Pore-space distribution and transport properties of an andesitic intrusion

- We characterize the pore space of an andesitic intrusion by X-ray microtomography.
- We observe a power law distribution of pore volumes.
- Power law scaling is attributed to coalescence of pores at crystal-melt boundaries.
- Weathering of the andesite is associated with preferential growth in the larger pores.
- Grain boundarity networks preferentially connect large pores to system boundaries.

The pore structure of magmatic rocks records processes operating during magma solidification and cooling. It has first order effects on the petrophysical properties of the magmatic rocks, and also influences mass transfer and mineral reactions during subsequent metamorphism or weathering. Here, the pore space characteristics of an andesitic sill intrusion were determined by multiscale resolution computed X-ray microtomography (μ-CT), and the 3D structure was used for transport modeling.Unaltered andesite has a power law distribution of pore volumes over a range of five orders of magnitude. The probability distribution function (PDF) scales with the inverse square of the pore volume (V), PDF∝V−2. This scaling behavior is attributed to the coalescence of pores at crystal-melt boundaries. Large pores are concentrated on the outer margins of amphibole and plagioclase phenocrystals. Incipient weathering of the andesite is associated with preferential growth of weathering products in the largest pores. This can be explained by a model in which diffusion of external components into the porous andesite is controlled by a random network of grain boundaries and/or microfractures. This network preferentially links the larger pores to the system boundaries and it is the major fluid transport pathway, confining incipient weathering into a small fraction of the rock volume only.