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Role of water in melt generation
The main sources of water for magma generation are the dehydration of hydrous silicates within the crust and volatiles transported into the crust from subducted oceanic crust and upper mantle in the form of hydrous basalts and andesites. Dehydration reactions of muscovite, biotite, and horn-blende are of particular significance. Anatectic granites may be partially classified in terms of the probable dehydration reaction responsible for their generation
The origin of granite: The role and source of water in the evolution of granitic magmas
JAMES A. WHITNEY
Basalts have low water contents, higher temperatures of solidification, generally low viscosity, and generally silicate poor chemistry. Granites have relatively high water contents, viscous magmas, presence of volatiles and large ion lithophile elements, high silicate contents, and generally lower temperatures of solidification.
The presence of water affects the temperature of melt generation and the characteristics of said the melts formed. In this essay I will firstly address the generation of melts at subduction zones and mid-ocean ridges, the effect of water in metamorphic meltduring anatexiss and finally the characteristic mineralogies mineral assemblages associated with the presence of water.
Ocean ridge magmatism is the consequence ofThe melting of dry peridotite at ~20kbar represents ocean ridge magmatism, and createswhich almost exclusively creates almost exclusively anhydrousnearly dry and basaltic liquid (containing ~2 wt.% H2O)melts. The composition of mantle melts can be described using the phase diagram below left. Melting of a mantle composition (marked M on the diagram) begins at the univariant point involving Fo + Di + En (marked X on the diagram). This composition is essentially an olivine tholeiite, and a thermal divide exists between olivine tholeiites and quartz tholeiites under these conditions. Fractionation of this liquid can never result in silica-enrichment. The addition of water changes the topology of the phase diagram to that shown on the right.: this The second phase diagram of the melting of wet peridotite is appropriate for melting at a subduction zone. In dry conditions the melt M will crystallise to the eutectic point on the left side of the thermal divide. This will create a silica-poor composition. UnderIn wet conditions the first melts to form from a peridotite (shown as M) will have the composition of the crystallise to the peritectic point at the centre of the phase diagram – this is considerably more silica-rich than the corresponding liquid formed by melting dry peridotite and lies in the quartz tholeiite field. Crystallisation of this liquid will drive the composition of the remaining liquid towards ever more silica-rich compositions. Assuming there is melt left it will continue on to the eutectic at the quartz boundary, creating a silica-rich composition. The presence of water during melting will thereforetend to lead to quartz normative melts that can then evolve to highly silica-rich compositions during differentiation.
Synthetic basalt system: Fo-Di-An-SiO2. Dry melts can’t cross thermal divide?
Granitic melts are generated from the melting of continental crust in subduction zones.Melting of the mantle can be achieved without a change in temperature or pressure by the addition of water to the mantle wedge overlying subducting oceanic crust. The circulation of water in the fractured hot rocks at mid-ocean ridges results in hydrothermal circulation systems penetrating to depths of several km. Hydration reactions form minerals such as amphibole, sheet silicates and epidote. Suduction increases temperature and pressure and these hydration reactions reverse, releasing water into the mantle above the slab.. This has the effect of lowering the solidus of the mantle, creating basaltic melt of the composition shown in the phase diagram above right. A further source of water is the relatively minor volumes of As the denser oceanic slab subducts under the less dense continental plate, oceanic sediments that are scraped off the slab and onto the continental wedge. However some sediments do remain with the oceanic plate and are subducted. Upon subduction the hydrous minerals (chlorite, amphibole etc) in the sediments and crust experience increased pressure and temperature. These minerals break down and release water into the overlying mantle wedge. The geotherm intersects the solidus, creating peridotite melt that rises through the crust.
It is important to note that it is not the subducting oceanic slab that is melting, but the continental wedge above it.
The melting of the continental wedge allows for a quartz-tholeiite melt to be formed. Subduction zone magmas tend to span a wide range of compositions, and may be rich in silica and contain water.
Mentioned earlier were the hydrous minerals found in the oceanic crust. They formed at mid-ocean ridges. As the plates pull apart, sea water penetrates the hot crust to depths of several km. Hydration reactions occur and form hydrous minerals, such as amphibole. It is the reversal of this process (the expulsion of water) that generates melt in the continental crust at subduction zones.
Melting of continental crust can occur by water-absent reactions (quartz+ feldspar = melt), water-present reactions (biotite = opx + melt, or muscovite + quartz = Kspar + melt) or water-fluxed reactions (quartz + feldspar + HO = melt). The temperatures at which these reactions happen are extremely high for the water-absent reactions, and lowest for the water-fluxed reactions. Since there is very little free water in the continental crust, due to its high mobility, and due to the extremely high temperatures required for water-absent melting, most melting occurs via the water-present reactions, essentially by breakdown of hydrous minerals such as the micas and amphiboles. However, it is possible to generate very large quantities of melt if water can be introduced to the continental crust This is believed to have happened at Trois Seigneurs, where a complete traverse from low-grade metamorphic rocks, through high-grade rocks and migmatites and into a major granite body is preserved. The source of the water is enigmatic but it is likely to have been sea-water brought to great depths in the crust along fractures generated during rifting.
Granulites are formed from the heating of country rock/metamorphosed rocks. The water in the hydrous minerals allows the rock to melt. However, the melting removes water from the hydrous minerals within the rock, altering the amphibolites to granulite. Even at high temperatures granulites won’t melt due to their very low water content.
The phase equilibria in the granite system are strongly influenced by the extent of solid solution between Na-rich and K-rich feldspars. This in turernm is dependent on pressure and water -content of the system. At low pressure and in dry systems there is complete solid solution between albite and orthoclase below the solidus. This will create a single, homogenous feldspar which may. U, if cooling is sufficiently slow, undergo exsolution. pon further cooling the feldspar might separate back out. However, in wet systems the solidus temperature falls and may sufficiently intersect the solvus, creating separate Na-rich and K-rich feldspars.
Presence of water in the melt will not only alter the melting temperature, it will also affect the types of minerals present if the melt crystallises. Hydrous minerals will form, such as include biotite, muscovite and other sheet silicates, and amphiboles.
The addition of water can not only create hydrous minerals, but alter the original mineralogy. Hydrous alteration is visible in thin section. Olivine alters to serpentine (brown in-filling of curving cracks) and biotite alters to chlorine (chlorine is green in plain polars).
Water can also change affect the viscosity of the melt. Water-bearing melts have a lower viscosity, so exsolved gases can escape easily. This leads to fewer vesicles. However, the solubility of volatiles such as water decreases as the melt rises, causing the gas to come out of solution in the first place. It seems that when the lava has been erupted the presence of water allows gases to escape, meaning fewer vesicles. If the melt has solidified sufficiently before eruption the vesicles will be trapped, leaving an increase in vesicles.
This essay lacks a conclusion. This need only be a few sentences but it needs to summarise the main points made in the body of the essay and bring them together. This is what I wrote for Jo:
In summary, water has a fundamental effect on the thermodynamics of melting, expanding some primary phase fields and shrinking others and hence changing the composition of liquids formed by low-degree melting of the mantle. It also affects the solidus, permitting melting at lower temperatures than would otherwise be possible. Water can change the rheology of melts by changing the atomic structure of the liquid. The presence of water in a melt affects the minerals which grow during solidification, promoting growth of hydrous phases such as amphiboles and sheet silicates, suppressing plagioclase, and causing alteration of early-formed non-hydrous phases. It is an essential player in the generation of melt in the Earth.