Mantle convection and early crustal evolution
Mantle convection and early crustal evolution
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DOI：
10.1016/0301-9268(84)90016-0
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发表时间：
1984-10
影响因子：
3.8
通讯作者：
I. Campbell;G. Jarvis
中科院分区：
文献类型：
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作者：
I. Campbell;G. Jarvis
太古宙中橄榄岩科马提岩的存在表明太古宙地幔比现代地幔要热得多。这一证据与基于变质岩记录的压力和温度的太古宙大陆地温梯度的估计相矛盾，这表明太古宙和现代大陆地温梯度之间没有显着差异。数值模型表明，地幔温度的微小变化可以对对流产生重要影响。如果上地幔的平均温度升高200℃，地幔内部的对流就会变得混乱，上地幔部分熔融带将环绕地球。在此期间形成的地壳成分为科马提岩，但不稳定，并会通过俯冲混合回地幔中。后来，当地幔温度降至目前水平以上100°C时，上地幔部分熔融带收缩远离俯冲带。这表明第一批原始长英质岩浆是在俯冲带产生的。这些约 3.8 Ga 岩浆的出现使得浮力大陆得以形成，并最终导致地壳增厚。由于这种增厚，由双峰式玄武岩和钠质花岗闪长岩组成的原大陆包含两种类型的潜在能量：（1）以 Th、K 和 U 等元素形式存在的放射性能量；（2）放射性能量。 (2) 大陆海拔高度产生的势能。大陆的势能导致沉积。太古代时期沉积速率的增加是由于地壳浮力的增加造成的。与此同时，地壳深处的放射性元素释放的热量在上地壳的绝缘层下积聚起来。这引起了一次重大的变质、交代和地壳熔融事件，产生了晚太古宙的钾质花岗岩。一旦放射性元素从下地壳中去除，该大陆的该地区的构造就会变得稳定。元古代陆架沉积物沉积在这些稳定克拉通的边缘。太古宙地幔的对流模型显示热底辟从核-地幔界面上方的边界层上升。我们认为这些底辟在约 450 公里的深度开始融化，产生了科马提岩浆。该模型要求太古宙上地幔的平均温度比现代地幔的平均温度高约 100°C。如果太古宙大陆形成于俯冲带上方，则可以解释太古宙和现代大陆地温梯度之间的相似性。将太古宙地幔的温度提高 100°C (1) 使海洋岩石圈的厚度减半，(2) 增加对流单元中点的海洋地温梯度，(3) 使地幔的粘度降低至少一个数量级。这些效应的结合导致太古宙岩石圈和地幔的强度显着下降。因此，可以预见太古代构造的形式与现代构造有很大不同。
The existence of peridotitic komatiites in the Archaean suggests that the Archaean mantle was significantly hotter than the modern mantle. This evidence is contradicted by estimates of Archaean continental geothermal gradients, based on the pressure and temperature recorded in metamorphic rocks, which suggest that there is no marked difference between Archaean and modern continental geothermal gradients.Numerical modelling shows that small changes in the mantle temperature can have an important influence on convection. If the average temperature of the upper mantle is increased by 200°C, convection within the mantle becomes chaotic and an upper mantle partial melt zone encircles the globe. The crust formed during this period will be komatiitic in composition but will be unstable and will be mixed back into the mantle by subduction. Later, when the mantle temperature falls to 100°C above its present level, the upper mantle partial melt zone contracts away from subduction areas.It is suggested that the first primitive felsic magmas were generated at subduction zones. The appearance of these magmas at ∼3.8 Ga permitted the formation of buoyant continents and eventually led to crustal thickening. As a consequence of this thickening the proto-continents, consisting of a bimodal suite of basalts and sodic granodiorites, contained two types of latent energy: (1) radioactive energy held in elements such as Th, K and U; and (2) potential energy resulting from the elevation of the continents above sea level. The potential energy of the continents led to sedimentation. The increase in the rate of sedimentation during the Archaean resulted from increased crustal buoyancy. At the same time heat released by radioactive elements in the deep crust built up under the insulating blanket of the upper crust. This caused a major metamorphic, metasomatic and crustal melting event which produced the potassic granites of the late Archaean. Once the radioactive elements had been removed from the lower crust, that region of the continent become tectonically stable. The Proterozoic shelf sediments were deposited at the margins of these stable cratons.Convection models of the Archaean mantle show hot diapirs rising from the boundary layer above the core—mantle interface. We suggest that these diapirs began to melt at a depth of ∼ 450 km, giving rise to komatiitic magmas. This model requires the average temperature of the Archaean upper mantle to be ∼ 100°C above that of the modern mantle. The similarity between Archaean and modern continental geothermal gradients can be explained if Archaean continents formed above subduction zones.Raising the temperature of the Archaean mantle by 100°C (1) halves the thickness of the oceanic lithosphere, (2) increases the oceanic geothermal gradient at the mid-point of a convection cell, (3) decreases the viscosity of the mantle by at least an order of magnitude. The combination of these effects produces a marked decrease in the strength of the Archaean lithosphere and mantle. Thus the form of Archaean tectonics can be expected to have been very different from modern tectonics.