Fluid-Mediated Metamorphism in Ultramafic Rocks from the Isua Supracrustal Belt: Evidence Against UHP Conditions

The Eoarchean Isua supracrustal belt (ISB) in southern West Greenland contains one of Earth's oldest rock records. Its tectono-metamorphic evolution remains debated, particularly regarding timing and peak conditions in mafic to ultramafic lithologies. Phase equilibria modelling suggested homogeneous peak metamorphic conditions (550-600 °C; 0.8-1.0 GPa), though recent studies of a dunite lens suggested ultra-high pressure (UHP) to prograde low-pressure deserpentinization.

This study examines metamorphism in ultramafic rocks from lens B in the northwestern ISB, analyzing four samples from rim to center using EPMA, SEM, EBSD, ICP-MS and thermodynamic modelling.

The lens rim shows strong foliation with antigorite (X_Mg=0.91) + magnesite + magnetite ± ilmenite. The center shows decreased deformation with antigorite (X_Mg=0.98) + fosterite (X_Mg=0.97) + magnesite + magnetite ± Ti-chondrodite/Ti-clinohumite.

We found Ti-clinohumite being replaced by Ti-chondrodite, indicating cooling at lower pressures. The presence of carbonate without brucite or talc emphasizes CO₂'s role in phase relations. Thermodynamic modelling shows antigorite stability is CO₂-dependent, limiting antigorite + magnesite stability to pressures < 1 GPa at XCO₂ > 0.005. EBSD analysis shows olivine deformation preceded antigorite growth, supported by magnetite, magnesite and Ti-humite phases associated with olivine breakdown.

This study suggests the lens exhibits a fluid-mediated reaction front achievable at homogeneous amphibolite-facies conditions, rather than requiring UHP conditions or deserpentinization.