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Anisotropy of self-diffusion in forsterite grain boundaries derived from molecular dynamics simulations

Wagner, Johannes and Adjaoud, Omar and Marquardt, Katharina and Jahn, Sandro (2016):
Anisotropy of self-diffusion in forsterite grain boundaries derived from molecular dynamics simulations.
In: Contributions to Mineralogy and Petrology, pp. 1-9, 171, (12), ISSN 0010-7999,
[Online-Edition: http://dx.doi.org/10.1007/s00410-016-1308-y],
[Article]

Abstract

Diffusion rates and associated deformation behaviour in olivine have been subjected to many studies, due to the major abundance of this mineral group in the Earth’s upper mantle. However, grain boundary (GB) transport studies yield controversial results. The relation between transport rate, energy, and geometry of individual GBs is the key to understand transport in aggregates with lattice preferred orientation that favours the presence and/or alignment of specific GBs over random ones in an undeformed rock. In this contribution, we perform classical molecular dynamics simulations of a series of symmetric and one asymmetric tilt GBs of Mg2SiO4 forsterite, ranging from 9.58° to 90° in misorientation and varying surface termination. Our emphasis lies on unravelling structural characteristics of high- and low-angle grain boundaries and how the atomic structure influences grain boundary excess volume and self-diffusion processes. To obtain diffusion rates for different GB geometries, we equilibrate the respective systems at ambient pressure and temperatures from 1900 to 2200 K and trace their evolution for run durations of at least 1000 ps. We then calculate the mean square displacement of the different atomic species within the GB interface to estimate self-diffusion coefficients in the individual systems. Grain boundary diffusion coefficients for Mg, Si and O range from 10−18 to 10−21m3/s, falling in line with extrapolations from lower temperature experimental data. Our data indicate that higher GB excess volumes enable faster diffusion within the GB. Finally, we discuss two types of transport mechanisms that may be distinguished in low- and high-angle GBs.

Item Type: Article
Erschienen: 2016
Creators: Wagner, Johannes and Adjaoud, Omar and Marquardt, Katharina and Jahn, Sandro
Title: Anisotropy of self-diffusion in forsterite grain boundaries derived from molecular dynamics simulations
Language: English
Abstract:

Diffusion rates and associated deformation behaviour in olivine have been subjected to many studies, due to the major abundance of this mineral group in the Earth’s upper mantle. However, grain boundary (GB) transport studies yield controversial results. The relation between transport rate, energy, and geometry of individual GBs is the key to understand transport in aggregates with lattice preferred orientation that favours the presence and/or alignment of specific GBs over random ones in an undeformed rock. In this contribution, we perform classical molecular dynamics simulations of a series of symmetric and one asymmetric tilt GBs of Mg2SiO4 forsterite, ranging from 9.58° to 90° in misorientation and varying surface termination. Our emphasis lies on unravelling structural characteristics of high- and low-angle grain boundaries and how the atomic structure influences grain boundary excess volume and self-diffusion processes. To obtain diffusion rates for different GB geometries, we equilibrate the respective systems at ambient pressure and temperatures from 1900 to 2200 K and trace their evolution for run durations of at least 1000 ps. We then calculate the mean square displacement of the different atomic species within the GB interface to estimate self-diffusion coefficients in the individual systems. Grain boundary diffusion coefficients for Mg, Si and O range from 10−18 to 10−21m3/s, falling in line with extrapolations from lower temperature experimental data. Our data indicate that higher GB excess volumes enable faster diffusion within the GB. Finally, we discuss two types of transport mechanisms that may be distinguished in low- and high-angle GBs.

Journal or Publication Title: Contributions to Mineralogy and Petrology
Volume: 171
Number: 12
Uncontrolled Keywords: Forsterite; Grain; boundary; Self-diffusion; Mg
Divisions: 11 Department of Materials and Earth Sciences > Material Science > Materials Modelling
11 Department of Materials and Earth Sciences > Material Science
11 Department of Materials and Earth Sciences
Date Deposited: 16 Dec 2016 11:56
Official URL: http://dx.doi.org/10.1007/s00410-016-1308-y
Identification Number: doi:10.1007/s00410-016-1308-y
Funders: This work has been funded by the Deutsche Forschungsgemeinschaft (DFG) in the framework of Research Unit FOR 741 (HE2015/11-1) as well as Grant Number MA 6287/3-1, and MA 6287/2-1 and the Project PD-043 within the Helmholtz Postdoc Programm. Parts of the, simulations were performed at John von Neumann Institute for Supercomputing in the framework of NIC project HPO15. Financial support by the Helmholtz graduate school GEOSIM was gratefully acknowledged.
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