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1. An ab-initio study on the thermodynamics of disulfide, sulfide, and bisulfide incorporation into apatite and the development of a more comprehensive temperature, pressure, pH, and composition-dependent model for ionic substitution in minerals

   https://doi.org/10.2138/am-2022-8250  

   Kim, YoungJae ; Konecke, Brian ; Simon, Adam ; Fiege, Adrian ; Becker, Udo ( November 2022 , American Mineralogist)

   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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2. Variable Modes of Formation for Tonalite–Trondhjemite–Granodiorite–Diorite (TTG)-related Porphyry-type Cu±Au Deposits in the Neoarchean Southern Abitibi Subprovince (Canada): Evidence from Petrochronology and Oxybarometry

   Xuyang Meng, Jeremy P. ( September 2022 , Journal of petrology)

   Most known porphyry Cu±Au deposits are associated with moderately oxidized and sulfur-rich, calc-alkaline to mildly alkalic arc-related magmas in the Phanerozoic. In contrast, sodium-enriched tonalite–trondhjemite–granodiorite–diorite (TTG) magmas predominant in the Archean are hypothesized to be unoxidized and sulfur-poor, which together preclude porphyry Cu deposit formation. Here, we test this hypothesis by interrogating the causative magmas for the ∼2·7 Ga TTG-related Côté Gold, St-Jude, and Clifford porphyry-type Cu±Au deposit settings in the Neoarchean southern Abitibi subprovince. New and previously published geochronological results constrain the age of emplacement of the causative magmas at ∼2·74 Ga, ∼2·70 Ga, and∼2·69 Ga, respectively. The dioritic and trondhjemitic magmas associated with Côté Gold and St-Jude evolved along a plagioclase-dominated fractionation trend, in contrast to amphibole-dominated fractionation for tonalitic magma at Clifford. Analyses of zircon grains from the Côté Gold, St-Jude, and Clifford igneous rocks yielded εHf(t)±SD values of 4·5±0·3, 4·2±0·6, and 4·3±0·4, and δ18O±SD values of 5·40±0·11  , 3·91±0·13  , and 4·83±0·12  , respectively. These isotopic signatures indicate that, although these magmas are mantle-sourced with minimal crustal contamination, for the St- Jude and Clifford settings the magmas or their sources may have undergone variable alteration by heated seawater or meteoric fluids. Primary barometric minerals (i.e. zircon, amphibole, apatite, and magnetite–ilmenite) that survived variable alteration and metamorphism (up to greenschist facies) were used for estimating fO2 of the causative magmas. Estimation of magmatic fO2 values, reported relative to the fayalite–magnetite–quartz buffer as  FMQ, using zircon geochemistry indicates that the fO2 values of the St-Jude, Côté Gold, and Clifford magmas increase from  FMQ –0·3±0·6 to  FMQ +0·8±0·4 and to  FMQ +1·2±0·4, respectively. In contrast, amphibole chemistry yielded systematically higher fO2 values of  FMQ +1·6±0·3 and  FMQ +2·6±0·1 for Côté Gold and Clifford, respectively, which are consistent with previous studies that indicate that amphibole may overestimate the fO2 of intrusive rocks by up to 1 log unit. Micro X-ray absorption near edge structure (μ-XANES) spectrometric determination of sulfur (i.e. S6+/ S) in primary apatite yielded ≥ FMQ−0·3 and FMQ+1·4–1·8 for St-Jude and Clifford, respectively. The magnetite–ilmenite mineral pairs from the Clifford tonalite yielded  FMQ +3·3±1·3 at equilibrium temperatures of 634±21 ◦C, recording the redox state of the late stage of magma crystallization. Electron probe microanalyses revealed that apatite grains from Clifford are enriched in S (up to 0·1 wt%) relative to those of Côté Gold and St-Jude (below the detection limit), which is attributed to either relatively oxidized or sulfur-rich features of the Clifford tonalite. We interpret these results to indicate that the deposits at Côté Gold and Clifford formed from mildly (∼ FMQ +0·8±0·4) to moderately (∼ FMQ +1·5) oxidized magmas where voluminous early sulfide saturation was probably limited, whereas the St-Jude deposit represents a rare case whereby the ingress of externally derived hydrothermal fluids facilitated metal fertility in a relatively reduced magma chamber (∼ FMQ +0). Furthermore, we conclude that variable modes of formation for these deposits and, in addition, the apparent rarity of porphyry-type Cu–Au deposits in the Archean may be attributed to either local restriction of favorable metallogenic conditions, and/or preservation, or an exploration bias. 

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3. Mobilization of S during subduction via metamorphic redox reactions

   Cruz-Uribe, AM ; Walters, JB ; Marschall, HR ( August 2018 , Goldschmidt Abstracts)

   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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4. High-temperature Stability of an Iron-rich Smectite: Implications for Smectite Formation on Mars

   Jenkins, D.M. ; DePasquale, B.M. ( December 2020 , American Geophysical Union meeting, Fall 2020 Meeting)

   null (Ed.)

