Solidification History of the Kitdl t Lens: Immiscible Metal and Sulphide Liquids from a Basaltic Dyke on Disko, Central West Greenland

A unique troilite-iron-cohenite-rich lens was found on a ledge in the sediment-contaminated Kitdlit dyke on Disko. The lens displays a mineralogical diversity and extreme liquid evolution not previously described from any single magmatic sulphide or metal liquid system. It was deposited at 1200 °C and fo2 = 10−13 as two immiscible liquids–a sulphide liquid and a C-rich metal liquid—which solidified from about 1100° to 300 °C as a closed system in a thermal gradient. The lower iron-rich half of the lens formed by crystallization of iron, cohenite, troilite, schreibersite and wüstite (in that order), with trace amounts of the phases found in the upper half of the lens. The approximate crystallization order of the upper troilite-rich half of the lens was: iron, cohenite, troilite, chromite, wüstite, fayalite (pseudomorphosed), high-T ‘chalcopyrite’, high-T ‘heazlewoodite’ (beta-(Ni, Fe, Cu, Co)3+×S2) and, finally, lead minerals. The latter comprise native lead, galena, altaite (PbTe), shandite (Ni3Pb2S2) and an unidentified phase. Additional immiscible liquids were formed during solidification. Oxysulphide liquid (roughly FeO with a minor FeS-component) exsolved from metal and sulphide liquid in equilibrium with iron and cohenite below 1100 °C. Later, FeO-rich silicate liquid, and Pb-rich liquid with Ni, Cu, S and Te, exsolved from the sulphide liquid. Segregation of oxysulphide and Pb-rich liquid may occur during core formation in planetoids, and there are thus important cosmochemical implications.