Recycling Bromine in subduction zones: New insights from X-ray absorption measurements on fluids, melts and glasses

Subduction zones, where sediments, oceanic crust and lithosphere are returned to the mantle are places of considerable importance to the Earth's chemical evolution. High-temperature fluids and hydrous melts generated by 1) dehydration and melting of the slab, 2) hydrous melting of the mantle wedge and 3) degassing of arc magmas are the main phases controlling the chemical fluxes from the subducting slab to the mantle wedge, volcanic arc and, ultimately, the atmosphere. While halogens are minor volatiles compare to H₂O and CO₂ in these mobile phases, their ability to complex with other elements makes them key agents of the chemical transfer ongoing at subduction zones [1]. Amongst all halogens, chlorine is the most studied element, mainly due to its role in the formation of porphyry Cu-Au-Mo deposits [2]. However, Bromine is also of particular interest as its volcanic degassing has been found, through the formation of BrO in volcanic plumes, to be significantly more effective in ozone destruction than more abundant Cl [3].

Despite their anthropogenic impact, estimates of halogens' input (from the slab) versus output (degassing along arcs) at subduction zones still lack precision [4], mostly due to the difficulty to recover the high P-T composition of fluids and melts from exhumed or erupted rock samples. In order to develop a better understanding of halogens' fate and role in subduction zones, we combined in situ X-ray fluorescence (SXRF) and absorption (XAS) measurements on fluids and melts in diamond anvil cells to high-energy resolution fluorescence detected (HERFD) XAS on silicate glasses to constrain Br distribution and speciation over a wide range of P-T conditions and compositions. The structural mechanisms behind Br incorporation in fluids and melts can be extended to Cl (and I) to explain fluid-melt partitioning trends [5, 6, 7] and halogens's transfer from the subducting slab to the volcanic arc.

References: [1]Aiuppa et al., 2008. Chemical Geology 263, 1-18. [2]Heinrich et al., 2005. Geological Society Special Publications 248. [3]Bobrovski et al., 2003. Nature 423, 273. [4] Barnes et al., 2018. in Harlov and Aranovich (eds.), Springer Geochemistry, 545-590. [5] Bureau et al., 2000. EPSL 183, 51-60 [6] Louvel, 2011. PhD thesis, ETH Zurich. [7] Cadoux et al., EPSL 498, 450-463.