Carbonic fluid inclusions in a garnet–pyroxene granulite from Austhovde in the Lützow–Holm Complex, East Antarctica: Implications for a decompressional P–T path

Abstract

A garnet–pyroxene granulite from Austhovde in the Lützow–Holm Complex, East Antarctica, contains two types of fluid inclusions: a small number of primary inclusions in garnet and quartz, and dominant secondary inclusions in garnet and plagioclase. The melting temperatures of the trapped fluids lie in a range of −57.2 to −56.4 °C, which are close to the triple–point temperature of pure CO₂. The primary inclusions are homogenized at +1.8 to +12.6 °C, which correspond to densities of 0.842–0.917 g/cm³. However, the estimated pressure conditions (3–5 kbar at 850 °C) from the fluid densities of the primary inclusions are not consistent with the peak P–T conditions recorded in the rock (840–860 °C and 8.3–8.7 kbar). Homogenization of the secondary inclusions into the liquid phase occurs within a range of +11.5 to +25.2 °C, which correspond to low CO₂ densities of 0.708 to 0.851 g/cm³. The results imply that fluid density decreased by partial leakage of the trapped fluid in the primary inclusions during post–peak exhumation stage, and the process is consistent with the occurrence of orthopyroxene + hornblende + plagioclase symplectite around garnet, suggesting post–peak rapid decompression.