Apatite is a superb mineral by which to investigate the nature of fluids that have passed through and altered a rock (metasomatic processes). Its ubiquity allows it to act as a reservoir for P, F, Cl, OH, CO2, and the rare earth elements. It is also a powerful thermochronometer and can be chemically altered by aqueous brines (NaCl–KCl–CaCl2–H2O), pure H2O, and aqueous fluids containing CO2, HCl, H2SO4, and/or F. Thus, apatite is the perfect tracker of metasomatic fluids, providing information on the timing and duration of metasomatism, the temperature of the fluids, and the composition of the fluids, all of which can feed back into the history of the host rock itself.