Redox conditions in a carbonatite-alkaline complex: Deciphering Fe- and S-XANES in melt inclusions with silicate‑carbonate immiscibility

Abstract

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. Hanang volcanism is characterized by CO2-alkaline-rich magmas and silicate‑carbonate immiscibility process at crustal level. This study describes microscale iron (Fe) and Sulfur (S) K-edge X-ray absorption near edge structure (μXANES) spectroscopy measurements on nepheline-hosted melt inclusions (MI) preserved in Hanang lavas. For the first time, the μXANES method has been used on melt inclusions composed of both silicate glass and carbonate phase. Silicate glasses from Hanang display relatively high Fe3+/ΣFe ratio (Fe3+/ΣFe = 0.31, indicative of oxidized conditions) and very low S6+/ΣS ratio (S6+/ΣS = 0.05–0.07, indicative of reduced conditions). The discrepancy of the oxidation state measured from iron and sulfur and thus, the redox conditions (fO2) inferred from these two values, either suggests that the oxidation state is affected by post entrapment processes, such as re-equilibration with the host-mineral or the immiscibility process with carbonate liquid, or it suggests a significantly different relation of Fe and S oxidation state to fO2 in this chemical system. The Fe3+/ΣFe in melt inclusions yields magma redox conditions (fO2) at around ∆FMQ + 1.4 for phonolitic liquid composition from evolved differentiation and immiscibility process between silicate melt and carbonatite (≤850C°). Sulfur speciation decoupling is attributed to silicate‑carbonate immiscibility, and as such, low S6+/ΣS does not provide primary redox conditions in this system. Hanang lavas at the early stage of East African Rift have one of the most oxidizing conditions, related to the presence of carbonate-rich alkaline magmatic system associated with carbonatites.