Discussion and Conclusions

The increasing spatial resolution and elemental sensitivity of micro beam techniques is leading to an increasing data set on chemical zonation in matrix grains. As distinct from porphyroblasts grains, which have been already the subject of numerous studies, there has been relatively little work to date on their zoning patterns. One advantage of working with matrix grains is that they have often been present throughout the metamorphic history of a rock, and thus, potentially, may be posses a more complete record.

These simulations suggest a simple set of criteria which may be used to distinguish between the different types of zoning patterns that may occur in grain aggregates. The criteria of smoothness of profile, truncations and concentricity are sufficient to distinguish between the various zoning patterns (Table 1, Figure 8). This table assumes that only one of these processes is active in a rock at the any time, or have not overprinted each other, and that there were no other active processes, such as resorption of crystals into a melt. It also assumes end member behaviour: if grain boundary migration and lattice diffusion are both active, the lattice diffusion will always have the effect of smoothing what were once sharp corners in concentration profiles.

Figure 8. Comparison of zoning patterns

Comparison of zoning patterns.

Comparison of zoning patterns predicted for four different grain boundary processes.

  1. Normal grain growth

  2. Volume energy driven grain boundary migration

  3. Diffusion

  4. Late stage crystallization from a liquid

Table 1. Comparison of zoning patterns predicted for four different grain scale processes

Process Profile Truncations Concentricity Other features
Grain Growth Discontinuities possible Possible Not concentric Widest at triple junctions
General Grain Boundary Migration Discontinuities possible Possible Not concentric Mostly uniform width zones parallel to grainboundaries
Diffusion Smooth profiles Not possible Concentric wrt to current grain boundaries  
Crystallisation Discontinuities possible Not possible Concentric wrt to cores