Barbey, 2007 Biotite breakdown

Barbey, 2007 Biotite breakdown - Contrib Mineral Petrol...

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ORIGINAL PAPER Diffusion-controlled biotite breakdown reaction textures at the solid/liquid transition in the continental crust Pierre Barbey Received: 2 January 2007 / Accepted: 17 May 2007 / Published online: 6 June 2007 ȑ Springer-Verlag 2007 Abstract Two-phase quartz intergrowths with garnet, cordierite and tourmaline occur commonly in prograde high-temperature migmatites, granulites, as well as in the last crystallization stages of biotite granites. Structural, microtextural and mineralogical data show that they result from the breakdown of biotite in the presence of a melt phase associated with incongruent dissolution of feldspars into the melt and silica release (giving quartz in silica saturated rocks). Biotite breakdown and growth of Al-rich ferromagnesian minerals, occurring at the solid–liquid transition in the crust (early melting or Fnal crystalliza- tion), is kinetically controlled by ±e and Mg mass trans- port, the network-forming cations Si and Al being locally compensated for by feldspar dissolution/crystallization. This process leads to signiFcant changes with respect to equilibrium dehydration-melting reactions wherein quartz is a reactant and K-feldspar a reaction product. Therefore, quartz inclusions commonly occurring in garnets from granulite-facies metapelites and metagraywackes are not simply grains passively included during garnet growth. They may also correspond to newly crystallized phases. Resorption of feldspar may lead to more alkaline melt and to crystalline residue richer in Al than expected under equilibrium conditions. Hence, excess alumina in granu- lite-facies rocks is not necessarily related to initial alumina- rich whole-rock compositions (as currently considered), but may be due, at least partly, to kinetics of melting. Keywords Chemical diffusion ± Intergrowths ± Garnet ± Cordierite ± Tourmaline Introduction Biotite, a common mineral of granites, is a fundamental phase in crustal anatexis and differentiation of the conti- nental crust (Brown and ±yfe 1970 ). It has been shown that the amount of water available in the deep continental crust is unlikely to account for the large volumes of granite emplaced in the middle and upper continental crust (e.g. Clemens and Vielzeuf 1987 ), with consequence that much granitic magmas are initially water-undersaturated (Clem- ens 1984 ). Biotite breakdown and melt water-undersatu- ration have been accounted for by the two well-known models of ‘‘carbonic metamorphism’’ and ‘‘dehydration melting’’ (see Vielzeuf and Vidal 1990 ). Carbonic meta- morphism (Newton et al. 1980 ; Touret 1971 , 1989 , 1992 ; Touret and Hartel 1990 ) involves the breakdown of OH bearing phases, notably biotite, in response to water activity change of the ²uid phase. This change results from dilution by a CO 2 -rich ²uid either formed in situ by min- eral reactions or streaming from deeper levels (e.g. ±riends 1981 ; Janardhan et al. 1982 ; Raith et al. 1990 ; Touret and Hansteen 1988 ). Dehydration melting (Burnham 1967 ; Thompson 1982 ; Clemens 1984 ; Grant 1985 ; Vielzeuf and Schmidt 2001 ) involves the breakdown of OH
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Barbey, 2007 Biotite breakdown - Contrib Mineral Petrol...

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