EKU Faculty and Staff Scholarship



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Eight samples of pantelleritic lava and tuff and a lithic inclusion of trachyte from Pantelleria, Italy, have been thoroughly analyzed with an electron microprobe. These samples reveal fi ve different mineral assemblages if classifi ed by the presence of fayalite, aenigmatite, ilmenite, and magnetite: (1) augite + fayalite + ilmenite + magnetite, (2) augite + fayalite + ilmenite, (3) hedenbergite or sodian hedenbergite + fayalite + ilmenite + aenigmatite + quartz, (4) sodian hedenbergite or aegirine-augite + ilmenite + aenigmatite + quartz ± ferrorichterite, and (5) aegirine-augite + aenigmatite + quartz. Alkali feldspar (Or35–37) is present as the dominant phyric phase in each assemblage. Whole-rock silica and peralkalinity correlate strongly with the mineral assemblage: assemblage 1 is found in the sample with the lowest agpaitic index [A.I. = molar (Na + K)/Al] and silica concentration (A.I. < 1.31, SiO2 < 64.8 wt%) and equilibrated at 991–888°C at an oxygen fugacity between 0.7 and 1.1 log units below the FMQ buffer (FMQ – 0.7 to FMQ – 1.1). Assemblage 2 is associated with a higher agpaitic index and silica concentration (A.I. = 1.42, SiO2 = 67.1%) and equilibrated at ~794°C at FMQ – 0.5. Assemblage 3 is associated with a still higher agpaitic index and silica concentration (A.I. in the range 1.55 – 1.63, 66.8 < SiO2 < 67.8%) and equilibrated at 764–756°C at FMQ – 0.5 to FMQ – 0.2. Assemblage 4 is associated with a slightly higher agpaitic index and yet higher silica concentration (1.61 < A.I < 1.75, 67.6 < SiO2 < 72.0%) and equilibrated between 740–700°C at oxygen fugacities at or just below the FMQ buffer. Assemblage 5 is associated with the highest agpaitic index and highest concentration of silica (A.I. = 1.97, SiO2 = 69.7%) and equilibrated at <700°C at an oxygen fugacity just above the FMQ buffer in a “no-oxide” fi eld. Despite the paucity of two-oxide, two-pyroxene, or two-feldspar pairs, it may be possible to accurately constrain temperature and oxygen fugacity in peralkaline rocks with QUIlF equilibria given an equilibrium assemblage of fayalite, ilmenite, and clinopyroxene.

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The Canadian Mineralogist