Textural Equilibrium in Layered Igneous Rocks

Abstract

Rocks of many layered intrusions, adcumulate rocks in particular, are in local textural equilibrium; the shape and contiguity of grains are controlled by surface energies. Existing cumulus theory takes no account of the effects of surface energy in determining the textures which form in layered intrusions. Textural equilibrium involves the balance and minimisation of local interface energies. At equilibrium, grain boundaries possess constant curvature and dihedral angles at triple junctions between combinations of phases are constant. Monomineralic adcumulates form the most easilly recognisable examples of equilibrium textures; however, bimineralic and polymineralic rocks are also in equilibrium and many of the features of poikilitic heteradcumulates are consistent with textural equilibrium.

At least in the larger intrusions, crystallisation probably takes place under conditions of textural equilibrium. In crystal suspensions, grain coalescence and grain growth are important processes. They are driven by differences in local grain boundary curvature; crystals change shape by a process of solution-reprecipitation with diffusive transport through the melt. Porosity may locally be reduced in an attempt to achieve a stable melt fraction dependant on the ambient melt-matrix dihedral angle. Such processes lead to heterogeneous porosities on a scale of cm and have important implications for notions of initial, settling and residual porosities but probably cannot produce pure adcumulate rocks. Diffusion and solutal convection may aid to reduce the porosity but compaction is required to explain monomineralic adcumulate rocks. Textural equilibrium is important for compaction since it provides a mechanism whereby the grains can change shape and the matrix deform and also remain permeable by maintaining an interconnected porosity. Providing sufficient time is available, textural equilibration will continue into the subsolidus. The shapes of grains will be dependant on solid-solid interface energies; their geometry will, in general, not be that when crystals first touched in the presence of melt and the late-stage geometry of the melt framework will not be preserved.