Amphibolite facies conditions recorded in CO2-bearing fluid-mediated reaction fronts preserved in an ultramafic lens from the Isua supracrustal belt, southern West Greenland

Abstract

The Eoarchean Isua supracrustal belt (ISB) in southern West Greenland exposes one of the oldest rock records on Earth. Its tectono-metamorphic evolution remains debated, particularly with respect to its timing and metamorphic peak conditions recorded by the various mafic to ultramafic lithologies. A first-order question is the presence or absence of metamorphic gradients within the belt. To that end, phase equilibria modelling combined with multiple geothermobarometric data by our group suggested a homogeneous distribution of peak metamorphic conditions (550-600 °C; 0.8-1.0 GPa). However, recent studies of a specific dunite lens from the ISB suggested different metamorphic conditions, varying from ultra-high pressure (UHP) to prograde low-pressure deserpentinization. In this study, the metamorphism affecting ultramafic rocks from the specified lens B in the northwestern limb of the ISB is investigated along a transect from the rim towards the center of the lens. A total of four samples are examined for their petrography, mineral assemblages, and reaction textures, using EPMA, SEM, EBSD, ICP-MS data together with thermodynamic modelling. Results reveal a strong foliation in the rim of the lens, with antigorite (XMg=0.91) + magnesite + magnetite ± ilmenite forming the stable assemblage. Deformation decreases towards the center of the lens, where antigorite (XMg=0.98) + fosterite (XMg=0.97) + magnesite + magnetite ± Ti-chondrodite/Ti-clinohumite form the observed assemblage. The presence of accessory Ti-phases has been used as an indicator for retrograde decompression after UHP metamorphism through the breakdown reaction of Ti-chondrodite to form Ti-clinohumite. We present textural evidence of Ti-clinohumite being replaced by Ti-chondrodite, pointing to a different reaction along a cooling path at lower pressures. The presence of carbonate instead of brucite together with the absence of talc highlights the role of CO2 – which must be assessed to accurately describe phase relations for the metamorphic evolution of ultramafic rocks. Thermodynamic modelling indicates that antigorite stability is strongly CO2-dependent, limiting the stability of antigorite + magnesite to pressures < 1 GPa at XCO2 > 0.005. EBSD analysis reveals clear evidence of olivine experiencing internal deformation, and thus preceding the growth of antigorite in the sample. A second observation further supports this finding: Magnetite, magnesite and Ti-humite phases are spatially associated with olivine breakdown reaction textures. In summary, this study suggests that UHP conditions or deserpentinization are not required to explain the metamorphic records of these mafic lenses, thus removing a strong argument for the ISB to exhibit a metamorphic gradient and UHP rocks. Instead, we interpret the lens to exhibit a fluid-mediated reaction front that can be readily achieved at the homogeneous, amphibolite-facies conditions reported for the rest of the belt.