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Effect of Sulfur Speciation on Chemical and Physical Properties of Very Reduced Mercurian Melts
Abstract
The NASA MESSENGER mission revealed that lavas on Mercury are enriched in sulfur (1.5-4 wt.%) compared with other terrestrial planets (<0.1 wt.%), a result of high S solubility under its very low oxygen fugacity (estimated ƒO2 between IW-3 and IW-7). Due to decreasing O availability at these low ƒO2 conditions, and an abundance of S²⁻, the latter acts as an important anion. This changes the partitioning behaviour of many elements (e.g. Fe, Mg, and Ca) and modifies the physical properties of silicate melts. To further understand S solubility and speciation in reduced magmas, we have analysed 11 high pressure experiments run at 1 GPa in a piston cylinder at temperatures of 1250 to 1475 °C and ƒO2 between IW-2.5 to IW-7.5. S K-Edge XANES is used to determine coordination chemistry and oxidation state of S species in highly reduced quenched silicate melts. As ƒO2 decreases from IW-2 to IW-7, S speciation goes through two major changes. At ∼IW-2, FeS, FeCr2S4, Na2S, and MnS species are destabilized, CaS (with minor Na2S) becomes the dominant S species. At ∼ IW-4, Na2S is destabilized, MgS becomes the dominant S species, with lesser amounts of CaS. The changes in S speciation at low ƒO2 affect the activities of SiO2, MgO and CaO in the melt, stabilizing enstatite at the expense of forsterite, and destabilizing plagioclase and clinopyroxene. These shifts cause the initial layering of Mercury’s solidified magma ocean to be enstatite-rich and plagioclase poor. Our results on S speciation at low ƒO2 are also applicable to the petrologic evolution of enstatite chondrite parent bodies and perhaps early Earth.
... Therefore, the remarkably low fO 2 conditions in Mercury have been invoked to explain the substantial solubility of sulfide in its lavas (Nittler et al., 2011;Namur et al., 2016a). Sulfide has an affinity for divalent cations (Ca 2+ , Mg 2+ , Fe 2+ ), forming complexes in various glasses (O'Neill and Mavrogenes, 2002;Fleet et al., 2005;Namur et al., 2016a;Anzures et al., 2020). Additional processes which may induce the formation of Ca or Mg sulfides at the surface of Mercury have been suggested, such as gas-solid interactions during volcanic and impact processes (Renggli et al., 2022). ...
... Our current understanding of the effect of S on the structure of silicate glasses and melts has benefitted significantly from industry-related research on simple binary or ternary systems. As reviewed elsewhere (Wilke et al., 2011), various spectroscopic techniques have been used to investigate the speciation and bonding of sulfur in alkali silicate glasses, including Raman scattering (Asahi et al., 1999;Tsujimura et al., 2004;Klimm and Botcharnikov, 2010;Klimm et al., 2012;Namur et al., 2016a;Watson and Martin, 2017;Lonergan et al., 2020), nuclear magnetic resonance (NMR) (Tsujimura et al., 2004;Watson and Martin, 2017), Xray photoelectron spectroscopy (Asahi et al., 1999), and X-ray absorption near-edge structure (XANES) spectroscopy (Fleet et al., 2005;Klimm et al., 2012;Anzures et al., 2020). Generally, simple glass compositions were targeted in these studies, and further investigations are needed to elucidate the role of S in complex (natural) glasses and under highly reducing conditions relevant to Mercury. ...
... Prior Raman spectra of glasses acquired for a slightly reduced (ΔIW ~ +2.5) Sbearing KSi 4 O 9 glass (Klimm and Botcharnikov, 2010) or for more reduced (ΔIW < − 1.5) (Mg,Ca,Fe)S-bearing silicate melts (Namur et al., 2016a) found no indication of Si-S bonds. In addition, we see no evidence for Na-S bonds, which can be explained by the weak interactions between sodium and sulfide, and by the absence of Na 2 S species at our highly reduced conditions (Anzures et al., 2020). ...
