HR: 0830h
AN: V41C-0312    [PDF]
TI: New igneous thermobarometers based on plagioclase + liquid equilibria
AU: * Putirka, K D
EM: kputirka@csufresno.edu
AF: California State University, Fresno, Department of Earth and Environmental Sciences, 2345 E. San Ramon Ave., MS/MH24, Fresno, CA 93740 United States
AB: Calculation of the pressures (P) and temperatures (T) at which minerals grow is crucial to our understanding of igneous processes. Thermometers based on plagioclase + liquid equilibria represent some of the earliest and most long-standing attempts at determining the conditions at which igneous minerals form. In spite of this attention, though, the issue of error on predicted values of temperature has only rarely been addressed. Lacking estimates of error, the usefulness of such models is limited. To address the issue, several recent thermometers and plagioclase saturation surface models were tested for their ability to recover T from their calibration data, and predict T from experiments not used for calibration (tests data). Pre-1990 models, which tend to rely heavily on early anhydrous 1-atm experiments, tend to perform poorly. More recent calibration efforts by Sugawara, (2001) and Ghiorso et al. (1995, 2002) provide much better results for their calibration data at T $>$ 1100 $^{o}$C, but do not perform as well at T $<$ 1050 $^{o}$C for both test and calibration data, and mixed results are evident for hydrous and high SiO$_{2}$ liquids. To narrow the magnitude of T uncertainty, a new plagioclase-liquid thermometer was calibrated: 10$^{4}$/T(K)=68.8-0.86ln[An/(Ca$^{liq}$(Al$^{liq}$)$^{2}$(Si$^{liq}$)$^{2}$)] + 179.2[Al$^{liq}$] -113.3[Al$^{liq}$/(Al$^{liq}+$Si$^{liq}$)] -7.92[AbAn] -0.0613[P(kb)] -91.6[Ca$^{liq}$Al$^{liq}$] -155[Si$^{liq}$] + 110.3[Si$^{liq}$]$^{2}$ -149[Al$^{liq}$]$^{2}$ In order to predict pressure, a barometer was also developed: P(kb)= -42.2 + 0.0494[T(K)] + 0.0116[T(K)ln[(AbAl$^{liq}$Ca$^{liq}$)/(AnNa$^{liq}$Si$^{liq}$)]] -382.3[Si$^{liq}$]$^{2}$ + 514.2[Si$^{liq}$]$^{3}$ -19.6ln[Ab] -139.8[Ca$^{liq}$] + 287.2[Na$^{liq}$] + 163.9[K$^{liq}$] In these models, T is in Kelvins and P is in kbar. An and Ab are the fractions of anorthite and albite in plagioclase, calculated as cation fractions: An = CaO/(CaO+NaO$_{0.5}$+KO$_{0.5}$) and Ab = NaO$_{0.5}$/(CaO+NaO$_{0.5}$+KO$_{0.5}$). Terms such as Al$^{liq}$ refer to the cation fraction of AlO$_{1.5}$ in the liquid. Errors on these models are comparable to those for clinopyroxene thermobarometers: In the first equation, R = 0.96 and the standard error of estimate (SEE) is 33 K; for the second equation, R = 0.94 and the SEE is 1.77 kbar. These models successfully recover mean pressures for experimental data that are not used for calibration, and are furthermore able to recover near-1-atm P estimates for volcanic rocks from Kilauea, Hawaii, which are thought, based on field and petrographic evidence (C. Thornber, pers. comm.), to have crystallized very near the surface.
DE: 3620 Crystal chemistry
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 3655 Major element composition
DE: 8434 Magma migration
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting