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Compositional variation of phlogopite in a marble sample: implications for geological thermobarometry |
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| Authors: | K BUCHER-NURMINEN |
| Institution: | Department of Geology, University of Oslo, P.B. 1047, Blindern, N-0316 Oslo 3, Norway |
| Abstract: | Abstract The textural and compositional features of phlogopites in a contact-metamorphic dolomite marble inclusion in the Bergell intrusion (central Alps) and in a metasomatic reaction vein cutting through this marble suggest different origins for vein phlogopites: (a) High-Al vein phlogopite represents former marble phlogopite which has been compositionally modified by reaction with the vein forming fluid. (b) Low-Al vein phlogopite represents phlogopite precipitated from the vein forming fluid. As both types of vein phlogopite were in contact with the same vein forming fluid at the same time, low-Al phlogopite most likely represents an equilibrium phlogopite composition, whereas high-Al phlogopite does not. High-Al vein phlogopite retained its Al-content from the contact-metamorphic marble parent phlogopite and only underwent Fe-Mg exchange with the metasomatic fluid. All the vein phlogopites studied are strongly enriched in Fe relative to marble phlogopite. The data may suggest in general that phlogopite Al/Si ratios may be retained from the conditions under which the phlogopites first formed, whereas the Mg/Fe-ratios may be substantially modified by exchange with other ferromagnesian solid phases and/or a metamorphic fluid at later stages in their metamorphic history. This may have significant effects on calculated pressures and temperatures from thermobarometers involving biotite. |
| Keywords: | marbles non-equilibrium phlogopite compositions reaction veins thermobarometry |