Symplectite formation during decompression induced garnet breakdown in lower crustal mafic granulite xenoliths: mechanisms and rates |
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Authors: | Júlia Dégi Rainer Abart Kálmán Török Enikő Bali Richard Wirth Dieter Rhede |
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Institution: | 1. Metals Research Department, Research Institute of Solid State Physics and Optics of the Hungarian Academy of Sciences, Budapest, Hungary 2. Lithosphere Fluid Research Group, E?tv?s University, Budapest, Hungary 3. Freie Universit?t Berlin, Berlin, Germany 4. E?tv?s Loránd Geophysical Institute of Hungary, Budapest, Hungary 5. Bayerisches Geoinstitut, University of Bayreuth, Bayreuth, Germany 6. GFZ German Research Centre for Geosciences, Helmholtz Centre Potsdam, Potsdam, Germany
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Abstract: | The complex microstructure of kelyphitic rims around garnet in lower crustal garnet granulite xenoliths from the Bakony–Balaton
Highland Volcanic Field, Central Pannonian Basin has been studied in order to identify controls on garnet breakdown. Symplectites
comprised of a vermicular intergrowth of submicron sized anorthite, orthopyroxene and spinel replace garnet at a sharp reaction
front. Based on element distribution maps the transformation of garnet to symplectite is isochemical. Phase diagram calculations
indicate that this reaction was induced by a pressure decrease and/or a temperature increase. In site-specific TEM foils prepared
by focused ion beam technique and oriented parallel and perpendicular to the reaction front 200 nm wide rods of anorthite
and 20 nm wide rods of spinel are identified. The rods are oriented approximately perpendicular to the replacement front and
are embedded in an orthopyroxene matrix. The regular spacing of the symplectite phases along the reaction front suggests that
their growth is controlled by diffusion. The kinetics of symplectite formation has been modelled based on irreversible thermodynamics.
During interaction of the xenolith with the host basalt the microstructure and chemistry of the An–Opx–Spl symplectite was
significantly modified and it was partially replaced by an olivine bearing symplectite. In contrast to primary symplectite
formation, these processes were metasomatic in nature including addition of sodium, titanium and some trace elements from
the basaltic melt and can clearly be discerned from the garnet breakdown. Based on these observations it is inferred that
symplectite formation took place within the deep crust during the extension of the Pannonian Basin between 15 and 30 km depth
at high temperature (850–1,050°C) prior to the volcanic transport to the surface. |
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