HR: 0800h
AN: V41C-1407    [Abstracts]
TI: Experimental Determination of Allanite Stability in High-Silica Rhyolite
AU: * Vazquez, J A
EM: jvazquez@csun.edu
AF: California State University, Northridge, Dept. of Geological Sciences, Northridge, CA 91330
AU: Manning, C E
EM: manning@ess.ucla.edu
AF: University of California, Los Angeles, Dept. of Earth and Space Sciences, Los Angeles, CA 90095
AU: Reid, M R
EM: mary.reid@nau.edu
AF: Northern Arizona University, Dept. of Geology, Flagstaff, AZ 86011
AB: The rare earth elements (REE) are powerful tracers of the thermochemical evolution of rhyolitic/granitic magmas because their absolute and relative concentrations are primarily controlled by saturation of accessory minerals with discrete stability conditions. Allanite is the most important accessory mineral controlling the behavior of the light rare earth elements in metaluminous granites and rhyolites, yet the thermochemical conditions controlling its stability are poorly defined. Recent experiments (Hermann, 2002) have established allanite saturation in dacitic partial melts (63-65 wt.% SiO$_{2}$), but extrapolation of the results to the typical REE concentrations of high-silica rhyolite/leucogranite melts ($>$75 wt.% SiO$_{2}$) predicts temperatures that are at variance with those observed for natural allanite-bearing rhyolites (Bishop, Toba, & Bandelier Tuffs). In order to constrain allanite saturation in granitic magmas, we are performing experiments to determine the stability of allanite in high-silica rhyolite/leucogranite compositions at 1.6 to 7 kbar and 740 to 800$\deg$C. Experiments were conducted using piston-cylinder apparatus (7 kbar) and cold seal vessels (1.6 kbar), with Au capsules at H2O saturation and fO$_{2}$ at ~NNO. Chips of natural allanite were sealed with Late Bishop Tuff glass (77 wt.% SiO$_{2}$). Results at 7 kbar indicate that allanite saturation occurs between 760-770$\deg$C. At $\geq$780$\deg$C, anhydrous chevkinite (REE-Ti-rich silicate) replaced allanite, suggesting that allanite saturation may be associated with a discontinuous net transfer reaction involving chevkinite consumption. Preliminary results for experiments using cold seal vessels at 1.6 kbar pressure indicate that allanite is stable at temperatures from 750 to 770$\deg$C; higher temperature experiments are in progress. The stability of allanite in high-silica rhyolite melts containing REE concentrations greater than predicted by the experiments using dacitic melts suggests that the major element composition of magmas may affect allanite solubility. The apparent thermal stability of allanite at 7 kbar is similar to the 765$\pm$10$\deg$C inferred from Fe-Ti oxide thermometry of allanite-bearing Bishop Tuff that equilibrated at 1-2 kbar pressure (c.f., Hildreth, 1979), suggesting that allanite stability may be relatively insensitive to pressure for mid- to upper crustal granitic magmas. Ongoing experiments are examining the effects of water, fO$_{2}$, and melt REE concentration on allanite stability. References J. Hermann, Chem Geol 192: 289 (2002) W. Hildreth, GSA Special Paper 180: 43 (1979)
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting