Spherulite crystallization induces Fe-redox redistribution in silicic melt

• Published in: Chemical Geology
• Main Authors: Castro, Jonathan M., Cottrell, Elizabeth, Tuffen, H., Logan, Amelia V., Kelley, Katherine A.
• Other Authors: Institut des Sciences de la Terre d'Orléans (ISTO), Université de Tours-Institut national des sciences de l'Univers (INSU - CNRS)-Université d'Orléans (UO)-Centre National de la Recherche Scientifique (CNRS), Department of Mineral Sciences Washington, Smithsonian Institution, Department of Environmental Science Lancaster, Lancaster University, Graduate School of Oceanography Narragansett, University of Rhode Island (URI)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2009
• Subjects: Obsidian; Spherulite; Oxidation–reduction; FTIR; XANES; [SDU.STU.VO]Sciences of the Universe [physics]/Earth Sciences/Volcanology; [SDE.MCG]Environmental Sciences/Global Changes; Krafla; Iceland
• Online Access: https://hal-insu.archives-ouvertes.fr/insu-00442797; https://hal-insu.archives-ouvertes.fr/insu-00442797/document; https://hal-insu.archives-ouvertes.fr/insu-00442797/file/Castro-ChemicalGeology-2009.pdf; https://doi.org/10.1016/j.chemgeo.2009.09.006

International audience Rhyolitic obsidians from Krafla volcano, Iceland, record the interaction between mobile hydrous species liberated during crystal growth and the reduction of ferric iron in the silicate melt. We performed synchrotron µ-FTIR and µ-XANES measurements along a transect extending from a spherulite into optically distinct colorless and brown glass zones. Measurements show that the colorless glass is enriched in OH groups and depleted in ferric iron, while the brown glass shows the opposite relationship. The color shift between brown and clear glass is sharp, suggesting that the colorless glass zone was produced by a redox front that originated from the spherulite margin and moved through surrounding melt during crystallization. We conclude that the most likely reducing agent is hydrogen, produced by magnetite crystallization within the spherulite. The Krafla obsidians dramatically capture redox disequilibrium on the micoscale and highlight the importance of hydrous fluid liberation and late-stage crystallization to the redox signature of glassy lavas.