Journal of Petrology Advance Access originally published online on March 22, 2006
Journal of Petrology 2006 47(7):1345-1373; doi:10.1093/petrology/egl012
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The Occurrence of Forsterite and Highly Oxidizing Conditions in Basaltic Lavas from Stromboli Volcano, Italy
1 SCHOOL OF EARTH AND ENVIRONMENT (EARTH SCIENCES), UNIVERSITY OF LEEDS LEEDS LS2 9JT, UK
2 DIPARTIMENTO DI SCIENZE DELLA TERRA, UNIVERSITÀ DEGLI STUDI DI FIRENZE VIA LA PIRA, 4, I-50121 FIRENZE, ITALY
RECEIVED FEBRUARY 16, 2005; ACCEPTED FEBRUARY 15, 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.
KEY WORDS: Stromboli; Mg-rich olivine; oxygen fugacity; redox state of magmas; irreversible processes
*Corresponding author. Present address: Department of Geology, University at Buffalo, 876 Natural Sciences Complex, Buffalo, NY 14260-3050, USA. E-mail: caco{at}buffalo.edu
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