Examination of a global suite of eclogite-facies metabasites and metasediments suggests that eclogites tend to exhibit reduced mineral assemblages relative to their protoliths. High-pressure rocks tend to lack sulfides and Fe3+-bearing oxides in the eclogite facies. We suggest that eclogite-facies mineral assemblages are consistent with prograde reactions that balance the oxidation of S2- or S- to S6+ by reducing Fe3+in silicates or oxides:
(1)8Fe3+Si O (OH) +S2-=8Fe2+Si O +SO 2-+(H O) abc de42f
The oxidation of one mole of S2-or S-is balanced by the reduction of 7 to 8 moles of Fe3+, and typical S concentrations in the oceanic crust are capable of fully reducing the entire Fe3+ budget of metabasites.
As most eclogite facies rocks do not preserve peak metamorphic sulfides, petrographic evidence for prograde S oxidation reactions are cryptic; however, textures associated with sulfate reduction in response to influx of external fluids are common (reaction 1 in reverse). These reactions produce Fe3+-rich phases and are observed in both metasedimentary and metabasic rocks across a range of retrograde P-T paths (blueschist to granulite facies). For example, high-P calc- schists exhibit reaction textures that suggest the breakdown of garnet and white mica to produce pyrite + chalcopyrite + epidote + biotite + magnetite. Our thermodynamic models of aS2 and aO2 at subduction zone P-T conditions suggest assemblages of this type are indicative of aO2 0.7 to 4.5 log units above the quartz-fayalite-magnetite buffer.
In rehydrated eclogites, pyrite is commonly associated with the breakdown of garnet + omphacite to amphibole + pyrite. Additionally, direct precipitation of sulfide from sulfate is observed in two samples: 1) The retrograde assemblage pyrite + ilmenite + gypsum occurs in one retrogressed metagabbroic eclogite, and 2) Coronas of secondary pyrite + barite + gypsum enclose early retrograde pyrite in a retrogressed garnet blueschist. In many eclogites, S- is reduced to S2- as pyrite is replaced by pyrrhotite, chalcopyrite, and mixed valence Co-Ni sulfides. These reactions are balanced by oxidation of divalent to trivalent Fe-Co-Ni. Reactions of this type are consistent with increasing aS2 during retrograde metamorphism. Thus, ample evidence exists for oxidized S-bearing fluids released from subducting slabs.
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Mobilization of S during subduction via metamorphic redox reactions
The large range in oxidation states of sulfur (-II to +VI) provides it with a large oxidation potential in rocks, even at relatively low concentrations. Most importantly, the transition from sulfide to sulfate species in rocks and silicate melts occurs in the same approximate fO2 region (for a given temperature) as the transition from ferrous to ferric iron, and reduced S species can coexist with oxidized Fe and vice versa. The result is a large potential for reactions involving sulfur to oxidize or reduce Fe in silicate minerals, since Fe only occurs in two oxidation states (+II and +III).
In order for sulfur to be released during slab dehydration, sulfur in sulfide must be converted into an easily dissolved species, such as SO42− or H2S, through either oxidation or reduction. We propose that oxidation of sulfur in sulfide follows the generalized reaction:
8Fe3+SiaOb(OH)c +S2− = 8Fe2+SidOe +SO42− +(H2O)f (1)
In this type of reaction, sulfur participates in the dehydration of greenschist- or blueschist-facies hydrous silicates during transition to the eclogite facies: ferric Fe in Fe-bearing silicates (chlorite, amphibole, epidote) is reduced to ferrous Fe in anhydrous ferromagnesian silicates (pyroxene, garnet). At the same time, the reaction consumes sulfide by oxidation of S2− to produce SO42−, which is readily dissolved in the fluid produced during dehydration. Additionally, a similar redox reaction could oxidize sulfur by reducing ferric Fe in oxides. It is important to note that one mole of S has the same redox potential as 8 moles of Fe. The molar ratio of 8 moles of Fe per 1 mole of S translates to a mass ratio of approximately 14; therefore, small concentrations of sulfur can have a large impact on reduction/oxidation of the silicate assemblage.
Our observations show that sulfide minerals that can be identified as primary or related to the peak metamorphic stage are rare in eclogites and restricted to inclusions in garnet, consistent with reaction (1). Thermodynamic modeling is currently underway to assess the influence of sulfur on the phase equilibria of silicate phases during high pressure metamorphism.