   Iron-rich phyllosilicates on Mars comprise nearly 90% of the H2O- and OH-bearing phases observed directly by rovers and remotely by orbiters (Chemtob et al., 2017, JGR). Theories concerning the possible origin of Fe-rich smectite during Mars’ earliest history (phyllosian) are hard to test because of limited knowledge of the upper-thermal stability of Fe-rich phyllosilicates. In this study we present data on the upper-thermal stability of a pure-iron smectite to put some minimum constraints on its possible high-temperature origin early in Mars history either from a primordial atmosphere or by hydrothermal activity. Smectite coexisting with quartz and magnetite was synthesized from the oxides in the system Na2O-FeO-Fe2O3-Al2O3-SiO2-H2O at 500°C and 2 kbar and fO2 near FMQ. Reversal experiments involved mixtures with equal portions of the smectite-synthesis and breakdown products (quartz, fayalite, albite, magnetite (mt) treated in the presence of about 10 wt% H2O over the range of 1-3 kbar and 530-640°C. The average composition (electron microprobe) of smectite formed both in synthesis and in reversal experiments was Na0.35(Fe2+2.28Fe3+0.31Al0.41)(Al1.07Si2.93)O10(OH)2·nH2O, where ferric iron was calculated by summing the octahedral cations to 3.0. Reversals for the reaction smec+mt1 = fayalite+albite+mt2+quartz+H2O were obtained at 538±8, 590±10, and 610±10°C at 1, 2, and 3 kbar, respectively, where mt1 and mt2 have the approximate compositions Fe2.8Si0.2O4 and Fe2.8Al0.1O4, respectively, with all other phases being pure. This smectite has up to 2 interlayer H2O at 25°C (and high humidity), losing 1 H2O at <50°C, and the second at 125 ± 25°C. Thermodynamic modeling of this reaction was used to extrapolate the upper-thermal stability of this Fe-smectite down to 10 bars and approximately 239°C. Applications of these results indicate the maximum temperature for forming Fe-smectite from a dense primordial atmosphere of 100 bars is 390 ± 25°C. Crustal storage of water in Fe-smectite ranges up to a maximum of 10.7 wt% at ~2 km and 40°C, 7.4 wt% at 6 km and 120°C, and 3.8 wt% H2O at 32 km and 625°C for a Noachian geotherm of 20°C/km. Plain language summary: This study presents experimental limits on the temperatures at which the clay mineral smectite might form on Mars, either from a dense primordial atmosphere (390°C at 100 bars) or by high-temperature hydrothermal activity (625°C at 32 km). Because this study deals with iron end-member clay, these are minimum temperatures; any solid solution with magnesium will increase these temperatures. 

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5. Sulfide and sulfate saturation of dacitic silicate melts as a function of oxygen fugacity.

   Kleinsasser, J. ( September 2022 , Geochimica et cosmochimica acta)

   Sulfur is a key element in terrestrial magmatic processes yet its geochemical behavior remains one of the most difficult to model due to its heterovalent chemistry. The maximum amount of sulfur a silicate melt can dissolve before saturating with sulfide (e.g., pyrrhotite) or sulfate (e.g., anhydrite) changes with the redox state of the system and has important implications for the sulfur budget of a magmatic system. Several empirical models have been developed to predict the sulfur content of a silicate melt at either sulfide (under reducing conditions) or sulfate (under oxidizing conditions) saturation, but only one model existed that systematically assessed how the sulfur content of a basaltic melt changes as a function of oxygen fugacity (fO2) across the transition from sulfide- to sulfate-dominated conditions. The applicability of that model to intermediate and felsic melts rests on the assumption that changes in melt composition do not affect how sulfide or sulfate dissolves in the melt. Here, we report new experimental data that constrain the sulfur concentration at sulfide saturation (SCSS) and the sulfur concentration at anhydrite saturation (SCAS) in a dacitic melt as a function of fO2. The experiments were conducted using a H2Osaturated natural dacitic melt at 1000  C, 300 MPa, and at log fO2 varying over four orders of magnitude encompassing the sulfide-sulfate transition (log fO2 = DFMQ 0.7, DFMQ+0, DFMQ+0.5, DFMQ+1, DFMQ+1.48, DFMQ+1.54, DFMQ +1.75, DFMQ+2.08 and DFMQ+3.3). New SCSS and SCAS data and modeling for dacitic melts reveals that the sulfidesulfate transition occurs at DFMQ+1.81 ± 0.56, defined by the following equations to predict the sulfur content of intermediate to evolved silicate melts as a function of fO2: SCSSdacitic = [S2 ] (1 + 10(2.00DFMQ – 3.05)) SCASdacitic = [S6+] (1 + e(1.26 – 2.00DFMQ)) The results presented here demonstrate that the basaltic-derived SCSS-SCAS model is not appropriate for dacitic melts and that the sulfide-sulfate transition is shifted to higher fO2 in more evolved silicate melts. Implications include the stability of sulfides to higher fO2 in more evolved silicate melts and the potential for a narrower transition from a sulfide- to a sulfate-dominated melt than that predicted by thermodynamics. 

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