Abstract. Understanding the behavior of halogens (Cl, Br, and I) in
subduction zones is critical to constrain the geochemical cycle of these
volatiles and associated trace metals, as well as to quantify the halogen fluxes
to the atmosphere via volcanic degassing. Here, the partitioning of bromine
between coexisting aqueous fluids and hydrous granitic melts and its
speciation in slab-derived fluids have been investigated in situ up to
840 ∘C and 2.2 GPa by synchrotron x-ray fluorescence (SXRF) and x-ray
absorption spectroscopy (XAS) in diamond anvil cells. The partition
coefficients DBrf/m range from ∼2 to
∼15, with an average value of 6.7±3.6 (1σ)
over the whole pressure–temperature (P–T) range, indicating a moderate Br
enrichment in aqueous fluids, in agreement with previous work. Extended x-ray-absorption fine-structure (EXAFS)
analysis further evidences a gradual evolution of Br speciation from
hydrated Br ions [Br(H2O)6]− in slab dehydration fluids to
more complex structures involving both Na ions and water molecules,
[BrNax(H2O)y], in hydrous silicate melts and supercritical
fluids released at greater depth (> 200 km). In denser fluids
(ρ > 1.5 g cm−3) containing 60 wt % dissolved
alkali–silicates and in hydrous Na2Si2O5 melts (10 wt %
H2O), Br is found to be in a “salt-like” structure involving the six
nearest Na ions and several next-nearest O neighbors that are either from
water molecules and/or the silicate network. Bromine (and likely chlorine
and iodine) complexing with alkalis is thus an efficient mechanism for the
mobilization and transport of halogens by hydrous silicate melts and
silica-rich supercritical fluids. Our results suggest that both shallow
dehydration fluids and deeper silicate-bearing fluids efficiently remove
halogens from the slab in the sub-arc region, thus favoring an efficient
transfer of halogens across subduction zones.
Behaviour of niobium during early Earth’s differentiation: insights from its local structure and oxidation state in silicate melts at high pressure