The most frequent interfaces in olivine aggregates: the GBCD and its importance for grain boundary related processes

Olivine is the most important phase in the Earth’s upper mantle, where the bulk rock composition varies on average from lherzolitic to harzburgitic, where the olivine fractions are normally >60 %. Olivine is of major importance for Earth’s mantle dynamics and the presence of grain boundaries in olivine and their type might partially be responsible for attenuation phenomena as well as thermal and diffusion anisotropy observed in seismic studies (Sobolev et al. 1996; Hammond and Humphreys 2000; Toomey et al. 2002; Villagomez et al. 2014). Furthermore, the importance of anisotropic grain boundary properties for fluid (including melt) percolation has been a field of controversial debate (Faul 2001; Hiraga et al. 2007; Gardés et al. 2012; Dai et al. 2013; Ghanbarzadeh et al. 2014). In this contribution, we focus on the full characterization of all 5 independent grain boundary parameters for both natural and synthetic polycrystalline olivine samples, with the goal to determine the anisotropy of olivine grain boundaries in olivine dominated rocks. In the introduction, we first highlight the general importance of grain boundaries and summarize the current state of knowledge on olivine grain boundaries followed by an overview on melt distribution on the grain scale as this is a topic where many studies have been performed. This is followed by an overview on how grain boundary properties are studied. The geometry of grain boundaries and how they can be characterized in their full parameter space is introduced in the third paragraph of the introduction, we aim to give the reader an insight into the most commonly used concepts to differentiate different grain boundary types. Finally, the last part introduces the methods to measure the grain boundary character distribution. Abstract Rocks consist of crystal grains separated by grain boundaries that impact the bulk rock properties. Recent studies on metals and ceramics showed that the grain boundary plane orientation is more significant for grain boundary properties than other characteristics such as the sigma value or disorientation (in the Earth’s science community more frequently termed misorientation). We determined the grain boundary character distribution (GBCD) of synthetic and natural polycrystalline olivine, the most abundant mineral of Earth’s upper mantle. We show that grain boundaries of olivine preferentially contain low index planes, in agreement with recent findings on other oxides (e.g. MgO, TiO2, Al2O3 etc.). Furthermore, we find evidence for a preferred orientation relationship of 90° disorientations about the [001] direction forming tilt and twist grain boundaries, as well as a preference for the 60° disorientation about the [100] axis. Our data indicate that the GBCD, which is an intrinsic property of any mineral aggregate, is fundamental for understanding and predicting grain boundary related processes.