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

Abstract

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 themore » 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.« less

Authors:
 [1];  [2];  [3];  [4];  [5];  [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
Publication Date:
Research Org.:
Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Sponsoring Org.:
USDOE National Nuclear Security Administration (NNSA)
OSTI Identifier:
1479057
Report Number(s):
LLNL-JRNL-736676
Journal ID: ISSN 0016-7037; 889349
Grant/Contract Number:  
AC52-07NA27344
Resource Type:
Accepted Manuscript
Journal Name:
Geochimica et Cosmochimica Acta
Additional Journal Information:
Journal Volume: 239; Journal Issue: C; Journal ID: ISSN 0016-7037
Publisher:
The Geochemical Society; The Meteoritical Society
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; Olivine; Diffusion; Isotope effect; Iron; Geospeedometry; Cooling history; Crystal growth

Citation Formats

Sio, Corliss K., Roskosz, Mathieu, Dauphas, Nicolas, Bennett, Neil R., Mock, Timothy, and Shahar, Anat. The isotope effect for Mg-Fe interdiffusion in olivine and its dependence on crystal orientation, composition and temperature. United States: N. p., 2018. Web. doi:10.1016/j.gca.2018.06.024.
Sio, Corliss K., Roskosz, Mathieu, Dauphas, Nicolas, Bennett, Neil R., Mock, Timothy, & Shahar, Anat. The isotope effect for Mg-Fe interdiffusion in olivine and its dependence on crystal orientation, composition and temperature. United States. doi:10.1016/j.gca.2018.06.024.
Sio, Corliss K., Roskosz, Mathieu, Dauphas, Nicolas, Bennett, Neil R., Mock, Timothy, and Shahar, Anat. Sat . "The isotope effect for Mg-Fe interdiffusion in olivine and its dependence on crystal orientation, composition and temperature". United States. doi:10.1016/j.gca.2018.06.024. https://www.osti.gov/servlets/purl/1479057.
@article{osti_1479057,
title = {The isotope effect for Mg-Fe interdiffusion in olivine and its dependence on crystal orientation, composition and temperature},
author = {Sio, Corliss K. and Roskosz, Mathieu and Dauphas, Nicolas and Bennett, Neil R. and Mock, Timothy and Shahar, Anat},
abstractNote = {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 Di/Dj = (mj/mi)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 Fo83.4-Fo88.8 composition pair, βFe 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 βMg = 0.09 ± 0.05 for this range of olivine composition. For the Fo88.8-Fo100 composition pair, βFe becomes anisotropic with βFe [100] = 0.11 ± 0.03, βFe [010] = 0.14 ± 0.03 (both within error of the value measured for the Fo83.4-Fo88.8 pair), and βFe [001] = 0.03 ± 0.03. For Fo# between 83.4 and 100, βFe [100] and βFe [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.},
doi = {10.1016/j.gca.2018.06.024},
journal = {Geochimica et Cosmochimica Acta},
number = C,
volume = 239,
place = {United States},
year = {2018},
month = {6}
}

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