... In larnite rock, oldhamite is associated with Fe 3+ -bearing minerals such as brownmillerite, Ca 2 FeAlO 5 (Figure 15a), and cannot be an indicator of the reduction conditions for the entire rock volume. Experimental studies indicate that caswellsilverite (and grokhovskyite) form in paralava at relatively higher oxygen activity 0 ≤ ΔIW< −2 in comparison with oldhamite [42]. This suggests that super-or high-reduction conditions (ΔIW ≈ −6-−2) at the contact zone of paralava with clasts of altered country rock change within a distance of a few centimetres (the central parts of paralava zones) to the reduction conditions near the Fe/FeO (ΔIW ≈ 0) buffer. ...
... In the studied phosphide-bearing breccia, a "meteoritic" sulphide, oldhamite, CaS, rarely encountered as small rounded inclusions in pseudowollastonite from the paralava contact zone, crystallizes from melt (Figure 15b), and can be an indicator of the reducing conditions. The investigation of sulphide genesis in "mercurian melt" showed that oldhamite is stable at about ∆IW ≈ −2 [42]. ...
... Caswellsilverite crystallizes in the central porous parts of paralava together with pyrrhotite (Figure 2b), whereas in non-porous fragments of paralava, small iron drops form (Figure 2a) that can indicate that the primary iron melt is enriched in sulphur carried by combustion gases. Experimental studies indicate that caswellsilverite (and grokhovskyite) form in paralava at relatively higher oxygen activity 0 ≤ ∆IW< −2 in comparison with oldhamite [42]. This suggests that super-or high-reduction conditions (∆IW ≈ −6-−2) at the contact zone of paralava with clasts of altered country rock change within a distance of a few centimetres (the central parts of paralava zones) to the reduction conditions near the Fe/FeO (∆IW ≈ 0) buffer. ...
For the first time, chromium disulphides, known from meteorites, such as caswellsilverite, NaCrS2; grokhovskyite, CuCrS2; and a potentially new mineral, AgCrS2, as well as the products of their alteration, such as schöllhornite, Na0.3CrS2∙H2O, and a potentially new mineral with the formula {Fe0.3(Ba,Ca)0.2} CrS2·0.5H2O, have been found in terrestrial rock. Layered chromium disulphides were found in unusual phosphide-bearing breccia of the pyrometamorphic Hatrurim Complex in the Negev Desert, Israel. The chromium disulphides belong to the central fragment of porous gehlenite paralava cementing altered host rock clasts. The empirical formula of caswellsilverite is (Na0.77Sr0.03Ca0.01)Σ0.81(Cr3+0.79Cr4+0.18V3+0.01 Fe3+0.01)Σ0.99S2·0.1H2O, and the end-member content of NaCrS2 is 76%. It forms single crystals in altered pyrrhotite aggregates. Grokhovskyite has the empirical formula {Cu+0.84Fe3+0.10Ca0.06 Na0.01 Sr0.01Ba0.01}Σ1.03(Cr3+0.94 Fe3+0.05 V3+0.05)Σ1.00S2·0.35H2O, and the CuCrS2 end-member content is 75–80%. A potentially new Ag-bearing chromium disulphide is characterised by the composition (Ag0.89Cu0.07)Σ0.96(Cr0.98 Fe0.03V0.01Ni0.01)Σ1.04S2. Caswellsilverite, grokhovskyite and AgCrS2 form in gehlenite paralava at high temperatures (near 1000 °C) and low pressure under reducing conditions. The structure of the layered chromium disulphides, MCrS2, is characterised by the presence of hexagonal octahedral layers (CrS2)1−, between which M-sites of the monovalent cations Ag, Cu and Na set. A low-temperature alteration of the layered chromium disulphides, when schöllhornite and {Fe0.3(Ba,Ca)0.2}CrS2·0.5H2O form, is reflected in the composition and structural modification of the layer with monovalent cations, whereas the octahedral layer (CrS2)1− remains unchanged.
... This is evident in the S and C depletion in Nathair Facula (NE Rachmaninoff), the only pyroclastic deposit large enough to have its composition measured by MESSENGER (Deutsch et al., 2021;Pegg et al., 2021;Weider et al., 2016). Since the solubility of S in basalts increases with decreasing f O 2 from IW-3 to IW-7 (Anzures et al., 2020b;Namur et al., 2016a;O'Neill and Mavrogenes, 2002), the amount of S released by volcanic degassing from erupted volcanic plains is approximately an order of magnitude larger at IW-3 compared to IW-7 (Deutsch et al., 2021). The resulting volcanic gas in the system C-O-S was likely dominated by the species CO, S 2 , CS 2 , and COS (Zolotov et al., 2013;Zolotov, 2011), which is sim-ilar to a lunar volcanic gas (Renggli et al., 2017). ...
