New paper unravels the composition of Earth’s continental crust 3.5 billion years back in time
The earliest crust on Earth was made of uniformly dark, magnesium and iron-rich mafic minerals. Present-day crust looks very different between land and ocean: continental crust on land is now lighter-coloured (felsic), rich in silicon and aluminium. The moment these two diverged is important, since the composition of minerals in the crust affects the flow of nutrients available to organisms. It also influences the stability of the continents and bears information about the prevailing tectonic regime on Earth. During his postdoctoral research at the University of Chicago, Dr. Nicolas Greber (now at University of Geneva) studied titanium isotopic composition in shales (fine-grained detrital sedimentary rocks). The element titanium does not dissolve in water nor is it absorbed by plants, so its concentration and isotope composition remains unaffected.
In the study of Greber et al., samples of shales of different ages from around the world were analysed for their titanium contents and isotope compositions. The titanium isotope composition in shales should shift as the rocks on the continents change from mafic to felsic. Instead, the scientist saw little change from the present day to 3.5 billion years ago, suggesting that the transition from dark, magnesium and iron rich continents to felsic, silica rich continents must have occurred at or before that moment. As plate tectonics with subduction of oceanic crust is seemingly necessary to produce large volumes of felsic rocks, the scientists concluded that Earth’s tectonic regime was already in operation by 3.5 billion years ago – and possibly even earlier!
“Titanium isotopic evidence for felsic crust and plate tectonics 3.5 billion years ago,” Greber et. al. Science, Sept. 21, 2017. doi: 10.1126/science.aan8086
September 25, 2017