A close look at dihedral angles and melt geometry in olivine-basalt aggregates: a TEM study

Olivine-basalt aggregates sintered at high P/T have been used as a simplest approximation of partially molten upper mantle peridotite. In the past, geometry of partial melt in polycrystalline olivine (and other materials) has been characterised by dihedral (wetting) angles which depend upon surface free energy. However, since olivine (like most other crystalline materials) is distinctively anisotropic, the simple surface energy balance de®ning the dihedral angles cos…H=2† ˆ cgb=2csl is not valid and melt geometry is more complicated than can be expressed by a single dihedral angle value. We examine in detail melt geometry in aggregates held at high temperature and pressure for very long times (240±612 h). We show the simple dihedral angle concept to be invalid via transmission electron microscope images. Olivine-basalt interfaces are frequently planar crystal faces (F-faces) which are controlled by the crystal structure rather than the surface area minimisation used in the simple dihedral angle concept. Nevertheless, the dihedral angles may provide useful insights in some situations. They may give a rough estimation of the wetting behaviour of a system, or be used to approximate the melt distribution if F-faces are not present (possibly at large grain size and very low melt fraction). Our measurements, excluding F-faces, give a range of dihedral angle values from 0 to 10° which is signi®cantly lower than reported previously (20±50°). The nature of 0° angles (®lms and layers up to 1 lm in thickness) is unclear but their frequency compared to dry grain boundaries depends on grain size and melt fraction (e.g. 70% for grain size 43 lm and melt fraction 2%). Introduction Some properties of partially molten material in the upper mantle are controlled by melt distribution at the scale of individual grains (Goetze 1977). For example, melt migration by porous ̄ow will be faster if melt-®lled intergranular space is well interconnected and simple in shape than if some melt remains trapped in isolated pockets (e.g. Faul 1997); seismic attenuation will increase if more melt resides in thin segregations between two grains (e.g. Schmeling 1985); and mechanical strength of mantle peridotite will be lower if more grain surfaces are wetted by melt (e.g. Hirth and Kohlstedt 1995). Despite its importance, relatively little is known about the grain-scale melt distribution in the upper mantle. In this paper, we build on the observations of Wa€ and Faul (1992), by presenting an electron microscopy study of the olivine-basalt aggregates sintered at high pressure and temperature for long times. Some important details of local melt geometry are described and the signi®cance of olivine surface energy anisotropy is stressed. Considerable emphasis is placed on the analysis of dihedral angles and discussion of their relevance in determination of melt distribution. Theoretical background Partially molten mantle peridotite can be viewed as an example of polycrystalline aggregates which contain a small fraction of liquid phase. Such aggregates have been extensively studied in the ®elds of metallurgy and ceramics (e.g. Martin and Doherty 1976; Sutton and Ballu 1995). Theories developed to explain the behaviour of these systems predict that, at chemical equilibrium and under hydrostatic conditions, textural evolution is driven by the minimisation of total surface free energy c, the extra free energy of atoms at interfaces M. Cmõ ral (&) á J.D. Fitz Gerald á U.H. Faul á D.H. Green Research School of Earth Sciences, Australian National University, Canberra ACT 0200, Australia Tel: 61-2-6249 3416, fax: 61-2-6249 5989 e-mail: [email protected] Editorial responsibility: J. Hoefs Contrib Mineral Petrol (1998) 130: 336±345 Ó Springer-Verlag 1998