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
The fluxes of volatile elements (water, carbon, sulfur, and halogens) in
subduction zones play a critical role in the Earth's chemical evolution;
however, the mechanisms and extent of their transfer from slab components to
the mantle wedge, the volcanic arc, and ultimately the atmosphere remain
poorly understood. Although halogens (F, Cl, Br, and I) are rather minor
volatiles compared to
In the last decade, new developments in quantification techniques on pore
fluids, fluid inclusions, and rocks as well as in detection methods for
halogens species in volcanic gases enabled better estimates of halogen
fluxes in subduction zones (Wallace, 2005; Pyle and Mather, 2009; John et
al., 2011; Kendrick et al., 2013, 2015; Chavrit et al.,
2016; Barnes et al., 2018). For example, comparisons of the input from the
subducted sediments, altered oceanic crust, and serpentinized oceanic
lithosphere to the output along volcanic arcs point to a significant
imbalance between fluorine input and output, suggesting that a significant
amount of F may be transferred to the deep mantle (Roberge et al., 2015;
Grutzner et al., 2017). In contrast, Cl, Br, and I appear to be
efficiently recycled up to the surface either through shallow loss of
fluids to the fore-arc region (Br and especially I) or deeper release upon
slab dehydration (especially Cl and Br, and to a lesser extent I) (Kendrick
et al., 2018). Yet, the poor understanding of the transfer mechanisms and
pathways of halogens limits the development of numerical models constraining
the role of fluids in the global cycling of elements in subduction zones
(Ikemoto and Iwamori, 2014; Kimura et al., 2016). There is, for instance,
virtually no constraint on the amounts of residual halogens that may be
stored in the dehydrated slab or lost to the continental crust through
hidden hydrothermal activity and passive degassing. Similarly, current
knowledge of halogen solubility and speciation in fluids and melts is
mostly limited to pressures below 0.3 GPa (equivalent to
Synthesis conditions and chemical compositions of the
EMPA: electron microprobe analysis; LA-ICPMS: laser ablation
inductively couple plasma mass spectrometry; RBS: Rutherford
backscattering spectroscopy.
The speciation and fluid–melt partitioning experiments were conducted using
3 wt % NaBr aqueous solutions and synthetic sodium disilicate (NS2:
Major element (Si, Al, K, and Na) contents and distribution in the glass were
measured by electron microprobe analysis (EMPA) using a JEOL JXA-8200
microprobe with an accelerating voltage of 15 keV, a 10 nA beam current, and
a defocused beam of 30
All experiments were conducted in Bassett-type hydrothermal diamond anvil
cells (HDACs; Bassett et al., 1993) widely used for in situ SXRF and XAS
measurements on aqueous fluids and silicate melts up to 1000
Microphotographs of the compression chamber of the HDAC showing
the haplogranite–
Fluid–melt partitioning experiments were conducted by loading the sample
chamber with a piece of Br-bearing Hpg glass and either pure
Bromine fluid–melt partition coefficients at different
The composition of the high-pressure fluids (wt % cations dissolved) and
melts (wt %
The SXRF and XAS measurements were performed at the microXAS beamline
(X05LA) of the Swiss Light Source (SLS, Paul Scherrer Institute; Borca et
al., 2009). Measurements at the Br K edge were conducted with an incident
energy of 13.6 keV tuned by a Si(111) double-crystal monochromator and
focused down to
2D SXRF Br K
2D SXRF maps were acquired across the sample chamber to qualitatively
monitor the distribution of Br between the coexisting aqueous fluid and
haplogranite melt (Fig. 2). Then, at least three fluorescence spectra were
collected from each phase to further determine the Br fluid–melt partition
coefficients
The fluid–melt partition coefficients,
XAS measurements were conducted on 3 wt % NaBr aqueous solution,
“solute-poor” fluids equilibrated with hydrous haplogranite melt (Fig. 1b),
supercritical fluids containing different amounts of dissolved NS2 (Fig. 1e),
and hydrous NS2 melt (Fig. 1f). XAS analyses on the haplogranite melt were
precluded by the lower Br concentration of these melts (< 0.2 wt %). For each composition, three to five XAS spectra were collected with
counting times of 1 s per point in the pre-edge region to 3 s in
the x-ray absorption near-edge-structure (XANES) and extended x-ray-absorption fine-structure (EXAFS) regions. The contribution of Bragg reflections arising
from the diamond anvils was avoided in the energy range of interest by
changing the orientation of the diamond anvil cell by 0.5 to 1
Data reduction was performed using the Athena and Artemis packages (Ravel
and Newville, 2005) based on the IFEFFIT program (Newville, 2001). Averaged
experimental spectra were normalized to the absorption edge height and
background removed using the automatic background subtraction routine AUTOBK
included in the Athena software. To minimize the contribution of features at
distances below the atom–atom contact distance, the
Evolution of the Br partition coefficients
The distribution of Br between aqueous fluids and silicate melts at high
Despite the current uncertainties, an important observation here is that the
At lower-pressure conditions relevant to fore-arc or crustal processes
(< 0.2 GPa), our in situ partition coefficients are slightly lower than
those obtained from quench experiments (Fig. 3). For instance, Bureau et al. (2000) and Cadoux et al. (2018) reported average
Structural parameters derived from Br K-edge EXAFS analysis for the reference aqueous solutions and silicate glasses at ambient conditions.
