Substitution mechanisms and solubility of titanium in phlogopites from rocks of probable mantle origin |
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| Authors: | M Arima A D Edgar |
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| Institution: | (1) Department of Geology, University of Western Ontario, N6A 5B7 London, Canada |
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| Abstract: | Previously proposed substitution mechanisms for Ti in phlogopites, based on experimental studies and crystal chemistry, have
been examined using data for 81 phlogopites from mantle-derived rocks (primarily as nodules in kimberlites and also from alkali
basalts, lamprophyres and carbonatites), 49 phlogopites from high-K rocks with basaltic affinities, and from 32 phlogopites
crystallized in high pressure experiments mainly on high-K rock compositions. For the majority of phlogopites from the kimberlite
group and for all those crystallized in the experimental studies, the substitution of Ti can be represented by a combination
of the mechanisms represented by 2MgVI]⇌ TiVI]□VI] and MgVI]2SiIV]⇌TiVI]2AlIV]. Some phlogopites in ultrapotassic rocks have only the former substitution mechanism. The Ti contents of phlogopites generally
increase with decreasing octahedral site occupancy and decreasing Si+AlVI]. For the phlogopites crystallized in the experiments on high-K rocks, the solubility of Ti increases with increasing fO2 and temperature, and possibly with decreasing pressure at constant fO2. The effect of the composition of the liquids used in the experimental studies from which these phlogopites have crystallized
has only minor effect on either the substitution mechanism or the solubility of Ti in phlogopites. This suggests that phlogopite
in high-K rocks may be a potential geothermometer and possibly a geobarometer. |
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