Date of Award
Doctor of Philosophy
The solubility of Ti in phlogopite 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) from 10 to 30 kbar and from 825 to 1300(DEGREES)C. Starting compositions 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) were used in order to evaluate the relative importance of the two substitution mechanisms 2Mg = Ti and Mg(2Si)('IV) = Ti(2Al('IV)). These two substitution mechanisms are represented by the joins from the phlogopite molecule (K(,2)Mg(,6)Al(,2)Si(,6)O(,20)(OH)(,4)) to the Ti-OSD (Octahedral Site Deficiency) molecule (K(,2)Mg(,4)TiAl(,2)Si(,6)O(,20)(OH)(,4)) and to the Ti-eastonite molecule (K(,2)Mg(,5)TiAl(,4)O(,20)(OH)(,4)), respectively.;The solubility of Ti in phlogopite increases with increasing temperature and decreases with increasing pressure. For a given starting composition the Ti-OSD/Ti-eastonite and the eastonite/phlogopite ratios in the phlogopite solid solution coexisting with rutile increase with decreasing Ti-content of the phlogopite. The variations in the proportions of the phlogopite and the Ti-OSD end members with variations in temperature and pressure are minor.;The compositions of the experimentally produced phlogopites in terms of the relative proportions of the end member molecules are similar to the compositions of phlogopites from mantle xenoliths, mantle derived, mafic and ultramafic igneous rocks and lower crustal granulites. Based on the experimental data, compositional diagrams including the two Ti-rich phlogopite end members and the sum of the Ti-free end members were contoured for temperature (isobaric diagrams) and pressure (isothermal diagrams). The application of such a phlogopite geothermobarometer to phlogopites from mantle xenoliths and lower crustal granulites gives temperatures and pressures in good agreement with the temperatures and pressures determined by independent geothermometers and geobarometers. The accuracy of the temperature determinations is better than that of the pressure determinations.;Since the geothermobarometer is based on a subsolidus experimental study, it can not be applied to phlogopites that crystallized from a melt. Although further refinements of the phlogopite geothermobarometer are needed, the relationships derived from the simplified experimental system form a basis for determining the physical conditions of phlogopite formation in the upper mantle and the lower crust.
Tronnes, Reidar Gjermund, "The Incorporation Of Titanium In Phlogopite In A Simplified, Synthetic System: A Potential Geothermobarometer For Upper Mantle And Lower Crustal Rocks" (1985). Digitized Theses. 1426.