Variable oxidizing capacity of slab-derived fluids: Insights from Fe and S speciation in glasses from the Troodos Ophiolite

L.M. Saper*, M. Brounce, D. Woelki, R. Cao, G. Bromiley

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

Abstract / Description of output

Oxygen fugacity (ƒO2) varies systematically across tectonic environments. The typically high ƒO2 recorded in arc settings relative to oceanic ridges is attributed to recycling of oxidized materials derived from subducting slabs into regions of the mantle that undergo partial melting. To evaluate further the relationship between ƒO2 and mantle metasomatism, Fe and S X-ray Absorption Near-Edge Structure measurements were conducted on volcanic glass wafers sampled across the extrusive section of the Troodos Ophiolite, Cyprus, which is a type locality for studying the influence of subduction on mantle melting processes and the generation of oceanic crust. The glasses record ƒO2 values relative to the quartz-fayalite-magnetite (FMQ) buffer of FMQ+0.13±0.16(1σ) to FMQ+0.74±0.23(1σ) upon quenching at the seafloor. At a given MgO content, the ƒO2 values recorded by the Troodos glasses do not vary systematically based on type (i.e., boninitic versus tholeiitic) nor by sampling location, indicating that the lavas were derived from primary melts and mantle sources that were indistinguishable in their initial ƒO2 levels. The glass ƒO2 values do not vary with dissolved H2O contents, nor with trace element ratios (e.g., Ba/La, Ce/Pb) and Pb and Sr isotopic compositions, all used to monitor interaction of slab-derived materials with the Troodos mantle source. The decoupling of ƒO2 and H2O and fluid-mobile elements renders the Troodos an important endmember for interpreting global variations in the redox state of mantle melts formed in subduction-influenced settings. For example, the Troodos glasses are reduced relative to Izu-Bonin boninites and melt inclusions from Mariana and Cascades arc volcanoes, despite overlapping and elevated H2O contents, Ba/La, and S/Dy relative to mid-ocean ridge basalts. The lack of correlation between ƒO2 and volatile contents and with proxies for subduction influence in the Troodos glasses is similar to basalts formed in back-arc environments and in near-trench settings in Izu-Bonin-Mariana. Such comparisons suggest that the slab-derived fluids that infiltrated the Troodos melting region were derived from a shallow slab and had low oxidizing capacity relative to fluids or melts which interacted with the mantle in the Izu-Bonin-Mariana arc and in the Cascades, which represent arc environments with comparable glass Fe XANES measurements. A simple framework accounts for local and global variability in ƒO2 in fractionation-corrected glasses, where fluids of variable oxidizing capacity interact with depleted mantle to produce hydrous melts. The inheritance of volatile and fluid-mobile elements in regions of the mantle that undergo partial melting does not necessarily lead to higher ƒO2 if the oxidizing capacity of the infiltrating hydrous fluids or melts is low. In back-arcs and near-trench environments, partial melts formed by interaction with fluids of low oxidizing capacity are similar to arc melts in their volatile contents, trace element, and isotopic signatures, but modestly oxidized relative to mid-ocean ridge basalts.
Original languageEnglish
Article number118560
JournalEarth and Planetary Science Letters
Volume627
Early online date9 Jan 2024
DOIs
Publication statusPublished - 1 Feb 2024

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