The corundum conundrum: Constraining the compositions of fluids involved in ruby formation in metamorphic melanges of ultramafic and aluminous rocks

Abstract Corundum, including the variety ruby, is found in numerous locations in the North Atlantic Craton of southern Greenland where high-grade metamorphic lithologies of contrasting chemistry interact. This study constrains the conditions of corundum formation and the compositions of the fluids involved for the Storo and Maniitsoq localities using thermodynamic modelling of mineral and mineral-fluid equilibria, and mineral solubility. Metasomatism took place at 650–725 °C and 7 kbar, which is indistinguishable from peak metamorphic conditions, and involved an acidic metasomatising fluid of low fO2, low X(CO2), and rich in B. Aqueous concentrations of Al are low and indicate that corundum saturation is the result of residual enrichment rather than Al mobilisation. An evolved magmatic intrusion is the likely source of B, and U Pb dating of rutile inclusions is consistent with a temporal link between corundum formation and emplacement of the Qorqut granite. Interaction with metadunite, and metapelite-hosted Fe-sulfides modified the granite-derived oxidized fluid, introduced SO4, and produced the required reduced, low pH, high XMg and high K/Na fluid as constrained from the corundum-bearing samples. These results highlight the complex interplay among lithologies involved in corundum-formation in a high-grade metamorphic-metasomatic setting, but also demonstrate that corundum formation is a predictable part of the geological history where a magmatic intrusion sends a pulse of fluid through a lithologically heterogeneous carapace that includes aluminous and Si-deficient units.