Iron-sulfur-carbon redox interactions in the continental subduction factory and their effect on volatile element storage in the mantle wedge

The ultramafic rocks of the Ulten Zone tectonic mélange experienced a well-documented multi-stage metamorphic history, from residence in a hot mantle wedge to down-dragging and trapping in a Variscan slab accompanied by amphibolitisation, and finally two-stage exhumation accompanied by chloritisation and serpentinisation. We use these rocks as a natural laboratory to investigate whether volatile element fluxes in continental subduction zones promote long-term volatile element storage in the overlying mantle wedge. Here, we obtained new data on the chemical composition, iron speciation, carbon concentrations and isotopic compositions of ultramafic bulk rocks, and the carbon-oxygen isotope composition of carbonates in samples from >10 ultramafic lenses, which we combine with previously published data for additional insights.

The carbonate stable isotope compositions show a distinct provinciality, whereby rocks from the little retrogressed ultramafic lenses in the NE Ulten Zone domain have lower average δ¹³C_V-PDB and δ¹⁸O_V-SMOW of –16.8‰ to –5.7‰ and +8.0‰ to +17.8‰, respectively, than those in the more retrogressed SW domain (–11.2‰ to 0.0‰ and +12.9‰ to +20.7‰, respectively), suggesting influx of distinct crustal fluids. Bulk-rock carbon contents range from 130 to 28,000 μg g^-1, exceeding estimates for the convecting mantle, and are on average higher in rocks from the NE domain (median 880 μg g^-1), which can be modelled as Rayleigh-style dolomite addition at ∼800–700 °C. Rocks from the SW domain have lower C contents (median 570 μg g^-1) which correlate positively with δ¹³C and can be modelled as Rayleigh-style calcite addition at ∼500–400 °C. The lowest δ¹³C and C contents point to dedolomitisation during low-temperature (400 °C) serpentinisation, and furthermore suggest the contribution of a ¹³C-depleted phase to the bulk-rock compositions.

After melt depletion during formation of the pre-Variscan continental lithosphere, the sulfur inventory was replenished during amphibolitisation near peak-metamorphic conditions, via sulphidation during interaction with siliceous fluid. Sulfur was markedly (re-)depleted during processes related to exhumation, reflecting low sulfur fugacity during chloritisation and serpentinisation. The available data suggest that the high bulk-rock Fe³⁺/ΣFe (median 0.18) resulted from reduction of some aqueous sulphate during amphibolitisation, accompanied by redox-neutral carbonation, and from carbonate reduction during chloritisation when sulfur fugacity was low.

Ignoring exhumation-related C loss and taking near-peak metamorphic conditions as representative, significant amounts of C in carbonates and water in amphiboles may be stored in continental mantle wedges. These are subsequently stabilised below collisional orogens, which cover vast areas of Earth's continents and possibly constitute particularly volatile element-rich lithospheric mantle reservoirs.