HR: 0830h
AN: V41C-0313    [PDF]
TI: Phlogopite Thermometer Calibration and Use: Magmatic History and Processes in the Roman Potassic Province
AU: * Roach, A
EM: Angela_Roach@brown.edu
AF: Department of Geological Sciences Brown University, Box 1846, Providence, RI 02912 United States
AU: Rutherford, M
EM: Malcolm_Rutherford@brown.edu
AF: Department of Geological Sciences Brown University, Box 1846, Providence, RI 02912 United States
AB: Previous investigations have shown that the partitioning of titanium between melt and phlogopite is temperature-dependent (e.g. Tronnes et al., 1985; Righter and Carmichael, 1996). For many compositions of the Roman Potassic Province, a Ti-in-phlogopite geothermometer is useful over a large crystallization interval and has the advantage of being independent of pressure. Righter and Carmichael (1996) present a single geothermometer of the form \\ lnD TiO$_{2}$ phl/liq = a/T + b \\where D is wt.% TiO$_{2}$ in phlogopite/wt.% TiO$_{2}$ in glass, T is in Kelvins, and a and b are constants. Our investigation indicates that titanium partitioning into phlogopite is lower in peralkaline melts than in metaluminous and peraluminous melts. These results are consistent with previous experiments which show that the activity coefficient of TiO$_{2}$ is decreased in peralkaline melts (Hess, 1995). We have calibrated two new geothermometers. For peraluminous and metaluminous compositions: \\ (1) ln D TiO$_{2}$ phl/liq = 16200/T - 11.02; r$^{2}$ = 0.85. \\This geothermometer was calibrated using data from our experiments on Agnano Monte Spina trachytes and the metaluminous and peraluminous data used by Righter and Carmichael. For peralkaline compositions: \\ (2) lnD TiO$_{2}$ phl/liq = 15180/T - 10.8; r$^{2}$ = 0.90. \\This geothermometer was calibrated with our experimental data from 79 A.D. Vesuvius compositions and the data of Guo and Green (1990). The molar (Na$_{2}$O+K$_{2}$O)/Al$_{2}$O$_{3}$ ratios of these compositions are 1.0 and 1.3 respectively; we have not yet investigated higher degrees of peralkalinity. These geothermometers constrain the pre-eruption temperatures of materials from the Roman Potassic Province. Geothermometer (1) gives a temperature of 929$^{o}$C for Layer B1 and 938$^{o}$C for Layer D1 trachytes of Agnano Monte Spina. These temperatures agree with water-undersaturated (CO$_{2}$-bearing) phase equilibria experiments on these compositions. The peralkaline geothermometer (2) gives a temperature of 857$^{o}$C for the white pumice of the 79 A.D. Vesuvius eruption; this agrees with phase equilibria obtained from water-saturated experiments. For the gray pumice of 79 A.D. Vesuvius, we calculate a temperature of 924$^{o}$C. This temperature is similar to that obtained from phase equilibria data if it is assumed that hornblende, sanidine, and leucite were not part of the pre-eruption phenocryst assemblage but instead were added during syn-eruptive mixing with phonolite. In contrast, the original geothermometer of Righter and Carmichael (1996) overestimated Vesuvius temperatures by 60-65$^{o}$; phase equilibria experiments suggest that phlogopite had not even begun to crystallize at these temperatures. Thus our new calibrations are more accurate for a broader range of compositions.
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 3694 Instruments and techniques
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting