Bromine speciation and partitioning in slab-derived fluids and melts: Implications for halogens recycling in subduction zones

Abstract. Understanding the behavior of halogens (Cl, Br, and I) in subduction zones is critical to constrain the recycling of trace elements and metals, and 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 X-ray fluorescence (SXRF) and absorption (XANES and EXAFS) spectroscopy in hydrothermal diamond-anvil cells. The partition coefficients Df/mBr range from 15.3 ± 1.0 to 2.0 ± 0.1, indicating the preferential uptake of Br by aqueous fluids at all investigated conditions. 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 invoving both Na ions and water molecules, [BrNax(H2O)y], in hydrous silicate melts and supercritical fluids released at greather depth (> 200 km). In dense fluids containing 60 wt % dissolved alkali-silicates and in hydrous Na2Si2O5 melts (10 wt % H2O), Br is found in a salt-like structure involving 6 nearest Na ions and several next-nearest O neighbors that are either from water molecules or the tetrahedral silicate network. Bromine (and likely chlorine and iodine) complexation with alkalis is thus an efficient mechanism for the mobilization and transport of halogens by hydrous silicate melts and supercritical fluids, which can carry high amounts of Br, up to the 1000 ppm level. Overall, 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 controling an efficient recycling of halogens in subduction zones.

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Bromine speciation and partitioning in slab-derived aqueous fluids and silicate melts and implications for halogen transfer in subduction zones

Solid Earth ◽

10.5194/se-11-1145-2020 ◽

2020 ◽

Vol 11 (4) ◽

pp. 1145-1161

Author(s):

Marion Louvel ◽

Carmen Sanchez-Valle ◽

Wim J. Malfait ◽

Gleb S. Pokrovski ◽

Camelia N. Borca ◽

...

Keyword(s):

Supercritical Fluids ◽

Subduction Zones ◽

Silicate Melts ◽

Water Molecules ◽

X Ray ◽

Average Value ◽

Hydrous Silicate ◽

Gradual Evolution ◽

Na Ions ◽

X Ray Absorption

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.

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Properties and effects of supercritical fluids

10.5194/egusphere-egu2020-1648 ◽

2020 ◽

Author(s):

Huaiwei Ni

Keyword(s):

Supercritical Fluids ◽

Subduction Zones ◽

Mutual Solubility ◽

Silicate Melts ◽

Mineral Deposits ◽

Computational Simulations ◽

Material Recycling ◽

Critical Curves ◽

Temperature And Pressure ◽

Deep Focus

<p>Enhanced mutual solubility between silicate and water at elevated temperature and pressure in Earth&#8217;s interior (subduction zones in particular) allows the formation of supercritical geofluids with composition intermediate between silicate melts and aqueous fluids. The forming conditions of supercritical fluids are controlled by the critical curves, the wet solidi curves and the second critical endpoints of rock-H<sub>2</sub>O systems. With unusual physicochemical properties, supercritical fluids have the potential to play a crucial role in mediating material recycling at subduction zones, mobilizing and enriching ore-forming elements, inducing intermediate to deep focus earthquakes, and modulating Earth&#8217;s habitability. Challenges in the study of supercritical fluids using experimental and computational simulations as well as natural rocks and mineral deposits demand breakthroughs in future development of transformative technologies.</p>

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Infrared spectroscopy of hydrous silicate melts to 1000 degrees C and 10 kbar; direct observation of H2O speciation in a diamond-anvil cell

American Mineralogist ◽

10.2138/am-1995-11-1223 ◽

1995 ◽

Vol 80 (11-12) ◽

pp. 1335-1338 ◽

Cited By ~ 108

Author(s):

Andy Shen ◽

Hans Keppler

Keyword(s):

Infrared Spectroscopy ◽

Direct Observation ◽

Diamond Anvil Cell ◽

Silicate Melts ◽

Hydrous Silicate ◽

Diamond Anvil

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A new type of high-temperature, high-pressure cell for spectroscopic studies of hydrous silicate melts

American Mineralogist ◽

10.2138/am-1996-11-1222 ◽

1996 ◽

Vol 81 (11-12) ◽

pp. 1507-1512 ◽

Cited By ~ 7

Author(s):

Marcus Nowak ◽

Harald Behrens ◽

Wilhelm Johannes

Keyword(s):

