A link between large mantle melting events and continent growth seen in osmium isotopes

Although Earth's continental crust is thought to have been derived from the mantle, the timing and mode of crust formation have proven to be elusive issues. The area of preserved crust diminishes markedly with age(1,2), and this can be interpreted as being the result of either the progressive accumulation of new crust(3) or the tectonic recycling of old crust(4). However, there is a disproportionate amount of crust of certain ages(1,2), with the main peaks being 1.2, 1.9, 2.7 and 3.3 billion years old; this has led to a third model in which the crust has grown through time in pulses(1,2,5-7), although peaks in continental crust ages could also record preferential preservation. The Re-187-Os-187 decay system is unique in its ability to track melt depletion events within the mantle and could therefore potentially link the crust and mantle differentiation records. Here we employ a laser ablation technique to analyse large numbers of osmium alloy grains to quantify the distribution of depletion ages in the Earth's upper mantle. Statistical analysis of these data, combined with other samples of the upper mantle, show that depletion ages are not evenly distributed but cluster in distinct periods, around 1.2, 1.9 and 2.7 billion years. These mantle depletion events coincide with peaks in the generation of continental crust and so provide evidence of coupled, global and pulsed mantle crust differentiation, lending strong support to pulsed models of continental growth by means of large-scale mantle melting events(6).