Hi Yong-Fei
Not sure I understand the last part of your statement: “water is released from these crustal rocks through the breakdown of hydrous minerals and the exsolution of structural hydroxyl and molecular water from nominally anhydrous minerals. While this leads to dehydration melting of the crustal rocks on the one hand, it also results in hydration of adjacent rocks for partial melting at temperatures on and above the wet solidus of these rocks on the other hand.”
It sounds as though you are suggesting that a melt produced by deydration melting releases water as it crystallises even while at a temperature above the wet solidus? If everything is happening at constant pressure and close to equilibrium, surely the water from the melt is just going to be incorporated into the hydrous minerals as they regrow on cooling? And if pressure has changed, for example so that a melt generated by muscovite breakdown now crystallises sillimanite + K-feldspar, the water still remains dissolved in the melt until it has crystallised to the point of water-saturation, i.e. the wet solidus. (In an experiment you can have excess water, melt the rock fully and still keep heating the resulting water-saturated melt above the wet solidus, but the real world does not work like that). If water released from anatectic melt as it crystallises on the wet solidus is going to flux water saturated melting of the adjacent rocks you have to have the special case of country rocks with a lower melting point, otherwise you contravene the second law of thermodynamics!
I think it is more likely in the muscovite melting example that the water released when the anatectic melt solidifies (assuming it hasn’t moved far) will be used up driving sub-solidus alteration of sillimanite + K-felspar to coarse, high-T muscovite (rather than typical low-T retrograde shimmer aggregate).
Bruce
Bruce Yardley
Emeritus Professor
School of Earth & Environment
University of Leeds
Leeds LS2 9JT
UK
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Dear John,
The physicochemical mechanisms for partial melting of crustal rocks include heating, decompression and adding water (hydration). There is no free water in crustal rocks when they were buried to middle to lower crust depths. Water in the crustal rocks of crystalline basement is present in the forms of structural hydroxyl and molecular water in both hydrous and nominally anhydrous minerals. By either heating or decompression, water is released from these crustal rocks through the breakdown of hydrous minerals and the exsolution of structural hydroxyl and molecular water from nominally anhydrous minerals. While this leads to dehydration melting of the crustal rocks on the one hand, it also results in hydration of adjacent rocks for partial melting at temperatures on and above the wet solidus of these rocks on the other hand.
In experimental petrology, the dehydration melting was designed to occur in the absence of aqueous solutions (water-absent melting), whereas the hydration melting was designed to proceed in the presence of aqueous solutions (water-present melting). In either case, water is preferentially partitioned into melts as soon as crustal anataxis takes place, leaving relatively anhydrous residues as granulites. As a consequence, granites crystallized from felsic melts are much more enriched in water than granulites. While granitic melts can be produced by either dehydration melting or hydration melting, the removal of water from granulites always proceeds through the dehydration melting.
Migmatites can be produced through either dehydration melting or hydration melting. The key is almost no physical separation of partial melts from parental rocks. Partial melting for migmatitization may occur in three phases: (1) decompressional exhumation of the deeply subducted crustal rocks from subarc to forearc depths in the late stage of collisional orogeny; (2) thinning of the collision-thickened crust during foundering of orogenic roots; (3) heating of the thinned crust in the postcollisional stage. In either case, leucosome is enriched in water than mesosome unless hydrous minerals are concentrated in the mesosome.
Best,
Yong-Fei
Granites tend to be fluid-absent melting phenomena.
In wet melting, as the sniff of H2O runs out, melting grinds to a halt, unless T keeps going up and you then reach the high-T mica and amphibole melting reactions.
University of Stellenbosch
I have ‘occasionally’ found both large-scale leucogranites and abundant migmatites along the entire arc of the collisional Himalayan orogeny, which would seem to suggest considerable crustal anatexis. Other orogens show similar features. There seems to be plenty of heat there…
What I find more interesting is the question of why the partial melt (sorry, magma) migrates out of some rocks but becomes ‘frozen’ in place in others. Extension may help but surely melt fraction has a role to play? Little bits of melt may get stuck; more melt will get connected and find a way to escape?
So maybe orogenic migmatites reflect generally smaller melt volumes and hence less chance of escape (and more chance of preservation!)? We see more of the process and less of the product…
Dear Martin,
Compressional settings for crustal thickening are characterized by low geothermal gradients and thus by Alpine-type HP to UHP metamorphism at blueschist to eclogite facies, so that no crustal anatexis can take place in the stage of crustal thickening during collisional orogeny. In contrast, extensional settings are produced by lithospheric thinning, resulting in shallowing of the lithosphere/asthenosphere boundary and thus upwelling of the asthenospheric mantle. This is able to transfer the high heat flow from the underlying asthenospheric mantle into the overlying thinned lithospheric mantle, making Buchan-type HT to UHT metamorphism at amphibolite to granulite facies. Both metamorphic dehydration and partial melting of crustal rocks are caused by asthenospheric heating of the thinned lithosphere, resulting in crustal anatexis in such extensional settings.As a consequence, the crustal anatexis is prominent subsequent to the lithospheric thinning and thus responsible for reworking of preexising collisional orogens in backarc and continental rifts (rifting orogeny).
Best,
Yong-Fei
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发件人:"Martin Hand" <[log in to unmask]>
发送时间:2018-06-22 16:32:12 (星期五)
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主题: Re: Crustal anatexis
Agreed, heat and/or volatiles. But why extensional settings would facilitate magma transfer more than other settings seems more paradigm, than actualistic.
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All you need is heat, if you want granulite-facies partial melting.
The extensional settings may help with getting mantle heat into the crust and with getting partial melt (or, more correctly, magma) out of the anatectic zone.
University of Stellenbosch
Does crustal anatexis mainly occur in extensional settings? In the current Earth, large-scale anatexis appears to be occurring in the Tibetan crust, and also at the base of magmatic arcs.
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At supersolidus temperatures, crustal rocks undergo partial melting to produce a sizable volume of anatectic melts. This process is common during high-grade metamorphism at amphibolite- to granulite-facies conditions. Migmatites are its typical products in which anatectic melts were not escaped from the anatectic systems.
Such a kind of crustal anatexis mainly occurs in extensional settings such as backarc and continental rifts, where the lithosphere is significantly thinned to cause upwelling of the asthenospheric mantle, transferring high heat flow into the lower crust for metamorphic dehydration and partial melting at crustal depths. While dehydrated and melt-extracted residues show HT to UHT granulitization, releassed fluids (aqueous solutions) result in amphibolitization of the overlying crustal rocks and escaped melts (hydrous, felsic) give rise to granitic intrusives in different sizes.
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发件人:"Mallickarjun Joshi" <[log in to unmask]>
发送时间:2018-06-21 22:06:08 (星期四)
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I wonder under what conditions a sizable volume of the anatectic melt formed as a consequence of high grade metamorphism in pelites would not be able to escape the system?
Admittedly, metamorphism outlasting the deformation should be a likely precondition.
Moreover, should it also shed some light on the tectonic set up other than that it is likely to have formed during the penultimate stages of the orogeny.
I would much appreciate your opinion and any suggestions for references to understand the problem.
Centre of Advanced Study in Geology
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Dr. Yong-Fei Zheng
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