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When Continents Formed

Island arc rocks provide a better constraint on when the continental crust was generated. When and how the continental crust was generated remains a fundamental question in Earth sciences. It has been widely believed that the trace element–enriched continental crust and the depleted upper mantle are complementary reservoirs, and that the continental crust has grown from the depleted upper mantle (1, 2). Model ages for neodymium (Nd) and hafnium (Hf) isotopes reflect when new continental crust was generated (2), and traditionally they have been calculated for crust derived from the depleted mantle (see the figure, left panel). The implication is that the isotope composition of the depleted mantle is similar to that of new continental crustal material as it is extracted from the mantle. However, the isotope composition of island arc rocks, and hence of new continental crust, is different from that of the depleted mantle (3, 4). We argue that model ages should be calculated using the composition of new continental crust, which is generally more enriched isotopically than the depleted mantle.