REVIEW OF FLUID AND MELT INCLUSIONS IN GEM MINERALS

Fluid and melt inclusions provide valuable information concerning the temperatures and pressures of formation of gemstone and the fluids responsible for their formation as well as fracture healing/heating treatment effects. Fluid and melt inclusions have been studied in a wide variety of gems, including diamond, emerald and corundum.

Fluid inclusions in fibrous diamonds have submicron size and are commonly referred to as high-density fluids, HDFs characterized by the presence of water, carbonates and minerals that precipitated during cooling. Fluid inclusion compositions indicate contributions from four distinct fluids/melts. The endmembers are: (1) hydrous-silicic melt enriched in H₂O, Si, Al and K; (2) Cl, K, Na brine; (3) C, Mg, Ca, Fe, K and Na carbonatitic melt; and (4) uncommon high-Mg carbonatitic melt. Typically fluid inclusions have compositions between either hydrous-silicic or carbonatitic melt and brine endmember, indicating immiscibility between silicic and carbonatitic endmembers. Within single deposit fluid inclusion composition may vary demonstrating melt evolution.

Fluid inclusions in emeralds show wide variability that reflects the environment of formation. Fluid inclusions in emeralds from pegmatites are characterized by low-salinity H₂O-CO₂-NaCl fluids, commonly including CH₄ (Franquera, Rila, Kianjavato deposits), that evolves to volatile-free low-salinity H₂O-NaCl. Shear zone-hosted deposits (Coscuez, Chivor, Vega and San Juan) are characterized by high-salinity, H₂O-NaCl-CaCl₂-KCl-CO₂-N₂ fluids.

Fluid inclusions in colored corundum are CO₂-bearing. Fluid inclusions in ruby often contain sulfur as H₂S, COS or S₈ phases. Often both fluid and melt inclusions occur in colored corundum (c.f., Denchai sapphire deposit, Weldborough and Barrington corundum fields), indicating magmatic and hydrothermal contributions during their formation.