... The high S abundance measured by XRS (X-ray spectrometry) on the surface of Mercury has commonly been assumed to be hosted in magmatic rocks, dissolved in silicates or crystallized as sulfide phases (Anzures et al., 2020b(Anzures et al., , 2020aDeutsch et al., 2021;Namur et al., 2016aNamur et al., , 2016b. This is based on the increasing solubility of S in basaltic melts with decreasing oxygen fugacity (Anzures et al., 2020b;Namur et al., 2016a;O'Neill and Mavrogenes, 2002). ...
... The high S abundance measured by XRS (X-ray spectrometry) on the surface of Mercury has commonly been assumed to be hosted in magmatic rocks, dissolved in silicates or crystallized as sulfide phases (Anzures et al., 2020b(Anzures et al., , 2020aDeutsch et al., 2021;Namur et al., 2016aNamur et al., , 2016b. This is based on the increasing solubility of S in basaltic melts with decreasing oxygen fugacity (Anzures et al., 2020b;Namur et al., 2016a;O'Neill and Mavrogenes, 2002). These models suggest that the magmas on Mercury erupted highly reduced, at oxygen fugacities of IW-4 or lower, and crustal oxidation of the magmas was limited. ...
The surface of Mercury is enriched in sulfur, with up to 4 wt.% detected by the NASA MESSENGER mission, and has been challenging to understand in the context of other terrestrial planets. We posit, that magmatic S was mobilized as a gas phase in volcanic and impact processes near the surface, exposing silicates to a hot S-rich gas at reducing conditions and allowing conditions for rapid gas-solid reactions. Here, we present novel experiments on the reaction of Mercury composition glasses with reduced S-rich gas, forming Ca- and Mg-sulfides. The reaction products provide porous and fragile materials that create previously enigmatic hollows on Mercury. Our model predicts that the gas-solid reaction forms Ca-Mg-Fe-Ti-sulfide assemblages with SiO2 and aluminosilicates, distinct from formation as magmatic minerals. The ESA/JAXA BepiColombo mission to Mercury will allow this hypothesis to be tested.
... Once the melt reached sulfur saturation, sulfides would exsolve. With little iron available, these sulfides would be dominantly magnesium (niningerite) and calcium (oldhamite) sulfide, as found in enstatite chondrites (Anzures et al., 2020;Keil, 1989). These Mg, Ca-rich sulfides would be much less dense than FeS and even less dense than the crystallized silicate phases. ...
... We predict the quantity of sulfide in Mercury's mantle and its implications for Mercury's OSS density structure. The sulfide composition and content in Mercury's mantle depend on the oxygen fugacity and temperature of its magma ocean, and under highly reducing conditions could have reached >10 wt.% sulfur dominantly speciated as MgS and CaS (Anzures et al., 2020). We assume that all dissolved sulfur eventually crystallized as sulfide and model the mantle mineralogy with one generic sulfide and one generic silicate with physical properties calculated based on MgS, CaS, enstatite, and forsterite. ...
... Higher densities require unexpectedly abundant dense phases. To convert sulfur to sulfide, we choose an atomic mass of 60 amu based on a Mg-dominated sulfide (predicted by Anzures et al. [2020]); more Ca would give a higher atomic mass. Alternative choices for the generic properties are evaluated in Text S3 of Supporting Information S1. ...
Plain Language Summary
Mercury's mantle is unusually rich in sulfur relative to the other terrestrial planets. That sulfur is not bound into the silicate rocks, but rather it is expected to take the form of calcium and magnesium‐rich sulfides. We estimate how much sulfide should be present in Mercury's mantle and what the presence of that sulfide means for the density of Mercury's mantle and core. We interpret recent measurements of the moment of inertia (MoI) of Mercury's mantle and of the whole planet in light of the possible sulfide content of Mercury's mantle. We find that if Mercury's mantle contains large amounts of sulfide, but not very much iron (which is what we expect), then the value of Mercury's polar MoI should be low. Alternatively, if Mercury's polar MoI is high, Mercury's mantle must not have much sulfide in it. If Mercury's mantle has little sulfide, either Mercury does not have much sulfur overall, or the sulfur is in Mercury's core. The latter implies that Mercury's interior chemistry (especially the amount of oxygen) is different from what is predicted from the abundance of elements and minerals measured at Mercury's surface.
