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Journal of Petrology Advance Access published online on March 22, 2006
Journal of Petrology, doi:10.1093/petrology/egl012
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© The Author 2006. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org
Received February 16, 2005
Accepted February 15, 2006

Article

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

JOAQUÍN A. CORTÉS 1 *, MARJORIE WILSON 1, ERIC CONDLIFFE 1, and LORELLA FRANCALANCI 2

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

* To whom correspondence should be addressed.
JOAQUÍN A. CORTÉS, E-mail: caco{at}buffalo.edu


  Abstract

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.

Keywords: Stromboli; Mg-rich olivine; oxygen fugacity; redox state of magmas; irreversible processes.
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