crust. This can be done through analysis of igneous rocks with the same isotopic composition as initial mantle rock. These igneous rocks are dated and assumed to be direct evidence of new continental crust formation. The resulting ages of isotopically juvenile igneous rocks give distinct peaks, representing an increased proportion of igneous rock and therefore increased crust growth, at 2.7, 1.9 and 1.2 Ga. The validity of these results is questioned as the peaks could represent periods of preservation rather than increased continental crust generation. This is reinforced by the fact that such peaks are not observed in recent geologic time where it is given that magmatism resulting from the plate subduction has strongly contributed to producing new crust. Crustal growth rates from igneous rocks can be compared to the rates generated from radiogenic isotope ratios in sedimentary rocks. Projections of growth rates using these techniques does not produce staggered peaks, instead smooth shallow curves presenting a more constant rate of crustal growth. Although representative of large periods of time, limitations are found where samples do not solely represent magmatic production events. Instead samples include the mixing of sediments which produces a mix of original and altered isotope ratios. Zircon dating Zircon minerals can be both detrital grains from sedimentary rocks and crystals in igneous rocks. Therefore, a combination of zircon forms can provide a more accurate estimate of crustal growth rates. Further to this, zircon minerals can be subject to Hf and O isotope ratio analysis. This is important as Hf isotopes indicate whether a rock originates from the mantle or an existing rock. High δ18O values of zircons represent rock recycled at the Earth's surface and thus potentially producing mixed samples. The outcome of this combined analysis is valid zircons showing periods of increased crustal generation at 1.9 and 3.3 Ga, the latter of which representing the time period following the commencement of global plate tectonics. Modern analog of early crust Iceland Geochemical features of early Archean rocks from the Acasta Gneiss complex have been compared to some modern intermediate-silicic rocks from Iceland and have been found to be very similar. Both have high total FeO, minor or absent Eu anomalies, high Na, and Sr/Y depletions. In addition, whole-rock trace-element data shows that shallow- rather than deep-level processes controlled the magmatic evolution of both the Acasta Gneiss complex, and silicic rocks on Iceland. References Geodynamics Earth Planetary science