High Pressure ◽

High Temperature ◽

Spectroscopic Studies ◽

Silicate Melts ◽

High Pressure Cell ◽

Pressure Cell ◽

Hydrous Silicate ◽

New Type ◽

High Temperature High Pressure

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A thermodynamic model for sulfur content at sulfide saturation (SCSS) in hydrous silicate melts: with implications for arc magma genesis and S recycling

10.7185/gold2021.4948 ◽

2021 ◽

Author(s):

Huijuan Li ◽

Lifei Zhang

Keyword(s):

Sulfur Content ◽

Thermodynamic Model ◽

Silicate Melts ◽

Sulfide Saturation ◽

Magma Genesis ◽

Hydrous Silicate ◽

Arc Magma

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Iron mobility in slab-derived hydrous silicate melts at sub-arc conditions

10.7185/gold2021.7509 ◽

2021 ◽

Author(s):

Carla Tiraboschi ◽

Arno Rohrbach ◽

Stephan Klemme ◽

Jasper Berndt ◽

Carmen Sanchez-Valle

Keyword(s):

Silicate Melts ◽

Hydrous Silicate ◽

Iron Mobility

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The paragenesis of upper-mantle xenoliths from the Quaternary volcanics south-east of Gees, West Eifel, Germany

Mineralogical Magazine ◽

10.1180/minmag.1991.055.378.08 ◽

1991 ◽

Vol 55 (378) ◽

pp. 95-112 ◽

Cited By ~ 20

Author(s):

F. E. Lloyd ◽

A. D. Edgar ◽

D. M. Forsyth ◽

R. L. Barnett

Keyword(s):

Upper Mantle ◽

Free Subgroup ◽

Mantle Xenoliths ◽

Silicate Melt ◽

Silicate Melts ◽

Hydrous Silicate ◽

Group I ◽

Group Ii ◽

West Eifel ◽

K Activity

AbstractGroup I xenoliths, orthopyroxene-rich and orthopyroxene-free, contain Cr-spinel and clinopyroxene ± phlogopite, and occur together with Group II clinopyroxenites ± Ti-spinel ± phlogopite in K-mafic pyroclastics southeast of Gees. The petrography and clinopyroxene chemistry of orthopyroxene-rich (opx-rich sub-group) Group I xenoliths is consistent with an ‘original’ harzburgitic mantle that has been transformed to lherzolite by the addition of endiopside. In harzburgites, orthopyroxenes are reacting to diopside + olivine + alkali-silicate melt, and, by inference, the orthopyroxene-free (opx-free subgroup) Group I, dunite-wehrlite series can be linked to the opx-rich sub-group via this reaction. Progressive enrichment of dunitic material in endiopside-diopside has resulted in the formation of wehrlite. Phlogopite is titaniferous and occurs as a trace mineral in opx-rich, Group I xenoliths, whereas substantial phlogopite vein-networks are confined to the opx-free sub-group (dunite-wehrlite series). Interstitial, alkali-felsic glass occurs are veins within, and as extensions of, the phlogopite networks. Clinopyroxenes in phlogopite-veined xenoliths are decreased in Mg/(Mg + FeTotal) (mg) and Cr and increased in Ti, Al and Ca, compared with clinopyroxenes in xenoliths which have trace phlogopite. It is proposed that harzburgitic and dunitic mantle has been infiltrated by a Ca- and alkalirich, hydrous silicate melt rather than an ephemeral carbonatite melt. Dunite has been transformed to phlogopite wehrlite by the invasion of a Ca-, Al-, Ti- and K-rich, hydrous silicate melt. Ca-activity was high initially in the melt and was reduced by clinopyroxene precipitation. This resulted in enhanced K-activity which led to phlogopite veining of clinopyroxene-rich mantle. Group II phlogopite clinopyroxenites contain Ti-spinel and salites that are distinct in their Ti, Al and Cr contents from endiopsides and diopsides in Group I xenoliths. It is unlikely that these Group II xenoliths represent the culmination of the infiltration processes that have transformed dunite to wehrlite, nor can they be related to the host melt. These xenoliths may have crystallised from Ca- and K-bearing, hydrous silicate melts in mantle channelways buffered by previously precipitated clinopyroxene and phlogopite. Gees lherzolites contain pyroxenes and spinel with distinctly lower Al contents than these same minerals in lherzolites described previously from other West Eifel localities, which may reflect a distinctive lithology and/or processes of modification for the Gees mantle.

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