CRYSTALLOGRAPHICALLY CONTROLLED VOID SPACE AT GRAIN BOUNDARIES IN EXHUMED METAMORPHIC ROCKS

Grain boundaries, the interfaces between minerals, are pathways for material transfer in metamorphic rocks. They facilitate reactions, act as pathways for the migration of elemental complexes, and serve as nucleation sites for minerals. Understanding the structure of grain boundaries is important, but it is unclear if this structure is locked in at peak pressure (P) and temperature (T) conditions, and how it changes during exhumation. We present examples from two contact aureoles (500-650°C, 4kbar) where there is a heterogeneous but non-random distribution of void space at certain mineral interfaces. To explain this distribution, we correlate the size and abundance of void spaces with calculated changes in the volume of neighboring minerals during exhumation from peak P-T conditions, using these mineral’s elastic properties (thermal expansion and bulk moduli).

An effectively monomineralic sample of the Harkness quartzite from the aureole of the Papoose Flat pluton, California (Law et al. 1992) was studied to improve understanding of how the anisotropic elastic properties of quartz may result in a relationship between the crystallographic orientation of neighboring grains and the development of void space between those grains during exhumation.

A metapelitic sample from the Nelson aureole, British Columbia (Pattison & Tinkham 2009) was studied to focus on the distribution of void space at garnet-quartz interfaces. The average measured void space of 18 garnet-quartz grain boundaries via SEM is 328 nm. The range in measured void spaces correlates with the orientation of quartz; smaller voids occur when the quartz c-axis is oriented perpendicular to the grain boundary. This agrees with the anisotropic elastic properties of quartz, with smaller changes parallel to the c-axis than parallel to the a-axis (Raz et al. 2002), leading to less void space created during exhumation when the c-axis is oriented perpendicular to the grain boundary.

Results suggest that for rocks exhumed from low P-medium T settings, the structure of grain boundaries is unlikely to be locked in at peak P-T conditions. Changes in mineral volumes efficiently redistribute grain boundary volume during exhumation, resulting in observable nano-porosity that may be poorly representative of porosity at peak P-T conditions.