Phase transition in the dynamics of the mantle : application to the Archean-Proterozoic boundary

针对S地幔的岩浆作用和地幔对流，包括板块构造，发展了一系列逼真的数值模式。这些数值模型显示了在地球S地幔的演化史上，什么条件下超强喷流能很好地发育，以及超强喷流的动力学行为是如何变化的。更具体地说，数值模式表明：(1)当放射性元素衰变的内部加热足够强时，超羽是由山脊火山作用的化学分异和板块构造的质量输送自然形成的。俯冲的洋壳在核地幔边界沉积，形成一个被衰变的放射性元素强烈加热的堆积；超羽成分致密，但热浮力较大，总体上是中性浮力。(2)由于强烈的内部加热，以这种方式形成的超羽在太古宙非常热。部分超强羽流脱离，以狭窄的羽流形式上升到地表。狭窄的岩柱在很深的地方诱发岩浆，当岩柱撞击岩石圈时，经常形成新的板块边界。频繁的板块边界形成使板块构造保持了相当的稳定。(3)随着地球历史上内部热源的衰变，超烟羽的活动也随之衰变。烟柱活动变得更加间歇性，新的板块边界的形成变得罕见。新板块边界的罕见形成也使板块构造具有间歇性。(2)深部岩浆的产生与太古代科马提岩的观察一致，间歇性超羽活动与大陆漂移的威尔逊旋回一致。作为一个整体，这里开发的数值模型很好地捕捉到了地球地幔演化的总体特征。

A series of realistic numerical models have been developed for the Earth' s mantle magmatism and mantle convection including plate tectonics. The numerical models show under what condition superplumes well develop in the Earth and how the dynamic behavior of superplumes has changed in the evolutionary history of the Earth' s mantle. More specifically, the numerical models show that:(1) Superplumes naturally develop from the chemical differentiation by ridge volcanism and the mass transport by plate tectonics when the internal heating by the decay of radioactive elements is strong enough. Subducted oceanic crusts sediment on the core mantle boundary to form a pile strongly heated by the decaying radioactive elements ; the superplumes are compositionally dense but thermally buoyant, and are neutrally buoyant as a whole.(2) The superplumes formed in this way were very hot in the Archean owing to the strong internal heating. Parts of the superplumes were detached and rose up to the surface as narrow plumes. The narrow plumes induced magma at great depth and frequently formed new plate boundaries when the plumes hit the lithosphere. The frequent plate boundary formation kept plate tectonics rather steady.(3) As the internal heat source has decayed in the Earth's history, however, the activity of superplumes has also decayed. The plume activity has became more intermittent and new plate boundary formation has became rare. The rare formation of new plate boundary made plate tectonics intermittent, too.The deep magma-generation in (2) is consistent with the observation of komatiiite in the Archean and the intermittent superplume activity is consistent with the Wilson cycle of continental drift. As a whole, the numerial models developed here well capture the overall features of the Earth's mantle evolution.