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

实验表明，典型的地壳岩石在含水矿物分解时熔融，熔体中聚集了SiO_2、K_2O和H_2O，Al_2O_3、FeO和MgO在固体中集中。陆壳上部比下部富含SiO_2、K_2O和H_2O。人们认为，下地壳部分熔融，熔体上升到上地壳，这一过程被称为壳内分异，或简单地称为地壳分异。目前尚不清楚熔体和关键成分K2O和H2O是如何通过这一过程重新分配的，因为没有地壳分异的清单。可以使用露头明显的花岗岩脉和较大的天体来清点熔体，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发生反应，如斜方辉石和石榴石，并在冷却过程中被实际上逆转了熔化反应的反应所消耗。然而，这一过程留下了以前熔体存在的结构和矿物学特征。例如，矿物通常被显眼的矿物黑云母所取代。库存必须包括另外两件物品。花岗岩熔体含有H2O，但花岗岩几乎是无水的；这是熔体在凝固时失去H2O的证据。被困在中地壳(18至28公里深)的花岗岩熔体在结晶时释放出H2O，这也必须得到解释。最后，它必须包括深部地壳中通过熔融以外的过程保留的K2O和H2O，例如边缘的形成。 他说，这项建议旨在通过确定在地壳受单一造山熔融事件影响的简单情况下，前熔体的丰度和分布以及中下地壳中的关键元素K2O和H2O，对整个地壳分异过程进行清点(也称为质量平衡)。提出了三个项目：1)一项研究，通过研究熔体在冷却过程中与其邻近的矿物反应并被其消耗时发生的矿物学和质地变化，确定深部地壳中仍有多少熔体。2)研究中地壳岩石被注入的花岗岩墙结晶时释放的水和其他元素的加入对岩石的变质程度，即中地壳持有多少水。3)边缘是不同岩石类型之间的反应带，对地壳深部保留K2O和H2O的作用进行研究。 科学家说，我们生活在陆壳上，这是地球上最古老、最复杂的部分。这项研究的结果将更详细地了解熔体和各种元素在地壳分异过程中如何在陆壳中重新分布。这些信息将被用来测试现有的模型，并开发关于大陆地壳如何演化的新模型。

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.