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

摘要

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, and this can be interpreted as being the result of either the progressive accumulation of new crust or the tectonic recycling of old crust. However, there is a disproportionate amount of crust of certain ages, 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, although peaks in continental crust ages could also record preferential preservation. The ~(187)Re-~(187)Os 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.%实践证明，地球大陆壳的形成机制及形成时间(如它是在地球历史的 早期一下子形成的、是逐渐生长的、还是通过几次大幅度的脉冲式生 长过程形成的)难以确定。Pearson等人对大量锇合金颗粒进行了分析， 以期确定熔岩从地幔中流出的年代分布。他们发现，这些年代不是均 匀分布的，而是集中在一些明显的时间段中，这些时间段距今分别约 12亿年、19亿年和27亿年，与大陆壳年龄的峰值相合。这一结果支持 认为大陆生长发生在与大规模地幔熔融事件相联系的脉冲中的理论。