Paragenesis and composition of laurite from chromitites of Othrys (Greece): implications for Os-Ru fractionation in ophiolitic upper mantle of the Balkan peninsula

Os-rich laurite and erlichmanite are the dominant PGM inclusions in chromit ites of the Othrys ophiolite complex. Close association of the PGM sulfides with enstatite, Na-rich pargasite, clinopyroxene, phlogopite, and Cu-Ni su lfides indicates crystallization at high temperature, in the presence of an alkali-rich fluid phase, under relatively-high sulfur fugacity. Because of the predominance of Ru-Os-Ir phases, the PGM assemblage of Othrys is simil ar to that of chromitites located in the mantle unit of other ophiolite com plexes of the Balkan peninsula (Vourinos, Skyros Island, Rhodope) although it is distinguished because of the Os-rich composition of laurite and the p resence of erlichmanite. Comparison among literature data for these complex es indicates that the Os/Ru ratios vary consistently in laurite and bulk ch romitite with respect to the chondritic Os/Ru value. This suggests that fra ctionation between the two elements occurred, being apparently registered i n the composition of laurite inclusions. Among other factors, fluctuation o f sulfur fugacity during fractionation, as well as variation of the Os/Ru r atio in the parent melts of the chromitites might be invoked to explain the Ru-Os decoupling.