Scientists had long debated when plate tectonics and subduction began, with estimates ranging from 0.85 to 4.2 billion years ago, more than two-thirds of the planet’s history. Finding out when plate subduction began means pinpointing when the Earth went from a planet dominated by transient land masses spanning the surface of the oceans to one made up of long-lived continents where long-term biogeochemical cycles are controlled by volcanic outgassing and recycling inside the Earth.
A finding that was intended to shed light on a hotly contested debate in Earth science: when plate subduction began, suggests that the process could have started 3.75 billion years ago. The findings also indicate that this process has reshaped the Earth’s surface and set the stage for a planet welcoming to life.
The study was conducted by scientists from the Scripps Institution of Oceanography at UC San Diego and the University of Chicago.
Clues to Earth’s earliest habitability are found in the elements that ancient rocks are made of, explicitly titanium. Scientists examined samples of the oldest known rocks on Earth from the Acasta Gneiss Complex in the Canadian tundra.
These rocks date back to the Hadean eon, which began towards the beginning of Earth’s formation and was characterized by hellish conditions on a planet that would seem strange to our modern eyes.
The scientists focused mainly on isotopes, which are variations of the same element based on its neutrons. Taking Acasta Gneiss samples provided by Jesse Reimink, an assistant professor at Penn State University, they crushed chunks of rock into a powder that was then heated to form a glass bead. This process allows the dissolution of the titanium you were looking to analyze.
Once cooled, the bead dissolved in acid and the titanium was chemically separated from other elements. The scientists were then able to determine the variations of the titanium isotopes present in the sample using a mass spectrometer.
They then compared the samples with new and modern rocks formed in subduction zones. In samples that are 3.75 billion years old, they noted similarities in structure and composition to modern ones, suggesting that plate subduction began around that time.
Lead study author Sarah Aarons said: “A lot of previous work has been done on these rocks to carefully date them and provide the geochemical and petrological context. We were fortunate to have the opportunity to measure the compositions of titanium isotopes, an expanding system of isotopes, in these samples. “
In four-billion-year-old rock samples, Aarons saw similarities to modern rocks that form in columnar settings, such as Hawaii and Iceland, where a land mass is moving over a hot spot. However, in rocks with an age of 3.75 billion years, he noticed a change in the tendency for rocks to form in modern subduction zones, suggesting that around that time in Earth’s history these areas began to form.
Aarons said: “While the trend in the titanium isotope data does not provide evidence that plate tectonics was happening globally, it does indicate the presence of wet magmatism, which supports subduction at this time.”
Magazine reference:
- Sarah M. Aarons et al., Titanium isotopes constrain a magmatic transition at the Hadean-Archean boundary at the Acasta Gneiss Complex, Science Advances Dec 09 2020: Vol. 6, no. 50, eabc9959. DOI: 10.1126 / sciadv.abc9959