High-pressure behavior of djerfisherite: Implication for its origin in diamonds and mantle xenoliths

Djerfisherite K6(Fe,Cu,Ni)25S26Cl is a widespread mineral in kimberlite-hosted mantle xenoliths and diamonds. It usually occurs as rims around primary Fe-Ni-Cu sulfides. It is assumed that metasomatic origin of djerfisherite is related to interaction of primary sulfides with the hypothetical K-Cl-rich fluid or melt. However, source and composition of metasomatic agent as well as PT-conditions of the interaction is unclear. There are two basic hypothesis of djerfisherite origin in diamonds and mantle xenoliths: (1) in-situ mantle metasomatism; (2) infiltration of kimberlite melt into mantle nodules during magma ascent. To date, djerfisherite was synthesized only at 1 atm and 350-650 oC (Clarke, 1979) and at 0.1 GPa and 470 oC (Gorbachev and Nekrasov, 1980). We performed first experimental study on the stability of this sulfide at high pressures. The experiments were performed at 3 GPa nd 800-1200 oC. Also we have planned to perform experiments at 6 GPa, however, it was not necessary due to instability of djerfisherite at lower pressures. As starting materials we used natural Cu-rich djerfisherite from peralkaline pegmatites of Khibina massif (Kola peninsula, Russia) and synthetic mixtures with the following compositions (in mol.%) (Fe18Cu2Ni5)25S26 + 5KCl + 3.5K2CO3 and (Fe18Cu2Ni5)25S26 + 6KCl + K2CO3 + 3.5Na2CO3 + 8Mg2SiO4. Potassium content was in excess by an addition of K2CO3. Sulfide portion of synthetic mixtures compositionally corresponds to djerfisherite in diamonds and mantle xenoliths. The results of experiments showed that djerfisherite is unstable at 3 GPa, or higher pressures. This indicates that djerfisherite in diamond, diamond-bearing garnet peridotite could not be crystallized in cratonic lithosphere mantle in-situ. Consequently, djerfisherite formation in mantle nodules is a result of their interaction with kimberlite melt during ascent of magma at the subsurface conditions. Instead of djerfisherite we synthesized two new potassium sulfides: K0.08-0.22Fe0.88-0.96Cu0.72-0.48Ni0.14-0.20S2 in synthetic mixtures; and KCu2FeS3 in natural djerfisherite system. These K-bearing sulfides can be considered as a possible concentrator of K in the mantle, whereas stability of djerfisherite should be studied in details at pressures below 3 GPa.