HR: 1340h
AN: MR13C-1396    [Abstracts]
TI: Li Diffusion in Olivine
AU: Dohmen, R
EM: ralf.dohmen@rub.de
AF: Institut fuer Geologie, Mineralogie und Geophysik, Ruhr Universitaet Bochum, Universitaetststr. 150, Bochum, 44780, Germany
AU: Kasemann, S
EM: simone.kasemann@ed.ac.uk
AF: Ion Microprobe Unit, Grant Institute of Earth Science, The University of Edinburgh, West Mains Road, Edinburgh, EH9 3JW, United Kingdom
AU: * Coogan, L
EM: lacoogan@uvic.ca
AF: School of Earth and Ocean Sciences, Petch building, University of Victoria, PO Box3055 STN CSC, Victoria, B.C., V8W 3P6, Canada
AU: Chakraborty, S
EM: sumit.chakraborty@rub.de
AF: Institut fuer Geologie, Mineralogie und Geophysik, Ruhr Universitaet Bochum, Universitaetststr. 150, Bochum, 44780, Germany
AB: We have carried out experiments to study the diffusion of Li along the [001] direction of oriented, polished single crystals of San Carlos olivine at 800 °C and atmospheric pressure under a controlled fO₂ of 10^{- 17} bars (~ WM buffer). Cubes of the crystals were annealed surrounded by a powder mix of the same olivine and ⁶Li enriched lithium silicate. After the experiments, the crystals were sectioned parallel to the diffusion direction and concentration profiles were measured using a Cameca IMS4f ion microprobe in the step scanning mode. The concentration profiles show (i) complex shapes with plateaus in between that are not amenable to treatment using standard solutions to the diffusion equation, and (ii) isotopic fractionation. The shapes of the profiles clearly indicate that at least two different species of Li are involved in the diffusion process. However, a time series of anneals (10 min, 5 h, 10 h, 40 h and 100 h) demonstrate that the shapes are reproducible and evolve systematically with time. We find that the concentration evolution can be described by the following set of processes: (i) incorporation of Li in olivine primarily in octahedral metal sites, (ii) a fast, homogeneous reaction involving metal vacancies (V^''_Me) that partitions Li between the octahedral site (Li^'_Me) and interstitial positions (Li^\bullet_i): Li^'_Me = V^''_Me + Li^\bullet_i, and (iii) diffusion of total Li as Li^'_Me as well as Li^\bullet_i, at very different rates. The concentration of Li^'_Me is at least 10 times higher than the concentration of Li^\bullet_i but the diffusion rate of Li^\bullet_i is about two orders of magnitude faster than of Li^'_Me. At the experimental conditions, we can fit the profiles using known values of metal vacancy diffusion coefficients in olivine D(V^''_Me) = 10^-14 m²/s, D(Li^'_Me) = 10^-15 m²/s and D(Li^\bullet_i) = 2.5 · 10^-13 m²/s. Once these parameters are obtained by fitting any one profile, calculated profiles for any other run duration are an excellent fit to the observed profile shapes, confirming the inferred mechanism. Experiments are in progress to study the temperature dependence of these processes.
DE: 1042 Mineral and crystal chemistry (3620)
DE: 3610 Geochemical modeling (1009, 8410)
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3904 Defects
SC: Mineral and Rock Physics [MR]
MN: 2007 Fall Meeting