A Seismic Shift – Earth’s Plate Tectonics Recently Underwent a Fundamental Change

Earth is absolutely unique among the planets of our Solar System. It boasts immense oceans and an abundance of species. However, Earth's uniqueness extends beyond its biodiversity and oceans—it is the only planet in our solar system to undergo plate tectonics, a process essential to the formation of its geological structure, temperature, and, possibly, the evolution of life itself.

The phrase 'plate tectonics' refers to the dynamic movement and complex interplay of tectonic plates throughout the Earth's crust. Convection, the excruciatingly slow but constant movement of Earth's mantle, propels these tectonic plates into motion. This mechanism carries heat from our planet's inner core to its surface.

Researchers think that convection in the mantle, which began shortly after the Earth's creation 4.5 billion years ago, happens on a global scale. When plates collide near the Earth's surface, one loses way and sinks into the heated mantle, eventually ending up in a plate cemetery on top of the metallic core.

However, a new research published in the journal Nature by the University of Copenhagen reveals that this type of plate tectonics may be a more recent part of Earth's geologic past.

"Our new results suggest that for most of Earth's history, convection in the mantle was stratified into two distinct layers, namely upper and lower mantle regions that were isolated from each other," says Zhengbin Deng, former assistant professor at the University of Copenhagen and the study's first author.

The transition between the upper and lower mantles occurs around 660 kilometers below the Earth's surface. Certain minerals go through a phase shift at this level. Deng and colleagues assume that the upper and lower mantle regions remained essentially segregated due to this phase shift.

"Our findings suggest that in the past, recycling and mixing of subducted plates into the mantle was limited to the upper mantle, where convection is strong." "This is very different from how we think plate tectonics works today, when subducting plates sink to the lower mantle," explains associate professor Martin Schiller, who is also involved in the new research.

To obtain these results, the scientists devised a novel method for measuring the isotopic composition of the element titanium in diverse rocks with extreme precision. Isotopes are slightly different mass variations of the same element. When the Earth's crust forms, the isotopic makeup of titanium changes. As a result, titanium isotopes may be used to monitor how surface material, such as the crust, is recycled in the Earth's mantle throughout geologic time. They used this novel approach to identify the composition of mantle rocks from 3.8 billion years ago all the way down to present lavas.

A primeval soup retained deep within the Earth?

If tectonic plate recycling and mixing were limited to the upper mantle, as proposed in the latest study, the lower mantle may hold undisturbed primordial material. A primordial mantle is a pool of mantle material that has remained substantially unmodified and conserved from the early phases of Earth's creation, around 4.5 billion years ago.

The concept of a primordial reservoir in the deep Earth is not novel; it has been proposed based on the isotopic composition of rare gases trapped in lavas from present deep-seated volcanoes. However, the interpretation of these data is controversial, with some suggesting that this isotope signal originates in the Earth's core rather than the deep mantle. Because titanium is not found in the Earth's core, it sheds new light on this long-standing issue.

"We can now confidently identify which modern deep-seated volcanoes sample Earth's primordial mantle thanks to our new titanium isotope data." This is interesting because it gives a temporal window into our planet's primordial makeup, perhaps allowing us to discover the source of Earth's volatiles that were required for life to form," says Professor Martin Bizzarro, who was also involved in the research.