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

Abstract

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 (▵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.