The nature of grain boundaries in slates implications for mass transport processes during low temperature metamorphism

Abstract

The structure of grain boundaries in some very low-grade slates has been studied with transmission electron microscopy. All phyllosilicate boundaries have structural widths of less than 1 nm. A range of structural types have been observed from apparently coherent basal layer chlorite-muscovite boundaries, semi-coherent chlorite-chlorite boundaries and incoherent boundaries which are commonly defined by a thin layer, 7–10 nm thick, of crystalline second phase. Remnants of isolated fluid inclusions are only found at quartz-quartz boundaries. The cleavage microstructures suggest that a large amount of volume loss occurred during cleavage development at low temperatures. This is most likely to have been achieved by diffusion and/or advection through a fluid-filled network present along grain boundaries or grain edges. The phyllosilicate grain boundaries in their present state could not have acted as the pathways for extensive fluid-assisted mass transport. This suggests that the grain boundary structure during cleavage formation was different from the present state. An interconnected fluid network may be maintained along grain boundaries during deformation by hydrofracturing or by grain boundary migration during dehydration reactions, but as deformation and reactions cease the grain boundaries develop an equilibrium structure with very narrow structural widths and restricted fluid distribution.