The bulk composition of fluid inclusions is generally determined by using the temperature of ice melting (i.e., freezing-point depression of H 2 O-NaCl) and relating this to the ice-melting curve in the two-component system H 2 O-NaCl. In reality, natural systems are more complex. Therefore, the bulk composition is referred to as equivalent wt.% NaCl to account for the presence of other, generally undetermined, cations (e.g., K, Ca, Fe, Mn, Mg). Inferences of a more complex chemical composition are generally made on the basis of the depression of the eutectic temperature for the H 2 O-NaCl system (-21.2°C), the other common solutes being CaCl 2 , MgCl 2 , and FeCl 2 . Such an approach is qualitative at best, and the inherent problem of metastability invites caution. An alternative, relatively simple and cost-effective method among the many analytical methods available, is to artificially decrepitate fluid inclusions and analyze the resulting evaporate mounds with an electron microprobe equipped with an imaging system. To illustrate the potential of the method, three granitic environments, in Nova Scotia and in Peru, were chosen, two mineralized (Sn, Cu, Zn) and one barren (tourmaline pegmatite). Standard fluid-inclusion thermometric data have been integrated with compositions determined from evaporate mounds in the same suite of samples. The following points emerge: (1) the inclusions are invariably multi-component, being dominated by Na-K-Ca, but in consistently different proportion that reflect protracted fluid:rock interaction and mixing of reservoirs; (2) the low first-melting temperatures (i.e., <-50°C) do not unequivocally indicate the presence of CaCl 2 in solution; (3) high concentrations of Fe and Mn are common within the mineralized systems examined, in addition to lesser amounts of Sr, Ba, P and Zn, and (4) the two methods complement each other, together providing a much more comprehensive account of the nature of the fluid and its evolution in space and time.Keywords: fluid inclusions, evaporate mounds, granites, pegmatites, tin mineralization. Nova Scotia, Peru.
SOMMAIRELa composition globale de la phase fluide piégée dans les cristaux est généralement établie au moyen de la température de fusion de la glace, c'est-à-dire, l'abaissement du point de congélation de l'eutectique du système H 2 O-NaCl, mis en relation avec la courbe décrivant la fusion de la phase solide dans le système binaire H 2 O-NaCl. En fait, les systèmes naturels sont plus complexes. Pour cette raison, la composition globale du fluide est exprimée en termes d'équivalents de NaCl afin de rendre compte d'autres cations, non déterminés, par exemple K, Ca, Fe, Mn, Mg. Des conclusions à propos de la composition chimique plus complexe sont généralement faites pour rendre compte de l'abaissement de l'eutectique en dessous de -21.2°C, sa valeur dans le système H 2 O-NaCl. Les autres solutés courants sont CaCl 2 , MgCl 2 , et FeCl 2 . Une telle approche est définitivement qualitative, et le problème de métastabilité mérite une att...