Other
Scientific paper
Nov 1985
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1985gecoa..49.2323t&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 49, Issue 11, pp.2323-2329
Other
5
Scientific paper
The solubility of Tio 2 in phlogopites has been experimentally determined in the system K 2 Mg 6 Al 2 Si 6 O 20 (OH) 4 -K 2 Mg 4 TiAl 2 Si 6 O 20 (OH) 4 -K 2 Mg 5 TiAl 4 Si 4 O 20 (OH) 4 between 825-1300°C and 10-30 kbar under vapour absent conditions. Starting compositions lie along the join K 2 Mg 6 Al 2 Si 6 O 20 (OH) 4 -K 2 Mg 4.5 TiAl 3 Si 5 O 20 (OH) 4 which represents a combination of the Mg [ VI ] 2 Si [ IV ] = Ti [ VI ] 2 Al [ VI ] and 2 Mg [ VI ] = Ti [ VI ] / [ VI ] substitution mechanisms for Ti in phlogopites. The results of the experiments indicate a systematic increase in solubility of Ti with increasing temperature and decreasing pressure for given bulk Tio 2 content. Under isobaric conditions high temperature Ti-saturated phlogopite breaks down to Ti -deficient phlogopite + rutile + vapour. Mass balance calculations suggest that the vapour phase may contain K 2 O dissolved in H 2 O and that the reaction is controlled by the vapour phase. Analyses of phlogopites coexisting with rutile and vapour can be represented in terms of the end-member components phlogopite [K 2 Mg 6 Al 2 Si 6 O 20 (OH) 4 ], eastonite [K 2 Mg 5 Al 4 Si 5 O 20 (OH) 4 ], an octahedral site deficient Ti-phlogopite (Ti-OSD) of composition K 2 (Mg 4 Ti )Al 2 Si 6 )O 20 (OH) 4 , and Ti-eastonite [K 2 Mg 5 TiAl 4 Si 4 O 20 (OH) 4 ]. With decreasing amounts of Ti in these phlogopites there is a decrease in the Ti-eastonite component and increase in the eastonite component. The general equation for the breakdown of Ti-phlogopite solid solution to Ti -free phlogopite + rutile + vapour is: 14 Ti -eastonite + 7 Ti - OSD 16 eastonite + 3 phlogopite + 21 rutile + 4 H 2 O + 2 K 2 O . Lack of knowledge of H 2 O and K 2 O activities in the vapour phase does not permit evaluation of thermodynamic constants for this reaction. The Ti solubility in phlogopites and hence its potential as a geothermobarometer under lower crustal to upper mantle conditions is likely controlled by common mantle minerals such as forsterite.
Arima Makoto
Edgar Alan D.
Tronnes Reidar G.
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