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.
