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Deciphering Fe- and S-XANES in melt inclusions with silicate-carbonate immiscibility: a case study from Hanang volcano (Tanzania)

Céline Baudouin 1, Hugo Moreira2,3, Charles Le Losq4,5, Max Wilke6, Fleurice Parat2

  • Affiliations: 1ISTeP - Earth sciences institute - CNRS - Sorbonne Université, France; 2Géosciences Montpellier - CNRS, Montpellier, France; 3School of the Environment and Life Sciences, University of Portsmouth, UK; 4Université de Paris Cité, Institut de physique du globe de Paris, CNRS, Paris, France; 5Institut Universitaire de France; 6Institut für Geowissenschaften, Universität Potsdam, Germany 

  • Presentation type: Poster

  • Presentation time: Thursday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 228

  • Programme No: 1.1.14

  • Theme 1 > Session 1

Abstract

For the first time, the K-edge X-ray absorption near edge structure (µXANES) method has been used on melt inclusions composed of both silicate glass and carbonate phase.  This study describes microscale iron (Fe) and Sulfur (S) µXANES measurements on nepheline-hosted melt inclusions preserved in Hanang lavas. Hanang is a volcano in the southern part of the east branch of the East African Rift (North Tanzania Divergence) and represents volcanism at the early stage of continental break-up; characterized by CO2-alkaline-rich magmas and silicate-carbonate immiscibility process at crustal level. Silicate glasses from Hanang display relatively high Fe3+/ΣFe ratio (Fe3+/ΣFe=0.30) and very low S6+/ΣS ratio (S6+/ΣS=0.06). The discrepancy of the oxidation state measured from iron and sulfur suggests that the oxidation state is affected by post entrapment processes or by the immiscibility process with carbonate liquid. In our case, we suggest that S speciation decoupling is attributed to silicate-carbonate immiscibility, and as such, low S6+/ΣS does not provide primary redox conditions in this system.  The Fe3+/ΣFe in melt inclusions yields magma redox conditions (fO2) at around ∆FMQ+1.4 for phonolitic liquid corresponding of one of the most oxidizing conditions within the East African rift.