Title: A new method to measure iron self-diffusion at high pressure and temperature

Presenter: Prof. Simon Clark

Institution: Australian Research Council Centre of Excellence for Core to Crust Fluid Systems (CCFS) and GEMOC, Department of Earth and Planetary Sciences, Macquarie University

Time: 20180405-15:00:00

Location: Room 2829, No. 2 Science Building.

Abstract

Diffusion plays a key role in planetary processes. It controls solid state fractionation which determines the distribution of elements and it is the rate limiting step for creep which determines the rheology. Many processes occur deep within planetary bodies so we need to understand the effect of pressure on diffusion rates. Iron is abundant throughout the Earth so an appreciation of the self-diffusion rate of iron is of particular relevance. Previous studies [1,2] have focussed on determining diffusion coefficients in iron-nickel alloys. These studies determined diffusivities by placing two metals in contact (iron and nickel), pressurizing in a piston cylinder or multi-anvil high-pressure cell, heating, quenching and then cutting and polishing the sample before measuring concentration as a function of distance using an electron micro-probe. These studies successfully quantified the effect of temperature on the diffusion coefficient but were unable to detect any effect of pressure possibly due to the relatively low pressure range achievable using piston cylinder and multi-anvil devices. To extend the pressure range we need to move to diamond anvil high-pressure cells but the extremely small sample size (typically 20-50μm in diameter) makes the method of using the electron microprobe (with a beam size of around 5μm) infeasible. This limitation has now been overcome with the advent of new instruments such as the nano-SIMS with a beam size of around 100nm. We have therefore carried out a demonstration measurement of iron self-diffusion in the bcc phase of iron using samples prepared at high-pressure and temperature in a laser heated diamond anvil cell. Samples were prepared by taking a 6μm thick sheet of 56Fe and coating it on one side with a 70nm thick layer of 57Fe. 60μm diameter disks were cut from this sheet and loaded into a 60μm hole in a Re gasket in a diamond anvil cell together with a 60μm diameter, 10 μm thick disk of KCl on either side as insulating layers. The sample was then pressurized and the centre of the sample was heated using the laser heating system in the Department of Earth Sciences, Bristol University. The pressure was released, the samples recovered and the KCl insulating layers removed. Slices were cut from the centre of the sample using a Helios NanoLab G3 CX focused ion beam mill and the concentration of 56Fe and 57Fe measured using a Cameca NanoSIMS 50L. Both of these instruments are housed in the Centre for Microscopy, Characterization and Analysis at the University of Western Australia. Diffusion coefficients of 1.46x10-14 m2/s-1 at 43.6GPa and 2100 K and 9.92x10-14 m2/s-1 at 43.6GPa and 2000 K were determined. These were combined with the previous data [1,2] and found to give a satisfactory fit to the Sammis and Smith model [3] with an activation volume of 1.43 cm3/mol and an activation energy of 431 kJ/mol.

Reference

[1] Goldstein, J., Hanneman, R., and Ogilvie, R., 1965, Diffusion in the fe-ni system at 1 atm and 40 kbar pressure(Interdiffusion coefficients for Fe-Ni alloy as function of composition in alpha and gamma phases at 1 atm and 40 kbar pressure): AIME, TRANSACTIONS, v. 233, p. 812-820.

[2] Yunker, M. L., and Van Orman, J. A., 2007, Interdiffusion of solid iron and nickel at high pressure: Earth and Planetary Science Letters, v. 254, no. 1, p. 203-213.

[3] Sammis, C. G., Smith, J. C., and Schubert, G., 1981, A critical assessment of estimation methods for activation volume: Journal of Geophysical Research: Solid Earth, v. 86, no. B11, p. 10707-10718.

CV

•Education

1985 - 1992: Department of Crystallography, Birkbeck College, University of London. PhD Crystallography.

1988 - 1983: Department of Crystallography, Birkbeck College, University of London. MSc Crystallography.

1976 - 1980: Chemistry Department, City University, London. BSc Industrial Chemistry.

•Work Experience

2012 - Present: Lecturer, Dept. Earth and Planetary Science, Macquarie University, Australia.

2001 - 2012: Staff Scientist, Advanced Light Source, Lawrence Berkeley National Laboratory, USA.

1987 - 2001: Beamline Scientist, Daresbury Laboratory, UK.

•Adjunct Positions

2011: Honorary Associate, Dept. of Earth and Planetary Sciences, Macquarie University, Sydney, Australia.

2006: Associate Adjunct Professor, Dept. of Earth and Planetary Sciences, University of California, Berkeley, USA.

2005: Associate Researcher, Dept. of Earth and Planetary Sciences, University of California, Berkeley, USA.

2000: Honorary Professor Dept. of Earth Sciences, The University of Manchester UK.

1994: Honorary Lecturer, Dept. of Earth Sciences, The University of Manchester, UK.

•Research

My research is based primarily around using x-ray scattering and diffraction to study interesting problems in materials and Earth science. The majority of my work has involved non-ambient conditions usually high-temperatures and high-pressures. I started out doing neutron diffraction at the ILL and ISIS before moving to synchrotron radiation. My current interests include using scattering to determine the atomic structure of amorphous materials and melts and using a combination of diffraction and imaging to measure defect densities as a function of pressure and temperature.

Homepage

https://directory.science.mq.edu.au/users/sclark

https://sites.google.com/site/simonmartinclarkpersonalinfo/cv