from the magmas found in oceanic hotspots. Yet they appear to have the same magmatic “ancestor.”

“This discovery is a real revolution in models of Earth’s chemical evolution and in the way we study element cycles on a global scale,” said Dr. Smit. “Not only is the mantle much more homogeneous than previously thought, it probably no longer contains ‘primordial reservoirs’, entities that were once needed to explain the data but could never really be reconciled with the concept of mantle convection.”

“This model explains the observations in a simple way and enables a myriad of new hypotheses for global geochemical research in the future,” said Dr Kooijman.