... Impact basin plains are also chemically distinct from each other, with the Caloris basin being more Al/Si enriched than Rachmaninoff, and conversely, Rachmaninoff being more Mg/Si rich than Caloris (Lawrence et al., 2017;Weider et al., 2015). Studying the properties of Mercury's magma ocean may lend insight into the compositional variability of Mercury's surface, which implies heterogeneous mantle sources that may relate to stratification of Mercury's mantle after magma ocean crystallization or processes such as cumulate overturn or the rising of buoyant material into shallower cumulates (Charlier et (Weisberg et al., 1988); however, the Fe and Na content assumed were modified to better resemble an estimated Mercurian composition (Anzures et al., 2020). We experimentally investigate a residual liquid after 80% crystallization of a Mercurian magma ocean with an ALH 85085 bulk composition because it falls within a proposed Mercurian magma ocean fractionation line (see Figure 3 in Anzures et al., 2020). ...
... Studying the properties of Mercury's magma ocean may lend insight into the compositional variability of Mercury's surface, which implies heterogeneous mantle sources that may relate to stratification of Mercury's mantle after magma ocean crystallization or processes such as cumulate overturn or the rising of buoyant material into shallower cumulates (Charlier et (Weisberg et al., 1988); however, the Fe and Na content assumed were modified to better resemble an estimated Mercurian composition (Anzures et al., 2020). We experimentally investigate a residual liquid after 80% crystallization of a Mercurian magma ocean with an ALH 85085 bulk composition because it falls within a proposed Mercurian magma ocean fractionation line (see Figure 3 in Anzures et al., 2020). This silicate liquid is rich in Mg and has an intermediate silica content, similar to an Fe-free andesite (Table 1). ...
... First, we address lower viscosities of the S-bearing experiments compared to the S-free experiments, which suggests that the presence of sulfur in reduced andesitic liquids decreases viscosity. This result is somewhat surprising, because in silicate liquids under low oxygen fugacity conditions, sulfur is thought to exist as an anion (S 2− ) that complex with metal cations (e.g., Anzures et al., 2020). The addition of sulfur to a silicate liquid may disrupt existing Mg-and Ca-bearing structural networks through the formation of MgS and CaS species (Anzures et al., 2020;Fogel, 1997;McCoy et al., 1999;. ...
Mercury has a compositionally diverse surface that was produced by different periods of igneous activity suggesting heterogeneous mantle sources. Understanding the structure of Mercury's mantle formed during the planet's magma ocean stage could help in developing a petrologic model for Mercury, and thus, understanding its dynamic history in the context of crustal petrogenesis. We present results of falling sphere viscometry experiments on late‐stage Mercurian magma ocean analogue compositions conducted at the Advanced Photon Source, beamline 16‐BM‐B, Argonne National Laboratory. Owing to the presence of sulfur on the surface of Mercury, two compositions were tested, one with sulfur and one without. The liquids have viscosities of 0.6–3.9 (sulfur‐bearing; 2.6–6.2 GPa) and 0.6–10.9 Pa·s (sulfur‐free; 3.2–4.5 GPa) at temperatures of 1600–2000°C. We present new viscosity models that enable extrapolation beyond the experimental conditions and evaluate grain growth and the potential for crystal entrainment in a cooling, convecting magma ocean. We consider scenarios with and without a graphite flotation crust, suggesting endmember outcomes for Mercury's mantle structure. With a graphite flotation crust, crystallization of the mantle would be fractional with negatively buoyant minerals sinking to form a stratified cumulate pile according to the crystallization sequence. Without a flotation crust, crystals may remain entrained in the convecting liquid during solidification, producing a homogeneous mantle. In the context of these endmember models, the surface could result from dynamical stirring or mixing of a mantle that was initially heterogeneous, or potentially from different extents of melting of a homogeneous mantle.
