HR: 1340h
AN: MR13C-1404    [Abstracts]
TI: Preliminary Results for Fluorine Diffusion in Biotite
AU: * Price, J D
EM: pricej@rpi.edu
AF: Rensselaer Polytechnic Institute, Dept. of Earth & Environmental Sciences, 110 8th St., JSC 1W19, Troy, NY 12180, United States
AU: Sallet, R
EM: sallet@geologia.ufrn.br
AF: Federal University of Rio Grande do Norte (UFRN), Dept. of Geology, Campus Universitário Caixa postal 1639, Natal, RN59078-97, Brazil
AB: Recent experiments evaluated the diffusion of F in biotite at 650, 700, 750 °C, 0.4 GPa of pressure. Experiments assembled a mixture of two naturally occurring, compositionally homogenous biotites separated from schists of the Seridó Fold Belt in northeastern Brazil. A number of ~0.1 mm long PPPL 18 biotite grains, with F=2.5wt% and an Mg-number of 0.36, were mixed with ~0.1 mm long St. Andre biotite grains, with F=0.3wt% and an Mg-number of 0.38. These were loaded into silver capsules with a powdered anorthite-fluorite-sillimanite- quartz (AFSQ) buffer and 4M HF acid. The capsules and their silver covers were positioned into piston-cylinder aparati, pressurized to 0.4 GPa, and held at that pressure for 30 minutes to insure a pressure weld between the capsule and its cover. The materials were then heated to either 650, 700, 750 °C at 70 °C/min and held for 74, 78, and 24 hours, respectively. The materials were quenched to room temperature in less than 1 minute, removed, sectioned, and polished. These were evaluated by electron microprobe analysis (EMPA) using a 1 μm spot size at 15 keV and 100 nA for mapping and 20 nA for spot analyses. X-ray mapping revealed that high-F PPPL 18 biotites were largely unaltered. However, the low-F St. Andre biotites were enriched in fluorine along edges adjacent to pore spaces. There were additional linear zones of F enrichment following defects perpendicular to the c axis. Presumably these defects are related to pre-run cleavage that only partially annealed at run conditions. Diffusion profiles were obtained from crystal edge-core EMPA traverses in areas significantly away from these linear defects. Profiles were fitted with a complementary error function to determine diffusivities. The diffusivities for biotite at 650, 700, and 750 °C are 1.31 ± 0.5 E-17, 1.70 ± 0.5 E-16, 2.71 ± 0.3 E- 15 m²/s, respectively. This yields an Arrhenius relationship with an activation energy (Ea) of 314.5 kJ and an intercept (Do) of 22.7 m²/s. The diffusivities and activation energy are much higher than those for F in tremolite over this temperature range. The diffusivities are also higher than those determined from down- temperature extrapolation of F-diffusion in apatite, while the Ea for apatite appears to be comparable to that for biotite.
DE: 1000 GEOCHEMISTRY
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3630 Experimental mineralogy and petrology
DE: 3660 Metamorphic petrology
DE: 3904 Defects
SC: Mineral and Rock Physics [MR]
MN: 2007 Fall Meeting