Istituto di Geoscienze e Georisorse     
Cortes J., Wilson M., Condliffe E., Francalanci L. The occurrence of forsterite and highly oxidizing conditions in basaltic lavas from Stromboli volcano, Italy. In: Journal of Petrology, vol. 47 (7) pp. 1345 - 1373. Oxford University Press, 2006.
We report the occurrence of unusual, high-magnesium (Fo96) olivine phenocrysts in a basaltic lava and an ejected lithic block from the Upper Vancori period (13 ka) and the recent activity (2002-2003) of Stromboli volcano, Italy. The samples that contain this distinctive mineral chemistry are a shoshonitic basalt and a basaltic andesite with anomalous bulk-rock chemical characteristics in which the iron is highly oxidized (6-8 wt % Fe2O3 and <1 wt % FeO). In other respects these samples are similar to the majority of Stromboli basalts, characterized by the coexistence of olivine, clinopyroxene, plagioclase and Fe-Ti oxides as phenocrysts, and clinopyroxene, plagioclase and Fe-Ti oxides in the groundmass. In the high-magnesium olivine samples, Fe-Ti oxides (pseudobrookite) typically occur as symplectitic intergrowths with the olivine phenocrysts, indicating simultaneous growth of the two phases. We propose, as a paragenetic model, that the Fo96 olivine phenocrysts crystallized from a highly oxidized basaltic magma in which most of the iron was in the ferric state; hence, only magnesium was available to form olivine. The highly oxidized state of the magma reflects sudden degassing of volatile phases associated with instantaneous, irreversible, transient degassing of the magma chamber; this is postulated to occur during periods of sudden decompression induced by fracturing of the volcanic edifice associated with paroxysmic activity and edifice collapse.
URL: http://https://academic.oup.com/petrology
Subject Stromboli
Mg-rich olivine
oxygen fugacity
redox state of magmas
irreversible processes

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