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- Award ID(s):
- 1725301
- NSF-PAR ID:
- 10108913
- Date Published:
- Journal Name:
- Goldschmidt Abstracts
- Page Range / eLocation ID:
- 502
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Sulfide breakdown during subduction releases oxidizing fluids that transport chalcophile and siderophile elements (CSE) such as Ni ,Co, and As. These fluids are reincorporated into high-pressure rocks such as eclogites during exhumation and rehydration along the slab-mantle interface. Evidence for these rehydration reactions takes the form of large sulfide (pyrite, pyrrhotite, chalcopyrite) grains (up to 5 mm) associated with hydrous Fe3+-bearing minerals. Here we present results of trace element determination by LA-ICP-MS coupled with mass balance calculations for sulfide-silicate reactions in rehydrated eclogites from the Mariánské Lázně Complex and Moldanubian Zone, Bohemian Massif, Czech Republic. One key texture observed in these rocks is the breakdown of garnet + omphacite in the presence of fluid to produce hornblende + diopside + plagioclase + pyrite. This rehydration reaction involves the oxidation of Fe2+ in garnet to Fe3+ in hornblende. In order to oxidize the iron from the garnet, we propose that sulfate is brought into the rock by an infiltrating fluid, where it is reduced to form pyrite, consistent with the observed textures. Trace element analyses reveal the Co distribution within rehydrated eclogite: Co is measurable in garnet (~50 μg/g), omphacite (~26 μg/g), hornblende (~80 μg/g), and pyrite (~5000 μg/g). Mass balance calculations suggest that of the total amount of Co present in the rehydration products, only ~35 % can be supplied by the breakdown of garnet and omphacite, leaving ~65 % of the Co to be supplied by another source. Average concentrations of Ni are: in garnet (1–4 μg/g), omphacite (~57 μg/g), hornblende (~90 μg/g), and pyrite (~2500 μg/g). Mass balance calculations suggest that of the total amount of Ni present in the rehydration products, ~70 % comes from the breakdown of garnet + omphacite, with the other 30 % supplied external to this reaction. Arsenic is not present in the silicate minerals, but is in the 10s of μg/g range in pyrite, and must be supplied externally to the rock, likely from a fluid. We conclude that the fluids released from subducting slabs carry sulfate and CSEs, which infiltrate the slab-mantle interface and eventually make their way into the sub-arc mantle, where they can be incorporated into the arc magmatic system.more » « less
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Abstract The mineral apatite, Ca10(PO4)6(F,OH,Cl)2, incorporates sulfur (S) during crystallization from S-bearing hydrothermal fluids and silicate melts. Our previous studies of natural and experimental apatite demonstrate that the oxidation state of S in apatite varies systematically as a function of oxygen fugacity (fO2). The S oxidation states –1 and –2 were quantitatively identified in apatite crystallized from reduced, S-bearing hydrothermal fluids and silicate melts by using sulfur K-edge X-ray absorption near-edge structure spectroscopy (S-XANES) where S 6+/ΣS in apatite increases from ~0 at FMQ-1 to ~1 at FMQ+2, where FMQ refers to the fayalite-magnetite-quartz fO2 buffer. In this study, we employ quantum-mechanical calculations to investigate the atomistic structure and energetics of S(-I) and S(-II) incorporated into apatite and elucidate incorporation mechanisms.
One S(-I) species (disulfide, S22−) and two S(-II) species (bisulfide, HS−, and sulfide, S2−) are investigated as possible forms of reduced S species in apatite. In configuration models for the simulation, these reduced S species are positioned along the c-axis channel, originally occupied by the column anions F, Cl, and OH in the end-member apatites. In the lowest-energy configurations of S-incorporated apatite, disulfide prefers to be positioned halfway between the mirror planes at z = 1/4 and 3/4. In contrast, the energy-optimized bisulfide is located slightly away from the mirror planes by ~0.04 fractional units in the c direction. The energetic stability of these reduced S species as a function of position along the c-axis can be explained by the geometric and electrostatic constraints of the Ca and O planes that constitute the c-axis channel.
