Some remarks on the movement of small melt fractions in the mantle
Some remarks on the movement of small melt fractions in the mantle
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DOI：
10.1016/0012-821x(89)90167-2
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发表时间：
1989-10
影响因子：
5.3
通讯作者：
D. McKenzie
中科院分区：
文献类型：
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作者：
D. McKenzie
在岩石圈下面可能存在非常富含钾的小熔体，当熔体的粘度为0.1 Pa s时，当熔体的粘度超过10 - 3%时，它们将从基质中分离出来。这种熔体运动不能传递热量，因此熔体在到达地表之前就会凝固。它的位置取决于岩石圈内的温度，而地幔结核的矿物学表明，如果机械边界层的厚度为165公里，而不是像典型的海洋地区那样为100公里，那么Kaapvaal克拉通下面的热结构可以用板块模型来描述。在温度随时间变化不大的板块中，熔体凝固形成交代层。这一层可能在太古宙岩石圈中发育最好，但也可能以某种形式存在于古老的大洋板块中。由于熔体分数非常小，在它们的源区发生了Rb从Sr和Sm从Nd的强烈分馏。交代层中87rb和147sm的衰变可以产生相对于1ga对流上地幔的较大同位素异常。当它被羽流的热传导加热或被延伸减压时，就会发生重熔。这种熔融对岩石圈温度的扰动比产生干熔融所需的扰动要小，并且可以解释在羽流附近和大陆伸展早期阶段产生的熔体的同位素比率和富k成分。虽然很难探测到，但在海洋下面可能存在一个类似的层，尽管可能不那么丰富。
Small melt fractions very rich in potassium are likely to be present beneath the lithosphere, and will separate from the matrix when there is more than 10−3% melt present if their viscosity is 0.1 Pa s. Such melt movement cannot transport heat, so the melt will solidify before it reaches the surface. Where it does so depends on the temperature within the lithosphere, and the mineralogy of mantle nodules is used to show that the thermal structure beneath the Kaapvaal Craton can be described by a plate model if the thickness of the mechanical boundary layer is 165 km, instead of being 100 km as is typical of oceanic areas. In a plate whose temperature changes little with time melt solidifies to produce a metasomatic layer. This layer is likely to be best developed in Archaean lithosphere, but is probably present in some form even within old oceanic plates. Because the melt fractions are so small, strong fractionation of Rb from Sr, and Sm from Nd, occurs in their source regions. The decay of87Rb and147Sm in the metasomatic layer can produce large isotopic anomalies relative to the convecting upper mantle in 1 Ga. Remelting of this layer occurs when it is heated by heat conduction from a plume, or decompressed by extension. Such melting requires a smaller perturbation to the temperature of the lithosphere than that which is necessary to produce dry melting, and can account for the isotopic ratios and K-rich composition of melts produced near plumes and in the early stages of continental stretching. Though it is more difficult to detect, a similar, though probably less enriched, layer may be present beneath the oceans.