Newswise — Earth stands out as an exceptional celestial body within our Solar System. Not only does it boast expansive water oceans and thriving life, but it also possesses a remarkable feature exclusive to itself—plate tectonics—a phenomenon that has profoundly shaped its geology, climate, and quite possibly played a significant role in the evolution of life.
The concept of plate tectonics entails the intricate dance and interplay of tectonic plates on Earth's surface. This fascinating movement is a consequence of the gradual, creeping motion of Earth's mantle, known as convection, which effectively transports heat from the planet's interior to its surface.
According to researchers, the process of mantle convection, which commenced shortly after Earth's formation around 4.5 billion years ago, has been occurring on a global scale throughout the mantle. Consequently, when tectonic plates collide on Earth's surface, one of them yields and descends into the hot mantle, ultimately finding itself in a kind of "plate graveyard" atop Earth's metallic core.
However, a recent study conducted by the University of Copenhagen and published in the journal Nature puts forth a thought-provoking proposition. This study suggests that the style of plate tectonics as we know it might be a relatively recent addition to Earth's geological history.
Zhengbin Deng, the former assistant professor at the University of Copenhagen and lead author of the study, revealed the team's latest findings. According to their research, throughout the majority of Earth's history, mantle convection displayed a stratified structure consisting of two distinct layers: the upper and lower mantle, which remained largely isolated from each other.
The separation between these two mantle regions occurs approximately 660 km below Earth's surface, where specific minerals undergo a phase transition. Deng and his colleagues propose that this phase transition might be the key factor responsible for maintaining the isolation between the upper and lower mantle regions.
The results of our research suggest that in the past, the recycling and blending of subducted plates into the mantle were mainly limited to the upper mantle, which experienced robust convection. This stands in stark contrast to the current understanding of how plate tectonics functions, where subducting plates descend into the lower mantle. These significant findings come from the study led by Associate Professor Martin Schiller, who played a crucial role in this investigation.
In their quest for understanding, the researchers devised an innovative approach, enabling them to achieve ultra-high precision measurements of the isotopic composition of titanium in a range of rocks. Isotopes, distinct variations of an element with slight differences in mass, come into play. The isotopic composition of titanium undergoes modifications during the formation of Earth's crust. This unique property of titanium isotopes proves invaluable in tracing the recycling of surface materials, such as crust, within Earth's mantle over vast geological periods.
Through the application of this groundbreaking method, the scientists successfully determined the composition of mantle rocks formed as far back as 3.8 billion years ago, spanning all the way to contemporary lavas. Their meticulous efforts shed light on the processes that have shaped Earth's geologic history across the ages.
A primordial soup preserved in the deep Earth?
According to the findings proposed in the new study, if the recycling and blending of tectonic plates were indeed confined to the upper mantle, it raises the possibility that the lower mantle might hold undisturbed primordial material. The notion of a primordial mantle pertains to a reservoir of mantle material that has endured relatively unchanged and preserved since the early phases of Earth's formation, approximately 4.5 billion years ago. In this scenario, the lower mantle could serve as a precious repository of ancient elements, offering valuable insights into the Earth's distant past.
The concept of a primordial reservoir within the depths of the Earth is not novel and has been previously proposed, relying on the isotopic composition of rare gases confined in lavas from present-day deep-seated volcanoes. Nonetheless, interpreting this data has been ambiguous, leading to some conjecture that the isotope signal originates from Earth's core rather than the deep mantle. However, the introduction of titanium as a factor brings a new angle to this enduring debate since titanium is absent in Earth's core. This fresh perspective opens up exciting possibilities for further exploration and understanding of the Earth's hidden geological history.
"Thanks to our latest titanium isotope data, we can now confidently pinpoint which modern deep-seated volcanoes actually tap into Earth's primordial mantle. This revelation is truly thrilling as it offers a precious time-window into the original composition of our planet, potentially granting us insights into the source of Earth's volatiles that played a crucial role in the development of life," concludes Professor Martin Bizzarro, who is also a key figure behind this study. The newfound ability to identify and study Earth's primordial mantle holds immense promise for unraveling the mysteries of our planet's ancient origins and its significance in the emergence of life.
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