Abstract
The opaque minerals in eclogite xenoliths from Stockdale Kimberlite are rutile, ilmenite, and a complex polysulfide assemblage. Rutile shows exsolutions of ilmenite and spinel. Discrete ilmenite contains up to 10 wt % MgO in solid solution and is a primary mineral, but not of kimberlitic origin. Pyrrhotite containing exsolved pentlandite is the major sulfide mineral, and is usually rimmed by chalcopyrite which may display exsolution of cubanite. A veneer of monosulfide solid solution (12 wt % Ni and 5 wt % Cu) forms a rim on the chalcopyrite-pyrrhotite masses.
The simple model of sulfide liquid immiscibility within a silicate melt may account for the origin of the pyrrhotite-pentlandite-chalcopyrite assemblage, but it fails to explain the occurrence within one and the same sulfide globule of a monosulfidess rim, separated from an exsolved pyrrhotite core by chalcopyrite.
The monosulfidess is probably a metastable phase produced by the partial melting of a preexisting sulfide assemblage of similar bulk chemical composition to that existing at present. The melting possibly took place instantaneously when the eclogite was incorporated into the rising hot kimberlitic magma. Fast cooling during the explosive ascent of the kimberlite could have led to the quenching of the monosulfide solid solution.
Rutile in the eclogite xenolith was unaffected by the heating, but secondary amphibole and biotite may have possibly formed during this event.
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Meyer, H.O.A., Boctor, N.Z. Sulfide-oxide minerals in eclogite from Stockdale Kimberlite, Kansas. Contr. Mineral. and Petrol. 52, 57–68 (1975). https://doi.org/10.1007/BF00378001
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DOI: https://doi.org/10.1007/BF00378001