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Contributions to Mineralogy and Petrology

, Volume 130, Issue 3–4, pp 390–396 | Cite as

Magnesium self-diffusion in orthoenstatite

  • Craig S. Schwandt
  • Randall T. Cygan
  • Henry R. Westrich
Article

Abstract

Magnesium self-diffusion coefficients were determined experimentally for diffusion parallel to each of the three crystallographic directions in natural orthoenstatite (En88Fs12). Experiments were conducted at 1 atm in CO-CO2 gas mixing furnaces, which provided oxygen fugacities equivalent to the iron-wüstite buffer. Diffusion of 25Mg was induced in polished samples of oriented orthoenstatite using a film of isotopically enriched 25MgO as the source material. Very short (<0.15 μm) diffusional penetration profiles were measured by ion microprobe depth profiling. The diffusion coefficients determined for four temperatures (900, 850, 800, 750 °C) provide the activation energies, E a , and frequency factors, Do, where D = Do exp (−E a /RT) for Mg self-diffusion parallel to each crystallographic direction: a-axis, E a  = 360 ± 52 kJ/mole and Do = 1.10 × 10−4 m2/s; b-axis, E a  = 339 ± 77 kJ/mole and Do = 6.93 × 10−6 m2/s and c-axis, E a  = 265 ± 66 kJ/mole and Do = 4.34 × 10−9 m2/s. In this temperature range, any possible anisotropy of cation diffusion is very small, however the activation energy for diffusion parallel to the c-axis (001) is the lowest and the activation energies for diffusion parallel to the a-axis (100) and b-axis (010) are higher. Application of these diffusion results to the silicate phases of the Lowicz mesosiderite meteorite provides cooling rates for the silicate portion of the meteorite (4–11 °C/100 years) that are similar, although slower, to previous estimates. These silicate cooling rates are still several orders of magnitude faster than the cooling rates (0.1 °C/106 years) for the metal portions.

Keywords

Silicate Activation Energy Cool Rate Depth Profile Oxygen Fugacity 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

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Copyright information

© Springer-Verlag Berlin Heidelberg 1998

Authors and Affiliations

  • Craig S. Schwandt
    • 1
  • Randall T. Cygan
    • 2
  • Henry R. Westrich
    • 2
  1. 1.NASA, Johnson Space Center, SN4, Houston, Tx, 77058-3696, USA Tel: (281)483-1368; e-mail: cschwand@ems.jsc.nasa.govUS
  2. 2.Geochemistry Department, Sandia National Laboratories, Albuquerque, NM, 87185-0750, USAUS

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