Introduction

Many igneous and metamorphic rocks contain a fluid component or evidence of the former existence of such a fluid. Fresh lavas and pumice blocks commonly contain vesicles, formed by the exsolution of gas from the silicate fluid. Metamorphic rocks may preserve evidence of partial melt pockets. The term ‘pore’ is used here to indicate space filled, or formerly filled, with fluid surrounded by a more viscous fluid (e.g. silicate melt, glass) or crystals. Pores may be considered as the fluid equivalent of grains or crystals in solid phases. Fluid inclusions in crystals are excluded here as their textures have yet to be studied quantitatively.

Pores do not always stay filled with their original fluid during lithification. For instance, vesicles may be filled with other minerals to become amygdules. In partially molten silicate rocks the fluid may quench to a glass, or a mixture of minerals, whose textures pseudomorph the original pores. Hence the porosity of a rock, as discussed here, is the original volume proportion of fluid at the time of interest. For example, the porosity of a lava sample is the volumetric proportion of vesicles at the moment of solidification; the porosity of a partially molten rock is the volumetric proportion of fluid when the texture is preserved and such a melt may have gas bubbles. Clearly, it is important to specify what feature is being examined.