The Occurrence of Forsterite and Highly Oxidizing Conditions in Basaltic Lavas from Stromboli Volcano, Italy

We report the occurrence of unusual, high-magnesium (Fo₉₆) olivinephenocrysts in a basaltic lava and an ejected lithic block fromthe Upper Vancori period (13 ka) and the recent activity (2002–2003)of Stromboli volcano, Italy. The samples that contain this distinctivemineral chemistry are a shoshonitic basalt and a basaltic andesitewith anomalous bulk-rock chemical characteristics in which theiron is highly oxidized (6–8 wt % Fe₂O₃ and <1 wt %FeO). In other respects these samples are similar to the majorityof Stromboli basalts, characterized by the coexistence of olivine,clinopyroxene, plagioclase and Fe–Ti oxides as phenocrysts,and clinopyroxene, plagioclase and Fe–Ti oxides in thegroundmass. In the high-magnesium olivine samples, Fe–Tioxides (pseudobrookite) typically occur as symplectitic intergrowthswith the olivine phenocrysts, indicating simultaneous growthof the two phases. We propose, as a paragenetic model, thatthe Fo₉₆ olivine phenocrysts crystallized from a highly oxidizedbasaltic magma in which most of the iron was in the ferric state;hence, only magnesium was available to form olivine. The highlyoxidized state of the magma reflects sudden degassing of volatilephases associated with instantaneous, irreversible, transientdegassing of the magma chamber; this is postulated to occurduring periods of sudden decompression induced by fracturingof the volcanic edifice associated with paroxysmic activityand edifice collapse. KEY WORDS: Stromboli; Mg-rich olivine; oxygen fugacity; redox state of magmas; irreversible processes