Crystallization processes in migmatites
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Alfons Berger
und
Gregory Roselle
Abstract
An important prerequisite for interpreting microstructures in plutonic and metamorphic rocks is understanding crystallization processes. This study uses crystal size distribution (CSD) and grain shape parameters to investigate mineral-reaction-controlled crystal growth and melt solidification in migmatites of the Bayerische Wald (Variscan Orogeny, Germany). Partially molten rocks were studied because they allow the simultaneous investigation of crystallization of a melt and the growth of solid products of a metamorphic reaction. CSD and shape factors were obtained for three different minerals: (1) cordierite in a diatexite, where the melt remained in the system; (2) K-feldspar in a segregated K-feldspar + quartz leucosome; and (3) plagioclase from a segregated plagioclase + quartz leucosome. Cordierite crystals exhibit a nearly linear CSD pattern and subeuhedral crystal shapes. K-feldspar grains display an approximately linear CSD except for a marked increase at large crystal size classes. K-feldspar grains are characterized by strongly lobate grain boundaries. The CSD of plagioclase crystals is nearly linear, with a slight decrease at small crystal size classes. Plagioclase shape factors also indicate lobate grain boundaries, however, with lower values than K-feldspar. Cordierite CSD and shape data are consistent with interface-controlled crystallization. The CSD indicates that nucleation and growth is the rate limiting step during the cordierite-producing reaction, in contrast to porphyroblast growth in solid rocks where mass transfer is typically rate limiting. CSD combined with information on the duration of crystallization yields growth rates of 10-17 to 10-18 m/s. The plagioclase data are consistent with magmatic crystallization and are well-described by the communicating neighbor theory. The K-feldspar data also indicate magmatic crystallization. The “overproduction” of large grains of K-feldspar reflects the interplay between nucleation and growth rates at the initial stage of crystallization. Because of different environments between solid and anatectic rocks, the CSD may help to decipher metamorphic from anatectic migmatites.
© 2015 by Walter de Gruyter Berlin/Boston
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- Partitioning of Sr between coexisting minerals of the hollandite- and piemontite-groups in a quartz-rich schist from the Sanbagawa metamorphic belt, Japan
- Crystallization processes in migmatites
- Amblygonite-montebrasite solid solutions as monitors of fluorine in evolved granitic and pegmatitic melts
- Reduction of water loss from gold-palladium capsules during piston-cylinder experiments by use of pyrophyllite powder
- Trace-element partitioning between immiscible lunar melts: An example from naturally occurring lunar melt inclusions
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- Assemblages with titanite (CaTiOSiO4), Ca-Mg-Fe olivine and pyroxenes, Fe-Mg-Ti oxides, and quartz: Part II. Application
- The stability of clinopyroxene in the system CaO-MgO-SiO2-TiO2 (CMST)
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- Al,Si order in the crystal structure of α-eucryptite (LiAlSiO4)
- TEM-EDX study of weathered layers on the surface of volcanic glass, bytownite, and hypersthene in volcanic ash from Sakurajima volcano, Japan
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- Crystal properties and energetics of synthetic kaolinite
- Structures and energies of AlOOH and FeOOH polymorphs from plane wave pseudopotential calculations
- XPS measurement of fivefold and sixfold coordinated sulfur in pyrrhotites and evidence for millerite and pyrrhotite surface species
- Structure of synthetic 6-line ferrihydrite by electron nanodiffraction
- Atomic structures of planar defects in oxybiotite
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