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

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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.