The isotope effect for Mg-Fe interdiffusion in olivine and its dependence on crystal orientation, composition and temperature

Journal Article · 30 June 2018 · Geochimica et Cosmochimica Acta

DOI: https://doi.org/10.1016/j.gca.2018.06.024 · OSTI ID:1479057

Sio, Corliss K. ^[1]; Roskosz, Mathieu ^[2]; Dauphas, Nicolas ^[3]; Bennett, Neil R. ^[4]; Mock, Timothy ^[5]; Shahar, Anat ^[4]

---

1. Carnegie Inst. of Washington, Washington, DC (United States). Geophysical Lab.; Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
2. Sorbonne Univ., Paris (France). Muséum National d'Histoire Naturelle, Inst. de Minéralogie, de Physique des Matériaux et de Cosmochimie
3. Univ. of Chicago, IL (United States). Origins Lab., Dept. of the Geophysical Sciences and Enrico Fermi Inst
4. Carnegie Inst. of Washington, Washington, DC (United States). Geophysical Lab.
5. Carnegie Inst. of Washington, Washington, DC (United States). Dept. of Terrestrial Magnetism

Isotopic fractionation associated with diffusion in crystals is the most reliable means of understanding the origin of mineral zoning in igneous and metamorphic rocks. We have experimentally determined the relative diffusivities of iron isotopes in olivine as a function of crystallographic orientation, composition, and temperature. For two isotopes i and j of an element, the isotope effect for diffusion is parameterized as D_i/D_j = (m_j/m_i)^β, where β is a dimensionless parameter, and D and m stand for diffusivity and mass, respectively. A series of single crystal diffusion couple experiments were conducted at an oxygen fugacity of QFM – 1.5 at temperatures of 1200, 1300, and 1400 °C. Here, for the Fo_83.4-Fo_88.8 composition pair, β_Fe is isotropic and a value of 0.16 ± 0.09 can be used to describe diffusion along all major crystallographic axes in olivine. Based on our experiments and previously reported coupled Mg-Fe isotopic data, we also estimate β_Mg = 0.09 ± 0.05 for this range of olivine composition. For the Fo_88.8-Fo100 composition pair, β_Fe becomes anisotropic with β_{Fe [100]} = 0.11 ± 0.03, β_{Fe [010}] = 0.14 ± 0.03 (both within error of the value measured for the Fo83.4-Fo88.8 pair), and β_{Fe [001]} = 0.03 ± 0.03. For Fo# between 83.4 and 100, β_{Fe [100]} and β_{Fe [010]} are thus independent of composition. The reason why βFe [001] transitions from ~0.16 to ~0.03 close to the Mg-endmember is unclear. Over the temperature range studied, a dependence of βFe on temperature was not resolved. General analytical expressions are introduced to calculate isotopic fractionation as a function of distance, time, β, and the concentration contrast between the diffusing media for spherical, cylindrical, and planar geometries.

• Research Organization: Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)
• Sponsoring Organization: USDOE National Nuclear Security Administration (NNSA)
• Grant/Contract Number: AC52-07NA27344
• OSTI ID: 1479057
• Report Number(s): LLNL-JRNL--736676; 889349
• Journal Information: Geochimica et Cosmochimica Acta, Journal Name: Geochimica et Cosmochimica Acta Journal Issue: C Vol. 239; ISSN 0016-7037
• Publisher: The Geochemical Society; The Meteoritical SocietyCopyright Statement
• Country of Publication: United States
• Language: English