The occurrence and distribution of Mo and molybdenite in unaltered peralkaline rhyolites from Pantelleria, Italy

Small hexagonal and triangular platelets of molybdenite (MoS₂), 5 to 25 μm in diameter, were identified in phenocrysts and matrix glass of unaltered felsic volcanic rocks from Pantelleria, Italy. The MoS₂ occurs commonly in pantellerites (peralkaline rhyolites), rarely in pantelleritic trachytes, and never in trachytes. The occurrence of euhedral MoS₂ platelets in all phenocryst phases, in matrix glass, and even in some melt inclusions indicates that MoS₂ precipitated directly from the peralkaline melt. Despite MoS₂ saturation, the melt (glass) contains greater than 95% of the Mo in Pantellerian rocks: X-ray fluorescence analyses of 20 whole rocks and separated glasses show that whole rocks consistently contain less Mo than corresponding matrix glasses, the differences being in proportion to phenocryst abundances. The Mo contents increase with differentiation from trachytes (2-12 ppm) to pantellerites (15-25 ppm) and correlate positively with incompatible elements such as Th, Y, and Nb. The Mo concentrations, as determined by secondary ion mass spectrometry, are essentially the same in matrix glasses and melt inclusions, showing that Mo did not partition strongly into a volatile fluid phase during outgassing. The high Mo contents of the pantellerites (relative to metaluminous magmas with 1-5 ppm) may be due to several factors: (1) the enhanced stability of highly charged cations (such as Mo⁶⁺, U⁴⁺, and Zr⁴⁺) in peralkaline melts; (2) the rarity of Fe-Ti oxides and litanite into which Mo might normally partition; (3) reduced volatility of Mo in low fO₂, H₂O-poor (1-2 wt%) peralkaline magmas. Geochemical modeling indicates that the precipitation of MoS₂ can be explained simply by the drop in temperature during magmatic differentiation. The occurrence of MoS₂ in pantellerites may result from their high Mo concentrations and low redox state ({up triangle, open}Ni/NiO=-2.5) relative to metaluminous magmas, causing them to reach MoS₂ saturation at magmatic temperatures. The apparent absence of MoS₂ microphenocrysts in more oxidized, metaluminous rhyolites may indicate that Mo is dissolved primarily as a hexavalent ion in those magmas.