Initial isotopic heterogeneity and secondary disturbance of the SmNd system in fluorites and fluid inclusions: A study on mesothermal veins from the central and western Swiss Alps

Abstract

This paper presents SmNd data on hydrothermal fluorites, fluid inclusions and leach experiments on samples from the Mont-Blanc and Gotthard units, Swiss Alps. Both areas have experienced Alpine greenschist-facies overprint (≤350° and ∼450°C, respectively). Mont-Blanc samples represent late Variscan fluorites from three individual veins whereas Gotthard samples were taken from one hand-specimen of probable late Variscan age. Nd concentrations of fluorites range from 3.6 to 23 ppm, 147 Sm 144 Nd ratios from 0.12 to 0.35. None of the samples sets yielded an isochron relationship. Imperfect correlations indicate apparent ages between late Variscan and Alpine, reflecting neither the formation nor the age of metamorphic overprint. Reasons are wall-rock influence during formation and post-formation disturbance. Secondary fluorite (Gotthard) shows the highest Nd concentration and a low Sm/Nd ratio, close to that of fluid inclusion fractions. A leach experiment showed the same trend even more pronounced: ∼7% of Nd and ∼2% of Sm were removed from fluorite by 0.18 N HCl at 100°C. Hence, secondary fluids are likely to leach and fractionate Sm and Nd, and may easily cause dissolution and reprecipitation of fluorite. Consequently, the system is highly suspicious if secondary fluid flow cannot totally be ruled out. This is in sharp contrast to previously published conclusions. However, evidence for rejuvenation of the SmNd system in fluorites is also found in published data sets.