The corundum conundrum: Constraining the compositions of fluids involved in metasomatic corundum formation.
<p>Corundum, including the variety ruby, is found in numerous locations in the Archaean North Atlantic Craton of southern Greenland. Corundum owes its occurrence to fluid-induced interaction among high-grade metamorphic lithologies of contrasting chemistry. Here, we present constraints on the conditions of corundum formation and the compositions of the fluids involved for the Stor&#248; and Maniitsoq ruby localities. We use thermodynamic modelling of mineral and mineral-fluid equilibria, and complement these with experimentally obtained data on mineral solubility to show that metasomatism took place at 650-725&#730;C and 7 kbar, involving a boron-rich, acidic fluid of low <em>f</em>O<sub>2</sub> and low X(CO<sub>2</sub>). Aqueous concentrations of aluminium are low and indicate that corundum saturation is the result of residual aluminium enrichment rather than aluminium mobilisation. Intrusion of the <em>ca.</em> 2.55 Ga Q&#244;rqut granite and associated fluid release is the likely source of boron, and U-Pb dating of rutile inclusions is consistent with a temporal link between ruby formation and granite emplacement. Interaction with meta-dunite and Fe-sulfides modified the oxidized magmatic fluid, introduced SO<sub>4</sub>, and produced the reduced, high X<sub>Mg</sub> and K-rich fluid recorded by the corundum-bearing samples. These results highlight a complex interplay among lithologies involved in corundum-formation, but also demonstrate that corundum formation is a predictable part of the geological history where a magmatic intrusion expels a pulse of fluid through its lithologically heterogeneous carapace.</p>