... The high abundance of S on the surface of Mercury measured by MESSENGER, at concentrations of up to 4 wt.% (Evans et al., 2012;Nittler et al., 2011), was therefore interpreted to be hosted in magmatic phases at highly reducing conditions (Anzures, Parman, Milliken, Namur, et al., 2020;Namur et al., 2016;Zolotov et al., 2013). For example, at 1350-1500°C, 4 wt.% of S could be dissolved in silicate melts at very reducing conditions of ∼IW-6 (Anzures, Parman, Milliken, Namur, et al., 2020;Namur et al., 2016). ...
... (Evans et al., 2012;Nittler et al., 2011), was therefore interpreted to be hosted in magmatic phases at highly reducing conditions (Anzures, Parman, Milliken, Namur, et al., 2020;Namur et al., 2016;Zolotov et al., 2013). For example, at 1350-1500°C, 4 wt.% of S could be dissolved in silicate melts at very reducing conditions of ∼IW-6 (Anzures, Parman, Milliken, Namur, et al., 2020;Namur et al., 2016). Yet, the FeO content in surface silicates on Mercury was determined 0.2 to 4.5 wt% (Evans et al., 2012;McClintock et al., 2008;McCoy et al., 2018), which would be too high for such highly reducing conditions, if the FeO was dissolved in a silicate melt, requiring the presence of Fe in sulfides or a metal phase (Zolotov et al., 2013). ...
We propose that the observed enrichment of sulfur at the surface of Mercury (up to 4 wt.%) is the product of silicate sulfidation reactions with a S‐rich reduced volcanogenic gas phase. Here, we present new experiments on the sulfidation behavior of olivine, diopside, and anorthite. We investigate these reaction products, and those of sulfidized glasses with Mercury compositions previously reported, by mid‐IR reflectance spectroscopy. We investigate both the reacted bulk materials as powders as well as cross‐sections of the reaction products by in situ micro‐IR spectroscopy. The mid‐IR spectra confirm the presence of predicted reaction products including quartz. The mid‐IR reflectance of sulfide reaction products, such as CaS (oldhamite) or MgS (niningerite), is insufficient to be observed in the complex run products. However, the ESA/JAXA BepiColombo mission to Mercury will be able to test our hypothesis by investigating the correlated abundances of sulfides with other reaction products such as quartz.
... Under highly reduced conditions, sulfur should also be highly soluble in silicate melts (Namur et al., 2016a), consistent with the high sulfur contents of Mercurian lavas (0.5-3.5 wt%; Peplowski et al., 2015;Weider et al., 2015;Namur et al., 2016a). Therefore, Mercury's silicate mantle most likely contains abundant sulfides, probably of the oldhamite (CaS)-niningerite (MgS) series (Namur et al., 2016b;Boukaré et al., 2019;Anzures et al., 2020), which are commonly observed in enstatite chondrites (Crozaz and Lundberg, 1995;Lehner et al., 2013) and are enriched in various trace elements, notably rare-earth elements and actinides (Hammouda et al., 2022) that otherwise behave as lithophile (Ingrao et al., 2019). Malavergne et al. (2010) first proposed that a solid FeS layer could be stable at the CMB and estimated that it should be <15 km thick if Mercury formed from S-poor Benccubinite chondrites or 100-220 km thick if accreted from S-rich enstatite chondrites. ...
... The Si/SiO 2 ratio was balanced accordingly to keep the total Si content of enstatite chondrites. In this way, the total silicon content of the starting material was the same in all experiments but the Si/SiO 2 ratio changed, leading to various fO 2 conditions between experiments (Namur et al., 2016a(Namur et al., , 2016bAnzures et al., 2020). The final powders were dried in an oven at 120 • C and stored in a desiccator until use. ...
... This hypothesis may be similarly invoked as an alternative explanation for the sulfur content on the surface; however, in this study, we assume that surficial sulfur originates from the planet's interior. Experimental data show that sulfur solubility in silicate liquids is increased at low oxygen fugacities (e.g., Anzures et al., 2020;Berthet et al., 2009;Mavrogenes & O'Neill, 1999), suggesting that sulfides may have precipitated from Mercury's magma ocean along with the silicates. Below, the potential for sulfide in pyroxene-bearing and dunite layers is used to assess the implications of sulfide for the density of the layers. ...
