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

包括板块构造在内的地幔岩浆活动和地幔对流的数值模拟已发展成一系列现实的数值模拟。数值模式显示了地球中超级地幔柱在什么条件下才能发育，以及在地幔演化历史中超级地幔柱的动力学行为是如何变化的。数值模拟结果表明：（1）当放射性元素衰变产生的内部加热足够强时，超级地幔柱是洋脊火山作用的化学分异和板块构造的物质输送共同作用的结果。俯冲洋壳沉积在核幔边界上，形成一个被衰变的放射性元素强烈加热的堆;超级羽流的成分密集，但具有热浮力，总体上是中性浮力的。(2)由于强烈的内部加热，以这种方式形成的超级羽流在太古代非常热。部分超级羽流被分离并上升到表面，成为狭窄的羽流。狭窄的地幔柱在很深的地方引发岩浆，当地幔柱撞击岩石圈时，经常形成新的板块边界。频繁的板块边界形成使板块构造相当稳定。(3)然而，随着地球历史上内部热源的衰减，超级羽流的活动也在衰减。地幔柱活动变得更加间歇性，新的板块边界形成变得罕见。新的板块边界的罕见形成也使板块构造具有间歇性，（2）中深部岩浆的生成与太古代科马提岩的观测一致，间歇性的超地幔柱活动与大陆漂移的威尔逊旋回一致。总体而言，本文所建立的数值模型较好地反映了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.