Geochemical Insight Into The Origin Of Plate Tectonics

Plate tectonics is responsible for the largest scale features of the planet

Plate tectonics defines our planet. Extension creates oceans and basins while collision creates mountains. Glaciers and rivers are certainly potent in their effects, but ultimately the slow dance of the continents is what renews our planet's surface.

Fossils, sedimentary markers and palaeomagnetics allow us to reconstruct past configurations of the Earth's landmasses. The oldest reconstructions date billions of years, showing that plate tectonics is an ancient process. Plate tectonics required the differentiation of an initially homogeneous Earth into a crust. mantle and core. Yet when the plates themselves began to move is less clear. A recent geochemical study, however, may shed some light on the matter.

The movements of the plates brings surface materials to great depth and mantle material to the surface, resulting in specific isotopic compositions of different layers of the Earth and making mixing key to the crust and mantle's geochemistry. 'You can't have continents without granite, and you can't have granite without taking water deep into the Earth," said Roberta Rudnick from University of California, Santa Barbara. 'At some point plate tectonics began and started bringing lots of water down into the mantle. The big question is when?'

The active plate tectonics on Earth give it a unique chemical signature compared to other planets in the solar system: the continental crust is depleted in magnesium. Early on in its history, however, the magnesium content was higher - closer to that of the other rocky planets. By finding the point in Earth's history when the magnesium content of the crust began to deplete should mark the beginning plate tectonics. The issue is that magnesium is easily weathered and leached from rocks when exposed at the surface, meaning that direct measurement would be inaccurate. Instead the researchers focused on trace elements which were not soluble in water. They found that higher ratios of nickel to cobalt and chromium to zinc both correlate to higher magnesium content in the original rock.

On the left the early Earth with a mafic, magnesium rich continental crust,
on the right the modern day Earth, a product of global plate tectonics

'To our knowledge, we are the first to discover this correlation and use this approach,' said Ming Tang from the University of Maryland. 'Because the ratios of these trace elements correlate to magnesium, they serve as a very reliable fingerprint of past magnesium content.'

By sampling a range of rocks dating from two to four billion years old, the researchers were able to create a computer model of how the magnesium content of the crust had changed over the course of time. At three billion years ago, the magnesium oxide (the oxide form is a suitable proxy for the element itself) content of the crust was 11% by weight, but in half a billion years had dropped to just 4%. Today it is just 2 - 3%. This is demonstrative of large scale tectonic processes at least three billion years ago.

'Because the evolution of continental crust is linked to many major geological processes on Earth, this work may provide a basis for a variety of future studies of Earth history,' Tang said. 'For example, weathering of this magnesium-rich crust may have affected the chemistry of the ancient ocean, where life on Earth evolved. As for the onset of plate tectonics, I don't think this study will close the argument, but it certainly adds a compelling new dimension to the discussion.'

Tectonics is a well understood process, but it is important to consider the wider implications of the process. Everything from the topographic to the subtle geochemical changes it produces have the potential to have a profound impact on the broader evolution of the planet and its biosphere.