Chapter 14. Metamorphic mineral reactions: Porphyroblast, corona and symplectite growth
F. Gaidies, R. Milke, W. Heinrich and R. Abart
Much of the Earth’s dynamics is related to mineral reactions in the solid-state. Classically, this is referred to as metamorphic crystallization (Kretz, 1994). Based on the chemical compositions of the phases involved in a metamorphic mineral reaction, two basic reaction types may be distinguished. Reactions that involve only structural re-arrangements, while the compositions of the reactant and product phases are identical, are referred to as partitionless and ‘polymorphic phase
transformations’. If, in contrast, one or more reactant phases are replaced by one or more product phases with different compositions, this implies that chemical components are supplied to or removed from the reaction interfaces separating the reactants from the product phases. In the absence of advective transport via a fluid or melt, the necessary chemical mass transport can occur only by diffusion. Accordingly, this reaction type is partitioning and is referred to as ‘diffusive
phase transformation’. Some treatments of the kinetics of mineral reactions are based on partitionless polymorphic phase transformations and are reviewed only briefly in this chapter. However, because most metamorphic mineral reactions are partitioning diffusive phase transformations, the following discussion will focus mainly on this reaction type.
In this chapter, three types of reactions that play a key role in metamorphic crystallization are addressed. During prograde metamorphism continuous supply of aqueous fluid by dehydration reactions may facilitate relatively rapid intercrystalline diffusion so that a state close to chemical equilibrium on the scale of mineral grains and beyond may be attained resulting in ‘porphyroblastic mineral growth’. Interface-reaction controlled and diffusion-controlled growth are two endmember models in the kinetics of porphyroblastic growth and differ in terms of the spatial extent of chemical equilibration and its influence on the distribution and compositional zoning of porphyroblasts. The first section of this chapter may serve as a review of some of the key works in metamorphic petrology addressing the factors that control the abundance and size distribution of porphyroblasts and their chemical zoning patterns.