HR: 0800h
AN: V51E-0838    [Abstracts]
TI: Bulk Diffusion and Isotopic Fractionation of Lithium in Olivine: an Experimental Study
AU: * Singletary, S
EM: ssingletary@uncfsu.edu
AF: Fayetteville State University, Dept. of Natural Sciences 1200 Murchison Road, Fayetteville, NC 28301, United States
AU: Bell, D R
EM: david.r.bell@asu.edu
AF: Arizona State University, School of Earth and Space Exploration PO Box 871404, Tempe, AZ 85287-1404, United States
AB: Lithium is the lightest of the lithophile elements, with high solid-state diffusivity (e.g. Giletti and Shanahan, 1997). Recent studies in synthetic and natural systems suggest that isotopic fractionation of lithium accompanies diffusion (Richter et al., 2003; Lundstrom et al., 2005; Beck et al., 2006; Teng et al., 2007; Jeffcoate et al., 2007). Lithium isotope variations therefore have the potential to constrain time scales of geological processes. We have performed a series of experiments to quantitatively assess the bulk diffusion of lithium in olivine and the degree of isotopic fractionation that can occur during diffusive transport. Single crystals of San Carlos olivine were surrounded by LiF and then held at temperature (800°C to 1000°C) for variable lengths of times (up to 20 hours at 800°C and as short as 2 hours at 1000°C). Measurements of the lithium concentration and isotopic composition of the experimental charges were conducted using the Cameca IMS 3f at Arizona State University. The lowest temperature experiments show no discernable diffusion of lithium into the olivine crystal, while in the highest temperature experiment, up to 650 ppm lithium is incorporated into the olivine. The lithium content in all experiments above 1000°C decreases smoothly from the crystal rim to the core; indicating diffusive transport of Li into the crystal. Lithium concentrations in the crystal cores range from background in the lowest temperature experiment, to ~100 ppm in the highest temperature experiment. Rim values, in contrast, vary from 650 ppm in the high temperature experiments to core concentrations in the low temperature experiment. All experiments in which diffusive transport into the olivine crystal occurred also display a fractionation of ⁶Li from ⁷Li. In the most extreme case, a variation of 50‰ in δ⁷Li was observed across the diffusion profile, which was generated in just under 2 hours. As lithium content decreases toward the crystal center, the δ⁷Li values become increasingly negative, reaching values of -45‰ in one experiment. These results confirm that solid state Li diffusion can be a major influence on the isotopic composition of Li in olivine, as shown previously for silicate melts (Richter et al., 2003). In one experiment, low δ⁷Li values were observed ahead of detectable changes in Li concentration, suggesting that isotopic exchange of Li in olivine may be more rapid than bulk Li diffusion. With experimentally determined values of Li diffusivity, it should be possible to place strict time constraints on geochemical processes that affect the lithium abundance and isotopic ratios in olivine.
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 3630 Experimental mineralogy and petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting