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Conditions for Creation

A deep-sea volcanic eruption at Brimstone Pit, a vent on the side of a large submarine volcano in the Mariana Arc – part of the “Submarine Ring of Fire” that circles the Pacific Ocean basin, where tectonic plates spread or collide. Credit: NOAA Submarine Ring of Fire 2006 (Volcanoes Unit MTMNM)

A deep-sea volcanic eruption at Brimstone Pit, a vent on the side of a large submarine volcano in the Mariana Arc – part of the “Submarine Ring of Fire” that circles the Pacific Ocean basin, where tectonic plates spread or collide. Credit: NOAA Submarine Ring of Fire 2006 (Volcanoes Unit MTMNM)

By Simon Turner, Tracy Rushmer, Mark Reagan & Jean-Francois Moyen

A sequence of the world’s oldest rocks in the depths of the Mariana Trench indicates that both plate tectonics and life may have commenced on Earth 4.4 billion years ago.

Plate tectonics is the process by which large parts of Earth’s outer plates slide past each other. This process of subduction causes the natural disasters we are all familiar with: earthquakes and volcanic eruptions. Subduction also created the continental crust upon which we live and cultivated a life-friendly environment for the Earth.

A key question that has long perplexed earth scientists is just when, during our planet’s 4.56 billion-year history, did this process begin? If the present is the key to the past then it could have been a very long time ago indeed. Unfortunately, rocks sufficiently ancient to test this hypothesis are exceedingly rare.

However, we have identified a sequence of the world’s oldest rocks in Canada that is almost the same as modern rocks formed deep beneath the ocean near the Mariana Trench. The discovery suggests that both plate tectonics, and even life itself, may have commenced 4.4 billion years ago – a billion years earlier than previously thought.

The discovery came after a major conference in San Francisco in December 2012, when I was having a drink with my friend and colleague Mark Reagan. He was describing a volcanic rock sequence between the Mariana islands and the Mariana Trench, and it sounded identical to very ancient volcanic rocks in northern Quebec that Tracy Rushmer had told me about. Tracy was undertaking melting experiments on these rocks to understand how the Earth’s earliest continental crust formed, but the rocks she was melting were basaltic, like the present-day crust that formed when the Pacific plate began subducting beneath the plate that now includes the Izu, Bonin and Mariana islands.

As the conversation evolved we recognised that this might be evidence that modern plate tectonics had begun as far back as 4.4 billion years ago – the age of Quebec’s Nuvvuagittuq Fold Belt, and not long after the Earth formed from the accretion of impacting asteroids.

The importance of plate tectonics cannot be emphasised enough. This process is why Earth has a water cycle and a carbon cycle, and developed an environment suitable for life while Mars and Venus are barren.

For this reason, earth scientists have long argued about when this process started. Current estimates place the beginning of subduction anywhere between about 1–4 billion years ago, and this uncertainly continues to spur much research across the earth sciences.

Mark told me he had been on a number of ocean cruises that had been collecting samples from the sea floor and using the Japanese submersible Shinkai 6500 to explore the rocks at the base of the Izu–Bonin–Mariana arc – a chain of subduction-related volcanoes. What he and his colleagues found was a sequence of volcanic rocks in which the abundances of diagnostic trace elements progressively changed from base to top. These changes record the plate tectonic progression from subduction initiation through to fully-fledged island arc volcanism.

The lowermost rocks are typical of those produced by decompression melting of the Earth’s mantle. On top of these were “boninites”, which form when the residues from the first melting event become re-melted upon the addition of water that bled off the Pacific plate as it began to subduct. Once subduction was fully underway, new mantle was brought into the region, melting in the presence of water to produce typical subduction-related volcanics called calc-alkaline rocks.

At much the same time, a group of Canadian scientists led by Jonathan O’Neil and Don Francis had been working on the Nuvvuagittuq volcanic rocks in northern Quebec. When they joined with a group of American scientists to date the rocks they found the surprisingly old ages of 4.2–4.4 billion years – older than any rocks yet dated on Earth.

While there remains some debate about the exact ages, a key observation that the Canadians made while mapping the region was that the rocks had a simple sequence of three main layers. When I looked at the chemistry they reported for these three successive sequences of volcanic rocks I found that they closely matched the progressive changes in chemistry that Mark had been telling me about from the Izu–Mariana–Bonin arc. It seemed highly unlikely that this could be just coincidence!

At this point Jean-Francois Moyen joined our team as an expert on the Earth’s earliest crust. Jean-Francois had been collecting geochemical data on similar rock types from all over the world. He heard a presentation by Tracy not long after the San Francisco meeting and discussed with her the implications for making the Earth’s first continental crust in a subduction setting. He then contributed his insights from his extensive database.

As we prepared our discovery for publication, Mark recalled a “Darwin Day” talk he had seen given by Steven Benner, a biologist interested in the origins of life. Many biologists now think that RNA may have been the crucial molecule involved in the beginnings of life.

Steven talked about how the element boron stabilises RNA in water and the importance of serpentine as an energy source. Serpentine is abundant in the modern Izu–Bonin–Mariana subduction zone, which hosts vents for boron-rich fluids that teem with life.

Similar conditions in the Earth’s early history could have provided hydrogen for energy and a friendly environment for RNA to form and persist long enough to replicate and catalyse the production of other organic molecules. From this Mark realised that we not only had new evidence for when subduction began on Earth but that this setting would have contained key ingredients necessary for the formation of the first life.

Much work remains to be done to further explore the Quebec rocks, their ages and whether, for example, they exhibit stable isotope signatures that are consistent with the presence of ancient life. Nevertheless, there is renewed hope that we may be closing in on the answer to one of the great questions about the evolution of our planet.

Simon Turner is a Professor in Geochemistry in the Department of Earth and Planetary Sciences at Macquarie University. Tracy Rushmer is an Associate Professor in the Department of Earth and Planetary Sciences at Macquarie University. Mark Reagan is Professor and chair of the University of Iowa’s Department of Earth and Environmental Sciences. Jean-Francois Moyen is a Professor at the University of Saint-Etienne.