Isotope geochemistry and fluid inclusion study of skarns from Vesuvius

We present new mineral chemistry, fluid inclusion, stable carbon and oxygen, as well as Pb, Sr, and Nd isotope data of Ca-Mg-silicate-rich ejecta (skarns) and associated cognate and xenolithic nodules from the Mt. Somma-Vesuvius volcanic complex, Italy. The typically zoned skarn ejecta consist mainly of diopsidic and hedenbergitic, sometimes "fassaitic" clinopyroxene, Mg-rich and Ti-poor phlogopite, F-bearing vesuvianite, wollastonite, gehlenite, meionite, forsterite, clinohumite, anorthite and Mg-poor calcite with accessory apatite, spinell, magnetite, perovskite, baddeleyite, and various REE-, U-, Th-, Zr- and Ti-rich minerals. Four major types of fluid inclusions were observed in wollastonite, vesuvianite, gehlenite, clinopyroxene and calcite: a) primary silicate melt inclusions (T_HOM = 1000-1050°C), b) CO₂ ± H₂S-rich fluid inclusions (T_HOM = 20-31.3°C into the vapor phase), c) multiphase aqueous brine inclusions (T_HOM = 720-820°C) with mainly sylvite and halite daughter minerals, and d) complex chloride-carbonate-sulfate-fluoride-silicate-bearing saline-melt inclusions (T_HOM = 870-890°C). The last inclusion type shows evidence for immiscibility between several fluids (silicate melt - aqueous chloride-rich liquid - carbonate/sulfate melt?) during heating and cooling below 870°C. There is no evidence for fluid circulation below 700°C and participation of externally derived meteoric fluids in skarn formation. Skarns have considerably variable ²⁰⁶Pb/²⁰⁴Pb (19.047-19.202), ²⁰⁷Pb/²⁰⁴Pb (15.655-15.670), and ²⁰⁸Pb/²⁰⁴Pb (38.915-39.069) and relatively low ¹⁴³Nd/¹⁴⁴Nd (0.51211-0.51244) ratios. The carbon and oxygen isotope compositions of skarn calcites (δ13C_V-PDB = -5.4 to -1.1‰; δ¹⁸O_V-SMOW = 11.7 to 16.4‰) indicate formation from a ¹⁸O- and ¹³C-enriched fluid. The isotope composition of skarns and the presence of silicate melt inclusion-bearing wollastonite nodules suggests assimilation of carbonate wall rocks by the alkaline magma at moderate depths (< 5 km) and consequent exsolution of CO₂-rich vapor and complex saline melts from the contaminated magma that reacted with the carbonate rocks to form skarns.