Carbonic fluids in ultrahigh-temperature metamorphism: Evidence from Raman spectroscopic study of fluid inclusions in granulites from the Napier complex, East Antarctica

We report the first quantitative compositional data on fluid inclusions in ultrahigh-temperature (UHT) granulites from the Napier Complex of Enderby Land, East Antarctica. Fluid inclusions in various high-grade minerals such as garnet, orthopyroxene and sapphirine from three UHT localities in the Amundsen Bay area were studied in terms of petrography and microthermometry as well as laser Raman spectroscopy. Measured melting temperatures of inclusions from all the three localities indicate that the trapped fluid phase is dominantly carbonic. Raman analyses confirmed a near pure CO₂ composition with only minor dilutants such as N₂ (<6.0 mol%), CH₄ (<0.3 mol%), and H₂O(<0.1 mol%). CH₄-bearing fluid associated with sapphirine granulites suggests low oxygen fugacity (f O₂) conditions for the rocks, whereas CH₄ was not detected from fluid inclusions in magnetite-bearing high-f O₂ garnet granulite. The range of CO₂ isochores computed from density measurements in fluid inclusions from the granulites pass through the peak P-T conditions of the Napier metamorphism (T = 1050-1150 °C, P = 9-11kbar) indicating synmetamorphic nature of the fluids. Inclusions in garnet from Bunt Island coexist with carbonate minerals (magnesite) and graphite along with dense CO₂-rich fluid, indicating probable derivation from deep-seated primary magmatic sources. The ubiquitous association of carbonic fluids in the UHT mineral assemblages suggests CO₂ influx during extreme crustal metamorphism of the Napier Complex. The carbonic fluid probably played an important role in transporting heat from mantle or mantle-derived magmas and in stabilizing the dry mineral assemblages.