Wetting behavior of olivine; a 2D and 3D analysis of melt pocket geometries

Abstract

The characteristics of the distribution of melt through peridotites can strongly influence melt migration, as well as properties like seismic attenuation and mechanical strength, which are of major importance in many geological studies of the Earth’s mantle. Melt distribution can be characterized by the wetting angle or dihedral angle, which quantifies the amount of protrusion of melt between individual melt pockets. Conventional studies of dihedral angles rely on the estimation of 3D dihedral angles from the distribution of 2D apparent angles. However, there are few studies on the relationship between the true dihedral angle and apparent angle distributions. Dihedral angle analyses have been performed on samples containing 95% forsterite (Mg2SiO4) and 5% enstatite (MgSiO3) to gain more insight of the dihedral angle in olivine, which is a major constituent of the Earth’s mantle. Imaged cross sections through true dihedral angles in different orientations result in images containing apparent dihedral angles in conventional data sets. To investigate the validity of different statistical approaches of the true dihedral angle, as well as the effects of faceting and melt content on the dihedral angle, a new method has been developed for both true dihedral angle and apparent dihedral angle analysis compatible with FIB-SEM imaging. Usage of the new method aims to reduce errors caused by magnification and resolution limitations as well as influences of arbitrary elements present in conventional methods. Accuracy is increased to up to 1-2° for apparent dihedral angles and ~3° for true dihedral angles. An arbitrary set of cross sections was created for the analysis of apparent angles. The dihedral angle estimated from the mean of these apparent angles is in good agreement with the true dihedral angle measured in the FIB-SEM datasets. Distributions of apparent angles suggest that faceting and the presence of zero-degree dihedral angles between grains have a major influence on dihedral angle distributions and with that, on statistical approaches of the dihedral angle. This could have implications in multiple fields of study on the Earth’s mantle.