Melt trapped deep in the continental crust - affect on crustal differentiation

实验表明，典型的地壳岩石在含氢矿物破裂时发生熔融；SiO2、K2O和H2O在熔体中聚集，Al2O3、FeO和MgO在固体中富集。大陆地壳上部SiO2、K2O和H2O含量高于下部。人们认为，下地壳部分熔化，熔融物上升到上地壳，这一过程称为地壳内分异，或简称地壳分异。由于没有地壳分化的资料，因此尚不清楚熔体和关键成分K2O和H2O是如何在这一过程中重新分布的。可以利用露头中明显的花岗质脉和较大的体来对熔体进行盘点，但这将排除留在地壳深处的熔体，这些熔体仍然分布在那里的矿物之间。这种未分离的熔体会与其周围的矿物质发生反应，如正辉石和石榴石，并在冷却过程中通过实际上与熔化反应相反的反应被消耗掉。然而，这一过程留下了熔体以前存在的结构和矿物学特征。例如，这些矿物通常被明显的矿物黑云母所取代。存货清单必须包括另外两个项目。花岗岩熔体含有水，但花岗岩几乎是无水的；这是它们凝固过程中失去的水融化的证据。被困在地壳中部（18至28公里深）的花岗岩熔体在结晶时释放出水，这也必须得到解释。最后，它必须包括K2O和H2O保留在地壳深处的过程，而不是融化，如形成边缘。

Experiments show that typical crustal rocks melt when H2O-bearing minerals breakdown; SiO2, K2O and H2O collect in the melt, Al2O3, FeO and MgO concentrate in the solid. The upper part of the continental crust is richer in SiO2, K2O and H2O than the lower. It is believed that the lower crust partially melted and the melt rose to the upper crust, a process called intracrustal differentiation, or simply crustal differentiation. It is not known in detail how melt and the critical components K2O and H2O were redistributed by this process because there is no inventory for crustal differentiation. An inventory for the melt could be made using the obvious granitic veins and larger bodies evident in outcrop, but this would exclude melt that remained behind in the deep crust still distributed between the minerals there. Such unsegregated melt would react with the minerals around it, such as orthopyroxene and garnet, and be consumed during cooling by reactions that, in effect, reversed the melting reactions. However, this process leaves a textural and mineralogical signature of the former existence of melt. For example, the minerals are commonly replaced by the conspicuous mineral biotite. The inventory must include two other items. Granitic melts contain H2O, but granites are almost anhydrous; this is evidence that melts lost H2O as they solidified. Granitic melt trapped in the middle crust (18 to 28 km deep) releases H2O as it crystallises, this too must be accounted for. Lastly, it must include K2O and H2O retained in the deep crust by processes other than melting, such as the formation of selvedges.       This proposal aims to make an inventory (also called a mass balance) of the overall crustal differentiation process by determining what the abundance and distribution of former melt and the critical elements K2O and H2O are in the middle and lower crust in the simple case of crust affected by a single orogenic melting event. Three projects are proposed: 1) A study to determine to how much melt remains in the deep crust by studying the mineralogical and textural changes that occur as melt reacts with, and is consumed by, the minerals adjacent to it during cooling. 2) A study of the extent to which rocks in the middle crust are modified by the addition of H2O and other elements released as the granite dykes injected into them crystallise, i.e. how much H2O is held by the middle crust. 3) A study of the role of selvedges, which are reaction zones between compositionally different rock types, have in retaining K2O and H2O in the deep crust.       We live on the continental crust which is the oldest and most complicated part of the earth. The results from this research will provide a more detailed understanding of how melt and various elements were redistributed in the continental crust during crustal differentiation. This information will be used to test existing models and develop new ones on how the continental crust evolved.