Palaeofluid chemistry of a single fluid event: a bulk and in-situ multi-technique analysis (LIBS, Raman Spectroscopy) of an Alpine fluid (Mont-Blanc)

Abstract

The only direct evidence of palaeofluids that circulated in the crust is contained within fluid inclusions. To properly understand such processes requires the complete anion and cation analysis of the fluid inclusions, which are examples of single fluid events. This analysis is a major analytical challenge when an average inclusion (<80 μm) contains only 10 −9 g of material. An integrated multi-technique approach has been applied to the analysis of single inclusions from the water–chloride (+CO 2) system. It includes, after a careful selection of the inclusion and its microthermometric study, the use of non-destructive techniques (Raman spectroscopy for Cl − content and gas ratios) followed by destructive techniques such as laser-induced breakdown spectroscopy for major element ratios, in parallel to a bulk determination using a miniaturised crush leach technique on the whole fluid population. The laser-induced breakdown spectroscopy technique was used in this study for the first time in a routine way for fluid inclusion analysis. It provided Na/K, Na/Ca and Na/Li ratios. Detection limits are low and adequate for the determination of ion concentrations in individual inclusions (10 ppm for Na and Li, 20 ppm for Ca and 750 ppm for K). The application was carried out on a single fluid event typical of Alpine retrograde metamorphism that affected the Mont-Blanc granite (Glacier du Géant, France). The fluids are mostly aqueous (H 2O is dominant, XCl around 1 mol% with X=Na (930±90 mmol/kg H 2O), K (135±35 mmol/kg H 2O) Li (30±8 mmol/kg H 2O), Ca (105±25 mmol/kg H 2O), and small amounts of CO 2+N 2 mol%). The reconstruction of the fluid composition, using major element ratios obtained by the laser-induced breakdown spectroscopy technique, is in good agreement with the estimate obtained by the crush leach method on the same sample that is dominated by a single fluid population. The low Na/Li and Na/K are indicative of interaction with the host rocks at high temperature, the latter being indicative of equilibrium with two alkali feldspars at the temperatures estimated for the fluid event.