doi:10.1016/j.gca.2004.02.024 Experimental determination of Ni diffusion coefficients in olivine and their dependence on temperature, composition, oxygen fugacity, and crystallographic orientation CHRISTOF PETRY, 1 SUMIT CHAKRABORTY, 2, * and HERBERT PALME 1 1 Institut für Mineralogie und Geochemie, Universität zu Köln, Zülpicher Straße 49b, 50674 Köln, Germany 2 Institut für Geologie, Mineralogie und Geophysik Ruhr-Universität Bochum, 44780 Bochum, Germany (Received July 11, 2003; accepted in revised form February 27, 2004) Abstract—Diffusion couple experiments were carried out with San Carlos olivine (Fo 90 ) and NiFe alloys (Ni 100 , Ni 97 Fe 3 , Ni 90 Fe 10 ) or other olivine compositions (Fo 100 , Fo 25 ) in order to determine the dependence on temperature, oxygen fugacity, composition and crystallographic orientation of Ni diffusion coefficient (D Ni ) in olivine. Experiments at 1 atmosphere total pressure cover a temperature range of 900 –1445°C with run durations from 48 to 2155 h at different oxygen fugacities. In an Arrhenius plot the best fit for all data for Fo 90 yields an activation energy (E D ) of 220  14 kJ/mol and an fO 2 dependence of (1/4.25)   log fO 2 =  log D Ni . The relationship between diffusion coefficients along different crystallographic axes at 1200°C is given by D [001]  6  D [100]  6  D [010] .D Ni depends strongly on the major element (i.e. Fe/Mg) composition of olivine and decreases by about 1 order of magnitude as the olivine composition changes from Fo 35 to Fo 90 . Thus, experimental investigations in Fe-free systems cannot be applied to natural samples. For calculation of residence times or cooling rates the present Ni data yield shorter timescales compared to those obtained using diffusion data published until now. In addition to Ni diffusion coefficients, Fe-Mg, Mn and Ca diffusion data were obtained from some of the same diffusion couples (Fo 90 -Fo 100 ). It is found that the activation energies, E D [Ni]  E D [Fe-Mg]  E D [Mn]  E D [Ca]. All diffusion coefficients are strongly dependent on the major element composition of olivine. Copyright © 2004 Elsevier Ltd 1. INTRODUCTION Diffusion coefficients in combination with element partition- ing data may be used to constrain thermal histories of a variety of terrestrial and extra terrestrial rocks. Geospeedometric tech- niques can be used to obtain cooling rates (e.g., Lasaga, 1983) or the durations of entire thermal events as well as just the heating or cooling parts of the thermal cycle (e.g., Chakraborty and Ganguly, 1991). The exchange of Ni and Fe between olivine and metal is strongly temperature dependent (e.g., see Petry, 1999) and is thus ideally suited for the determination of thermal histories of metal bearing silicate meteorites, that con- stitute the majority among all meteorite types. To develop this tool, it is necessary to quantify (a) the partitioning of Ni between metal and olivine and (b) the diffusion rates of Ni in olivine. We have carried out an experimental study to address both issues (Petry, 1999). In this paper we report the results of the measurement of diffusion coefficients in olivine. The results on element partitioning will be discussed elsewhere. Nickel, as a strongly compatible element, becomes concen- trated in olivine through terrestrial magmatic processes. There- fore, modelling the compositional zoning of Ni in terrestrial olivine samples has been used to constrain thermal histories of volcanic and plutonic processes (e.g., Nakamura, 1995). De- tailed measurements of Ni diffusion rates in olivine can quan- tify and improve the understanding of such processes. Our experimental approach results in the simultaneous determina- tion of diffusion rates of additional divalent cations in Fe- bearing olivine besides Ni (e.g., Fe-Mg, Mn and some limited information on Ca). This allows the verification of existing diffusion data and most importantly, allows study of subtle differences between the diffusion rates of various cations in olivines. We have used olivine (Fo 90 )-metal (Ni 100 , Ni 97 Fe 3 , Ni 90 Fe 10 ) or olivine (Fo 90 )-olivine (Fo 100 , Fo 25 ) single crystal diffusion couples to obtain Ni diffusion coefficients between 900 and 1445°C as a function of temperature, oxygen fugacity, composition and crystallographic orientation, all at a total pres- sure of 1 atm. 2. EXPERIMENTAL METHOD 2.1. Starting Material and Anneals The experimental methods were similar to those of Chakraborty (1997) and details of crystals used, sample preparation, experimental setup and annealing procedures may be found there as well as in Petry (1999). All experiments were carried out using single crystals of olivine that were oriented using X-ray diffraction. A surface perpendicular to the direction in which diffusion rates were to be measured was then polished using diamond compounds, followed by a final step where a colloidal silica compound was used. For individual diffusion anneals, pieces 2 mm on the side were sawed from the larger pieces using a fine diamond saw. Compositions of all starting materials used are given in Table 1. Diffusion couples were held in a vertical gas mixing furnace under defined temperatures and oxygen fugacities (maintained by a flowing gas mixture of CO-CO 2 ) which were continuously monitored using Pt-Rh thermocouples and ZrO 2 sensors, respectively. Run conditions were chosen to be within the stability field of both partners of a diffusion couple. Two experimental approaches were used to check consistency as well as to determine the dependency of diffusion rates on different variables. In the first approach, oriented single crystals of San Carlos olivine (Fo 90 ) and metal alloys of different compositions (Ni 100 , * Author to whom correspondence should be addressed, at Wilhelms- höher Allee 289, 34131, Kassel, Germany (sumit.chakraborty@ruhr- universität-bochum.de). Pergamon Geochimica et Cosmochimica Acta, Vol. 68, No. 20, pp. 4179-4188, 2004 Copyright © 2004 Elsevier Ltd Printed in the USA. All rights reserved 0016-7037/04 $30.00 + .00 4179