PVT parameters of fluid inclusions and the C,O, N,and Ar isotopic composition in a garnet lherzolite xenolith from the Oasis Jetty,East Antarctica

Orthopyroxene, clinopyroxene, and olivine from a metasomatized mantle xenolith of garnet lherzolite in alkaline rocks at the Jetty Oasis, East Antarctica, contain numerous carbon dioxide-dominated composite melt-fluid and fluidized sulfide-silicate (±carbonate) inclusions. Although the maximum pressure under which the inclusions were captured by rock-forming minerals was evaluated at 13 kbar, its actual value should have been much higher, judging by the fact that the inclusions have lost part of their material (decrepitated) when the xenolith was brought to the surface. Two major fluid populations are distinguished. The fluids entrapped during the earlier episode have a more complicated composition. Dominated by CO₂, these fluids contain much N₂ (0.1–0.2 mole fractions), H₂S, and perhaps, also H₂O and are hosted by sulfide-silicate (±carbonate) inclusions produced by liquid immiscibility. As these inclusions evolved, they enriched in CO₂ and depleted in H₂S and N₂. Although the concentrations of N₂, H₂S, and H₂O were generally relatively low, these components played an important role in mantle metasomatism, as is reflected in the geochemistry of the derived magmas. The fluids of the younger episode (pressures lower than 7 kbar) are notably richer not only in CO₂ but also in H₂O (up to the appearance of inclusions with a liquid aqueous phase and the formation of CO₂ gas hydrate when cooled in a cryometric stage by liquid N₂). The effect of fluids on the mantle source in two discrete episodes is also confirmed by isotopic-geochemical data. Isotopic data on gases obtained immediately from fluid inclusions in minerals by the stepwise crushing technique provide evidence of the evolution of elemental and isotopic ratios of the gases in the course of the metasomatic processes. The high-pressure fluid inclusions of the earlier episode have low C/N₂, C/Ar, and N₂/Ar ratios, isotopically heavy N₂, and somewhat elevated (to 530) ⁴⁰Ar/³⁶Ar ratios. The younger fluids typically have higher (by two to three orders of magnitude) C/N₂ and C/Ar ratios, lower δ¹³C of CO₂, and N₂/Ar and ⁴⁰Ar/³⁶Ar ratios close to the atmospheric values. The nitrogen and argon isotopic compositions and elemental ratios suggest that the younger fluids could have been produced by two-component mixing in the mantle-atmosphere system. Comprehensive analysis of the data and in particular the ⁴⁰Ar/³⁶Ar ratios, which are atypical of the mantle, and an increase in the H₂O concentration, suggests a subduction-related nature of the fluids.