The thermodynamics of incorporation of disulfide and bisulfide into apatite is evaluated by using solid-state reaction equations where the apatite host and a solid S-bearing source phase (pyrite and Na2S2(s) for disulfide; troilite and Na2S(s) for sulfide) are the reactants, and the S-incorporated apatite and an anion sink phase are the products. The Gibbs free energy (ΔG) is lower for incorporation with Na-bearing phases than with Fe-bearing phases, which is attributed to the higher energetic stability of the iron sulfide minerals as a source phase for S than the sodium sulfide phases. The thermodynamics of incorporation of reduced S is also evaluated by using reaction equations involving dissolved disulfide and sulfide species [HnS(aq)(2−n) and HnS(aq)(2−n); n = 0, 1, and 2] as a source phase. The ΔG of S-incorporation increases for fluorapatite and chlorapatite, and decreases for hydroxylapatite, as these species are protonated (i.e., as n changes from 0 to 2). These thermodynamic results demonstrate that the presence of reduced S in apatite is primarily controlled by the chemistry of magmatic and hydrothermal systems where apatite forms (e.g., an abundance of Fe; solution pH). Ultimately, our methodology developed for evaluating the thermodynamics of S incorporation in apatite as a function of temperature, pH, and composition is highly applicable to predicting the trace and volatile element incorporation in minerals in a variety of geological systems. In addition to solid-solid and solid-liquid equilibria treated here at different temperatures and pH, the methodology can be easily extended to different pressure conditions by just performing the quantum-mechanical calculations at elevated pressures.
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The mineral apatite, Ca10(PO4)6(F,OH,Cl)2, incorporates sulfur (S) during crystallization from S-bearing hydrothermal fluids and silicate melts. Our previous studies of natural and experimental apatite demonstrate that the oxidation state of S in apatite varies systematically as a function of oxygen fugacity (fO2). The S oxidation states –1 and –2 were quantitatively identified in apatite crystallized from reduced, S-bearing hydrothermal fluids and silicate melts by using sulfur K-edge X‑ray absorption near-edge structure spectroscopy (S-XANES) where S6+/ΣS in apatite increases from ~0 at FMQ-1 to ~1 at FMQ+2, where FMQ refers to the fayalite-magnetite-quartz fO2 buffer. In this study, we employ quantum-mechanical calculations to investigate the atomistic structure and energetics of S(-I) and S(-II) incorporated into apatite and elucidate incorporation mechanisms. One S(-I) species (disulfide, S22−) and two S(-II) species (bisulfide, HS−, and sulfide, S2−) are investigated as possible forms of reduced S species in apatite. In configuration models for the simulation, these reduced S species are positioned along the c-axis channel, originally occupied by the column anions F, Cl, and OH in the end-member apatites. In the lowest-energy configurations of S-incorporated apatite, disulfide prefers to be positioned halfway between the mirror planes at z = 1/4 and 3/4. In contrast, the energy-optimized bisulfide is located slightly away from the mirror planes by ~0.04 fractional units in the c direction. The energetic stability of these reduced S species as a function of position along the c-axis can be explained by the geometric and electrostatic constraints of the Ca and O planes that constitute the c-axis channel. The thermodynamics of incorporation of disulfide and bisulfide into apatite are evaluated by using solid-state reaction equations where the apatite host and a solid S-bearing source phase (pyrite and Na2S2(s) for disulfide; troilite and Na2S(s) for sulfide) are the reactants, and the S-incorporated apatite and an anion sink phase are the products. The Gibbs free energy (ΔG) is lower for incorporation with Na-bearing phases than with Fe-bearing phases, which is attributed to the higher energetic stability of the iron sulfide minerals as a source phase for S than the sodium sulfide phases. The thermodynamics of incorporation of reduced S are also evaluated by using reaction equations involving dissolved disulfide and sulfide species [HnS2(aq)(2–n) and HnS(aq)(2–n); n = 0, 1, and 2] as a source phase. The ΔG of S-incorporation increases for fluorapatite and chlorapatite and decreases for hydroxylapatite as these species are protonated (i.e., as n changes from 0 to 2). These thermodynamic results demonstrate that the presence of reduced S in apatite is primarily controlled by the chemistry of magmatic and hydrothermal systems where apatite forms (e.g., an abundance of Fe; solution pH). Ultimately, our methodology developed for evaluating the thermodynamics of S incorporation in apatite as a function of temperature, pH, and composition is highly applicable to predicting the trace and volatile element incorporation in minerals in a variety of geological systems. In addition to solid-solid and solid-liquid equilibria treated here at different temperatures and pH, the methodology can be easily extended also to different pressure conditions by just performing the quantum-mechanical calculations at elevated pressures.more » « less
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