Metamorphic Diffusion

Migration, by diffusion, of materials from one part of a rock mass to another during metamorphism. Diffusion may involve chemically active fluids from magmatic sources, hot pore fluids, or fluids released from hydrous minerals or carbonates.

Diffusion is the transfer of mass via the motion of individual atoms or molecules.
Diffusion within crystals is often discussed in terms of point defects, which are missing atoms (vacancies) or extra atoms in the crystal lattice. Every crystal has an equilibrium number of vacancies that allows the crystal to be in a lower free energy state than if it were perfect crystal-this is important because it means that crystals are always "ready to go" when it comes to diffusion. Fig 1.
 

diffusion: mass transfer by the migration of point defects

Diffusion Pathways

Different types of diffusion pathways include

• intergranular (volume) diffusion
• grain-boundary film (grain-boundary diffusion)
• bulk fluid
• grain-scale defects
• Fig 2.

Let's examine three of these processes in more detail:

• Aside from veins, metamorphic rocks have essentially zero porosity, such that diffusion through bulk fluids is not an important process except at shallow levels in the crust
• volume diffusion: diffusion within a single grain. Volume diffusion only becomes significant at high temperature. Fig.3 
   
• grain-boundary diffusion: diffusion along a grain boundary. Grain-boundary diffusion is orders of magnitude faster than volume diffusion, meaning that grain boundaries are the pathways by which material is transported through and within rocks of low porosity. Fig.4
   

Metamorphic Effects of Diffusion

compositional zoning: Spatial variation in mineral composition results from slow volume diffusion (diffusion through the mineral lattice). In garnets, for example, diffusion of the divalent cations is negligible at low and medium grade, so that the interior of a garnet is isolated from the rock matrix, and as the P and T change, the garnet changes composition in response, and develops zoning. Garnets with prograde growth zoning tend to have Mn and/or Ca-rich cores, with Mg# increasing steadily toward the rim. Volume diffusion becomes fast enough at high grade for originally zoned garnets to become compositionally homogenized. 

diffusional/retrograde zoning: zoning in which a pre-existing grain is modified in composition by exchange of material with the rock matrix. The usual result in a garnet is a grain with a depletion in Mg, and commonly also an enrichment in Mn, at the rim. This is found in garnets which have experienced temperatures >600°C.