The XANES and EXAFS spectra collected at ambient conditions from the 3 wt % NaBr aqueous solution and Br-bearing silicate glasses are respectively reported
in Figs. 4 and 5, together with data for a KBr aqueous
solution from Ferlat et al. (2002). These spectra were employed to validate
the theoretical backscattering amplitude and phase shift functions for Br–O
and Br–Na scattering paths used in EXAFS modeling. The XANES spectrum of the
3 wt % NaBr aqueous solution is characterized by an absorption edge at
13.474 keV and a white line that peaks at 13.478 keV (Fig. 4). It displays
close similarities to that of the KBr aqueous solution from Ferlat et al. (2002) and overall resembles other alkali bromide aqueous solutions reported
in the literature (Wallen et al., 1997; Ferlat et al., 2001; Evans et al.,
2007). The EXAFS spectra from the KBr and NaBr aqueous solutions are
accurately modeled with a hydration shell of
Normalized Br K-edge XANES spectra collected on Br-bearing silicate glasses, aqueous fluids, and hydrous silicate melts at various pressure and temperature conditions. Spectra are offset for clarity. The vertical dashed line is a visual guide to appreciate phase shifts. The black arrow shows the pre-edge feature in the haplogranite glass spectrum corresponding to the 1s to 4p transition in Br (Burattini et al., 1991).
EXAFS spectra collected on NS2 and Hpg glasses at room conditions display
distinct oscillations, with a new feature at 2.2 Å
Normalized
Bromine K-edge XANES spectra of high
Br K-edge EXAFS analysis of experimental high
Evolution of bromine coordination numbers with oxygen (from
The structural parameters derived from the quantitative EXAFS analysis are
reported in Table 4. Comparably to room conditions, the EXAFS spectra of the
NaBr aqueous solution at high pressure–temperature conditions are well
matched by an octahedral hydration shell including multiple scattering
contributions from the
There are no significant changes in Br speciation in the aqueous fluids
equilibrated with haplogranitic melts, which contain only a few weight percent of
dissolved silicate components, and in fluids containing up to 30 wt %
dissolved NS2 (Fig. 6; Table 4). The first noticeable changes are only found
for fluids containing at least 50 wt % dissolved NS2, with a small
decrease in the average Br coordination number (
The new partitioning and speciation data derived for bromine in the present
study provide direct insights on the transport mechanisms of halogens (Cl,
Br and I) in subduction zones. Our results suggest that the mobilization of
Br (and likely Cl and I) in subduction zones is affected by the
chemistry of the slab-derived mobile phases. These phases, in turn, are
essentially controlled by the slab composition, the depth of fluid
extraction, and hence by the
General similarities between Cl, Br, and I speciation in aqueous solutions
and silicate glasses (Evans et al., 2008; McKeown et al., 2011, 2015;
Shermann et al., 2010) suggest that the speciation and partitioning trends
found in our study for Br may extend to Cl and I. Therefore, while early
dehydration fluids should release large amounts of halogens to the fore-arc
and the mantle wedge (100–200 km of depth), hydrous slab melts and
supercritical fluids play a critical role in recycling the residual halogens
dragged by the subducting slabs to greater depths. Such efficient recycling,
whereby most of the Cl and Br subducted is transferred to the mantle wedge and
ultimately returned to the surface through arc magmatism, is further
supported by the recent quantification of halogens in subducted sediments,
serpentinites, and altered oceanic crust. Mass balance calculations indeed
show a close match, within errors, between worldwide influx to the mantle
wedge,
In situ SXRF and XAS have been applied to quantify Br fluid–melt partition
coefficients and speciation in aqueous fluids, supercritical fluids, and
hydrous silicate melts up to 840
All materials are available from the corresponding author(s) upon request.
CS-V designed the study; ML, WJM, CS-V, CNB, and DG conducted the experiments; ML, GSP, WJM, and CS-V processed the data and interpreted the results. ML and CS-V wrote the paper with contributions from all co-authors.
The authors declare that they have no conflict of interest.
This article is part of the special issue “Exploring new frontiers in fluids processes in subduction zones”. It is a result of the EGU Galileo conference “Exploring new frontiers in fluids processes in subduction zones”, Leibnitz, Austria, 24–29 June 2018.
This work was supported by the Swiss National Science Foundation (grants 200021-120575 and 200020-132208 to Carmen Sanchez-Valle) as well as by the Swiss Academy of Sciences (SATW) and the Ministères des Affaires étrangères et européennes (MAEE) et de l'Enseignement Supérieur et de la Recherche (MESR) through the Partenariat Hubert Curien (PHC). We thank Max Doebeli and Jung-Hun Seo for conducting the RBS and LA-ICPMS analysis, respectively. The Paul Scherrer Institute (PSI) and the Swiss Light Source (SLS) are acknowledged for providing beam time for the experiments. Two anonymous reviewers and the topical editor Nadia Malaspina are thanked for their help in improving the clarity of the article.
This research has been supported by the Swiss National Science Foundation (grant nos. 200021-120575 and 200020-132208).
This paper was edited by Nadia Malaspina and reviewed by two anonymous referees.