INTRODUCTION

Approximately 5 wt% H2O must be driven from a shale to convert it to a high-grade metapelitic rock. Conversely, several wt% H2O must be added to an anhydrous basalt to convert it to a chlorite schist. Clearly, metamorphism in these cases must involve the transport of considerable quantities of H2O. Similarly, CO2 released by decarbonation reactions must also be transported away from the rock. At the temperatures and pressures normally extant during metamorphism, H2O forms a supercritical fluid, which has a density similar to that of water at room temperature and pressure. The fluid has a low viscosity, and can therefore flow through fractures or an interconnected pore network. This hot fluid, in equilibrium with a rock, is certainly not pure H2O but, instead, contains considerable amounts of dissolved ions. The loss or addition of such a fluid phase during metamorphism can therefore change the bulk composition of a rock in ways other than simply modifying its content of H2O and other volatiles.

When a metamorphic rock undergoes significant changes in composition, the process of change is referred to as metasomatism. In extreme cases, metasomatism can completely change the composition of a rock, as, for example, in the conversion of limestone to magnetite ore bodies at contacts with igneous intrusions. Despite clear evidence for such major compositional changes, metasomatism was the subject of much controversy during the first half of the twentieth century.