Posted on July 11, 2013
A team of geologists led by the Université libre de Bruxelles (Belgium), and from Macquarie University, University of Houston, University of California in Davis and Lunar and Planetary Institute reveal that the Earth may have been very different in the past compared to what we can see now.
In Earth Sciences, it is common to use natural radioactivity, where a parent element is decays to a daughter element, to date geological events. Parent elements with short half-lives of radioactive decay that were present when the solar system began to condense into planets, can date very ancient events near or at the time when Earth began to form and undergo differentiation from 4.55 to 4.2 billion years ago. After the parent element goes extinct geological materials can retain the fingerprint of the decay of these short lived radioactive elements. Different materials in the Earth´s mantle that have different abundances of the parent element would then have distinct daughter element abundances as well, and should remain that way unless the different materials mix and become homogenized. An efficient way to generate mixing within the Earth is by convection in the terrestrial mantle, with hot material rising and cold material sinking. This process should have been very efficient during the early times of Earth when our planet was hotter, and the mantle was thus convecting faster.
Led by the Université libre de Bruxelles, a team of scientists from ULB, Macquarie University, University of Houston, University of California in Davis and Lunar and Planetary Institute, discovered that a small anomaly in 142Nd, resulting from the decay of 146Sm during the first 350 million years (Myr) of Earth history, was still present in an ancient lava flow of 2.7 billion years (Gyr) located in the Ontario Province, Canada. This is the youngest lava sample presenting this anomaly. “This is paradoxical, says Vinciane Debaille, Université libre de Bruxelles, because the anomaly in 142Nd created by ancient geological process during the first 350 Myr of the Earth, should have been mixed and erased very rapidly by convection, while here, we can still observe this anomaly 1.8 Gyr after the formation of the Earth”.
By using numerical modeling, the international team found out that the mixing process can be slow, even in a highly convective mantle, if tectonic plates are not moving at the surface of the Earth. “This has major implications for our knowledge of ancient Earth, according to Craig O’Neill, Macquarie University, because this means that the Archean Earth (older than 2.5 Gyr) was very different of the present-day situation. It was more like the planet Mars, with no plate moving at the surface. Sometimes, a short disruption of this unique plate may have occurred, but the continuous motion of tectonic plates as we observe now probably started after 2.7 Gyr. A major change in the functioning of the Earth has already been postulated by other studies, but this is the first time that we are able to apprehend the plate tectonic regime of the Archean by using geochemistry. This finding has also implications for other planets such as Mars, for which the mantle is thus not well mixed and still preserves evidences of the geological processes that formed the planet”. “The fundamental change in Earth´s tectonics and its link to how the mantle convected also has ramifications to how the continents grew over time”, comments Alan Brandon of the University of Houston. “There has been debate on whether the early Earth geologic record of continental growth is preserved or if significant portions are missing from erosion. The Nd isotope models in this study indicate that there is no more room to add additional major crustal growth events in the Archean, so we now have a strong constraint on when crustal growth occurred and can now move forward with the bigger picture of Earth´s evolution.”
This research is published in Earth and Planetary Sciences Letters.