Earth's plate tectonics underwent a big change lately

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In a surprising revelation, scientists suggest that Earth's plate tectonics might have operated much differently in the distant past, according to a study published in Nature yesterday (27 July).

The new findings challenge the traditional belief that mantle convection started soon after Earth's formation 4.5 billion years ago, occurring across the entirety of the mantle.

Instead, the study argues plate tectonics were confined to the upper mantle in Earth's early history.

What's more, suppose this stratification was to be true, in that case, the lower mantle may harbor undisturbed primordial material, offering insights into Earth's original composition and a potential source of essential volatiles necessary for the development of life.

Plate tectonics is all about the movement and interaction of tectonic plates on Earth's surface. They are the reason behind all earthquakes, volcanoes, and mountain building (to name a few).

These plates are part of the Earth's crust, and their slow motion is driven by the convection currents in the mantle — the layer beneath the crust. As a refresher, here's an image of Earth's internal structure:

Srimadhav/Wikimedia Commons

According to traditional understanding, mantle convection has been ongoing since Earth's formation 4.5 billion years ago and has operated as a single layer.

However, a new study now argues this 'single-layer scenario' is a relatively recent feature of Earth's geological history.

"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," explained lead author Zhengbin Deng from the University of Copenhagen, in a statement.

The authors believe stratification would have occurred at about 660 kilometers (km) beneath Earth's surface, where certain minerals undergo a phase transition.

"Our findings indicate that in the past, recycling and mixing of subducted plates into the mantle was restricted to the upper mantle, where there is strong convection," added co-author associate professor Martin Schiller.

"This is very different from how we think plate tectonics operates today, where subducting plates sink to lower mantle."

The researchers developed a new method to analyze the isotopic composition of titanium in rocks, allowing them to study mantle rocks dating back 3.8 billion years to modern lavas in Australia.

Titanium isotopes are particularly useful because they change when Earth's crust forms. These help scientists trace how surface material gets recycled in the mantle over time.

The study's findings also have intriguing implications for the existence of a "primordial mantle" – a reservoir of mantle material preserved since Earth's early formation.

If the recycling and mixing of tectonic plates were limited to the upper mantle, the lower mantle might contain undisturbed primordial material.

"Our new titanium isotope data allows us to robustly identify which modern deep-seated volcanoes sample Earth's primordial mantle," highlighted co-author professor Martin Bizzarro.

"This is exciting because it provides a time window into our planet's original composition, possibly allowing us to identify the source of Earth's volatiles that were essential for live to develop."

While more research is needed to grasp these discoveries' implications fully, the study represents a significant step forward in understanding Earth's geological past and unique features.

The complete study was published in Nature on July 27 and can be found here.

Study Abstract:

Earth’s mantle has a two-layered structure, with the upper and lower mantle domains separated by a seismic discontinuity at about 660 km (refs. 1,2). The extent of mass transfer between these mantle domains throughout Earth’s history is, however, poorly understood. Continental crust extraction results in Ti-stable isotopic fractionation, producing isotopically light melting residues3,4,5,6,7. Mantle recycling of these components can impart Ti isotope variability that is trackable in deep time. We report ultrahigh-precision 49Ti/47Ti ratios for chondrites, ancient terrestrial mantle-derived lavas ranging from 3.8 to 2.0 billion years ago (Ga) and modern ocean island basalts (OIBs). Our new Ti bulk silicate Earth (BSE) estimate based on chondrites is 0.052 ± 0.006‰ heavier than the modern upper mantle sampled by normal mid-ocean ridge basalts (N-MORBs). The 49Ti/47Ti ratio of Earth’s upper mantle was chondritic before 3.5 Ga and evolved to a N-MORB-like composition between approximately 3.5 and 2.7 Ga, establishing that more continental crust was extracted during this epoch. The +0.052 ± 0.006‰ offset between BSE and N-MORBs requires that <30% of Earth’s mantle equilibrated with recycled crustal material, implying limited mass exchange between the upper and lower mantle and, therefore, preservation of a primordial lower-mantle reservoir for most of Earth’s geologic history. Modern OIBs record variable 49Ti/47Ti ratios ranging from chondritic to N-MORBs compositions, indicating continuing disruption of Earth’s primordial mantle. Thus, modern-style plate tectonics with high mass transfer between the upper and lower mantle only represents a recent feature of Earth’s history.