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

摘要

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 N-2 (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 N-2. Although the concentrations of N-2, 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 N-2). 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/N-2, C/Ar, and N-2/Ar ratios, isotopically heavy N-2, and somewhat elevated (to 530) Ar-40/Ar-36 ratios. The younger fluids typically have higher (by two to three orders of magnitude) C/N-2 and C/Ar ratios, lower delta C-13 of CO2, and N-2/Ar and Ar-40/Ar-36 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 Ar-40/Ar-36 ratios, which are atypical of the mantle, and an increase in the H2O concentration, suggests a subduction-related nature of the fluids.