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

困在大陆地壳深处的熔体——对地壳分异的影响

• 批准号: RGPIN-2015-03694
• 负责人: Sawyer, Edward
• 金额: $ 1.6万
• 依托单位: Université du Québec à Chicoutimi
• 依托单位国家: 加拿大
• 项目类别: Discovery Grants Program - Individual
• 财政年份: 2019
• 资助国家: 加拿大
• 起止时间: 2019-01-01 至 2020-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=678778
• 关键词: Melt; trapped; deep; continental; crust

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.

实验表明，当含H2 O的矿物分解时，典型的地壳岩石会熔化; SiO2、K2 O和H2 O聚集在熔体中，Al 2 O3、FeO和MgO富集在固体中。陆壳上部富SiO_2、K_2O和H_2O，下部富SiO_2、K_2O和H_2O。一般认为，下地壳部分熔融，熔融物上升到上地壳，这一过程称为壳内分异，或简称为地壳分异。由于没有地壳分异的详细资料，因此不清楚熔体和关键成分K2 O和H2O是如何通过这一过程重新分配的。可以利用明显的花岗岩脉和露头中明显的较大岩体来编制熔体清单，但这将排除留在地壳深处仍然分布在那里的矿物之间的熔体。这种未分离的熔体会与周围的矿物（如斜方辉石和石榴石）发生反应，并在冷却过程中被反应消耗，实际上，这种反应逆转了熔融反应。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，这些矿物通常被明显的矿物黑云母所取代。库存必须包括另外两个项目。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，这也必须考虑在内。最后，它必须包括通过熔化以外的其他过程，如形成边缘，保留在地壳深部的K2 O和H2O。      这一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，前熔体和关键元素K2 O和H2O在中地壳和下地壳中的丰度和分布情况，对整个地壳分异过程进行盘点（也称为质量平衡）。建议开展三个项目：1）通过研究熔融物在冷却过程中与邻近的矿物发生反应并被其消耗时发生的矿物学和结构变化，确定有多少熔融物留在地壳深部。2)研究中地壳中的岩石由于注入其中的花岗岩脉结晶时释放出的H2O和其他元素而被改变的程度，即中地壳中有多少H2O。3)边缘是不同组成岩石类型之间的反应区，研究边缘在地壳深部保留K2 O和H2O方面的作用。      我们生活在大陆地壳上，这是地球上最古老和最复杂的部分。研究结果将有助于更深入地了解地壳分异过程中熔体和各种元素在陆壳中的重新分布。这些信息将用于测试现有的模型，并开发关于大陆地壳如何演变的新模型。