Transport properties of olivine grain boundaries from electrical conductivity experiments
Grain boundary processes contribute significantly to electronic and ionic transports in materials within Earth’s interior. We report a novel experimental study of grain boundary conductivity in highly strained olivine aggregates that demonstrates the importance of misorientation angle between adjacent grains on aggregate transport properties. We performed electrical conductivity measurements of melt-free polycrystalline olivine (Fo90) samples that had been previously deformed at 1200 °C and 0.3 GPa to shear strains up to γ = 7.3. The electrical conductivity and anisotropy were measured at 2.8 GPa over the temperature range 700–1400 °C. We observed that (1) the electrical conductivity of samples with a small grain size (3–6 µm) and strong crystallographic preferred orientation produced by dynamic recrystallization during large-strain shear deformation is a factor of 10 or more larger than that measured on coarse-grained samples, (2) the sample deformed to the highest strain is the most conductive even though it does not have the smallest grain size, and (3) conductivity is up to a factor of ~ 4 larger in the direction of shear than normal to the shear plane. Based on these results combined with electrical conductivity data for coarse-grained, polycrystalline olivine and for single crystals, we propose that the electrical conductivity of our fine-grained samples is dominated by grain boundary paths. In addition, the electrical anisotropy results from preferential alignment of higher-conductivity grain boundaries associated with the development of a strong crystallographic preferred orientation of the grains.
KeywordsGrain boundaries Olivine Shear strain Electrical conductivity Electrical anisotropy
The authors wish to thank Bruce Watson and Tim Grove for thoughtful and insightful reviews of this manuscript. This study benefited from funding by the National Science Foundation through Cooperative Studies of the Earth’s Deep Interior projects number EAR 14-61594 to the University of California San Diego and number EAR 15-20647 to the University of Minnesota. This research was also partially supported by the Consortium for Materials Properties Research in Earth Sciences under National Science Foundation Cooperative Agreement EAR 11-57758. Anne Pommier acknowledges support from the Alexander von Humboldt Foundation and thanks the Bayerisches GeoInstitut for hosting her during part of the study. Florian Heidelbach acknowledges support through Deutsche Forschungsgemeinschaft Grant He3285/2-1. Stephen Mackwell acknowledges support from the National Aeronautics and Space Administration under CAN-NNX15AL12A. We are grateful to Jed Mosenfelder for his help with the FTIR, and Cameron Meyers for doing some EBSD measurements. We thank Katharina Marquardt for fruitful discussions, Mark Zimmerman for his help on the starting materials synthesis, Henrietta Cathey and Katherine Armstrong for assistance with microprobe and SEM analyses at Arizona State University and Bayerisches GeoInstitut, respectively, and Matej Pec for SEM analyses at the University of Minnesota.
- O’Neill HStC (1986) Mo–MoO2 (MOM) oxygen buffer and the free energy of formation of MoO2. Am Miner 71:1007–1010Google Scholar
- Paterson MS (1982) The determination of hydroxyl by infrared absorption in quartz, silicate glasses and similar materials. Bull Miner 105:20–29Google Scholar
- Underwood E (1970) Quantitative stereology. Addison-Wesley, ReadingGoogle Scholar
- Watson EB (2002) Mobility of siderophile elements in grain boundaries of periclase and periclase/olivine aggregates. EOS Trans AGU 83 Spring Meet Suppl Abs V52B-03Google Scholar
- Yang X (2012) Orientation-related electrical conductivity of hydrous olivine, clinopyroxene and plagioclase and implications for the structure of the lower continental crust and uppermost mantle. Earth Planet Sci Lett 317–318:241–250. https://doi.org/10.1016/j.epsl.2011.11.011 CrossRefGoogle Scholar