Taking the pressure out of the Archean Metamorphism: The role of CO2 in assessing metamorphic conditions of the Isua supracrustal belt, W-Greenland

The Eoarchean Isua Supracrustal Belt (ISB), West Greenland exposes one of the oldest rock records on Earth. Its tectonic setting is still subject of debate with interpretations ranging from plate tectonics to vertical tectonics. Information on the origin (mantle rocks vs. cumulates) and metamorphic P-T-X evolution of ultramafic lenses exposed in the Western part of the ISB may hold the key to shed substantial light on the geodynamic evolution. While the geochemical signatures are non-conclusive in view of the origin, there is a general agreement of high to ultrahigh pressure conditions for the metamorphic overprint based on the presence of titanian clinohumite und titanian chondrodite, phases typically reported as indicative for high pressure metamorphism in ultramafics.

In this contribution we present results assessing the role of CO2 using a combination of textural analysis and thermodynamic calculations. It is shown that the stability fields of the observed mineral assemblage is strongly controlled by the fluid composition (XCO2). While high pressure conditions (> 2 GPa) are limited to a narrow range of fluid composition, thermodynamic modelling reveals that the formation of the observed reaction textures including the formation of the Ti-phases can be readily achieved via cooling at amphibolite facies conditions (580 to 500 °C at 1 GPa) in the presence of a CO2-bearing fluid in agreement with the PT conditions determined for the rest of the ISB. Hence, the general wisdom of Ti-clinohumite and Ti-chondrodite being unequivocal indicative of high to ultrahigh pressure conditions in ultramafic rocks has to be abounded.