The Acasta Gneiss Complex, located at the Slave Craton in the Northwest Territories of Canada, contains 3.9-billion-year-old Archean rocks that, according to Roth et al., can be explained by crust extraction during the first few hundred million years after Earth formed and by relatively slow recycling of Earth’s crust.
The researchers collected multiple samples of two types of Archean rocks: 11 granitoid gneisses and 2 hornblende-plagioclase schists. Using a mass spectrometer at ETH Zurich, they measured the concentrations of the elements samarium and neodymium (Nd) in each rock and also examined Nd isotope ratios.
Their results indicate a deficit of a particular Nd isotope called 142Nd—this is the first time anyone has discovered this type of deficiency in gneiss rock from the Acasta Gneiss Complex. Their discovery suggests that the reservoir from which the samples came formed early during Hadean times, about 4.3 billion years ago.
From this result, the researchers conclude that their study site developed soon after Earth formed through slow recycling of Earth’s young crust into the lower mantle and crust extraction. Their explanation is compatible with the existence of plate tectonics throughout most of Earth’s history. (Geochemistry, Geophysics, Geosystems, doi:10.1002/2014GC005313, 2014)
—Jessica Orwig, Writer
Citation: Orwig, J. (2014), Formation process of Archean rocks in the Acasta Gneiss Complex, Eos, 95, doi:10.1029/2014EO021275. Published on 31 December 2014.