... Considering the low fO 2 environment and anticipated saturation state of sulfides in a Mercurian magma ocean, we evaluate what we consider probable density structure scenarios for a cumulate mantle formed after magma ocean solidification. Because of the low solubility of Fe sulfide in silicate melts at Mercurian fO 2 s (Anzures et al., 2020), if saturated in Fe-rich sulfide, a Mercurian magma ocean would precipitate the phase early, such that the highest concentration would be at the core-mantle boundary or in the lower mantle, increasing the density of deeper cumulates (Figure 5; e.g., Boukaré et al., 2019). As for low density sulfides (e.g., oldhamite), it is likely they would precipitate late, after fractional enrichment of alkali elements in the residual magma ocean liquid. ...
Mercury has a compositionally diverse surface exhibiting geochemical terranes that represent different periods of igneous activity, suggesting diverse mantle source compositions. Mercury's juvenile mantle likely formed after fractional solidification of a magma ocean, which produced distinct mineralogical horizons with depth. To produce the diversity of observed volcanic terranes, dynamic mixing of materials from distinct mantle horizons is required. One process that could dynamically mix the juvenile cumulate pile is cumulate mantle overturn, where dense layers in shallow planetary mantles sink into deeper, less dense layers as Rayleigh‐Taylor instabilities. Gravitationally unstable density stratification is a requisite starting condition for overturn; solidification of the Mercurian magma ocean is likely to have produced such a density inversion, with a relatively dense clinopyroxene‐bearing pyroxenite layer atop lower density dunite and harzburgite layers. Sulfides are present in abundance on Mercury's surface and would be additional mantle phase(s) if they are indigenous to the planet's interior. Sulfides have variable densities; they could potentially enhance the formation of gravitational instabilities or prevent them from developing. Exploring physically reasonable mantle density and viscosity structures, we evaluate the potential for cumulate mantle overturn in Mercury and predict the possible timing, scale, and rate of overturn for plausible physical parameter combinations. Our analysis suggests that overturn is possible in Mercury's mantle within 100 Myr of magma ocean solidification, providing a mechanism for producing the mantle sources that would melt to form surface compositions on Mercury, and overturn may control the spatial scale of volcanic provinces observed on the surface today.
... Both sulfur and carbon have been implicated as potential components driving explosive volcanism on Mercury (Kerber et al., 2009;Zolotov, 2011;Weider et al., 2016;McCubbin et al., 2017), but these volatiles contribute to this process through very different mechanisms. Sulfur solubility in silicate melts is strongly enhanced at low f O 2 and high temperatures (Namur et al., 2016;Anzures et al., 2020), and the efficient delivery of sulfur to Mercury's surface is evidenced by remarkably high S concentrations in surface materials (∼1-6 wt%; Nittler et al., 2011;Weider et al., 2012;Starr et al., 2012;Evans et al., 2012). The relatively high carrying capacity of sulfur compared to carbon in reduced magmas (Namur et al., 2016;Anzures et al., 2020) along with observations of elevated abundances of S at the surface have led to the conclusion that sulfur is one of the main components driving explosive volcanism on Mercury (Kerber et al., 2009;Weider et al., 2016). ...
... Sulfur solubility in silicate melts is strongly enhanced at low f O 2 and high temperatures (Namur et al., 2016;Anzures et al., 2020), and the efficient delivery of sulfur to Mercury's surface is evidenced by remarkably high S concentrations in surface materials (∼1-6 wt%; Nittler et al., 2011;Weider et al., 2012;Starr et al., 2012;Evans et al., 2012). The relatively high carrying capacity of sulfur compared to carbon in reduced magmas (Namur et al., 2016;Anzures et al., 2020) along with observations of elevated abundances of S at the surface have led to the conclusion that sulfur is one of the main components driving explosive volcanism on Mercury (Kerber et al., 2009;Weider et al., 2016). ...
Here we present the results of experiments designed to reproduce the interaction between super-solidus mercurian magmas and graphite at high temperatures (ramped up from ambient temperature to 1195–1390 °C) and low pressure (10 mbar). The compositions of resultant gases were measured in situ with a thermal gravimeter/differential scanning calorimeter connected to a mass spectrometer configured to operate under low pressures and reducing conditions. Solid run products were analyzed by electron microprobe and Raman spectroscopy. Three magma starting compositions were based on the composition of the Borealis Planitia region (termed NVP for the Northern Volcanic Plains) on Mercury ± alkali metals, sulfur, and transition metal oxides. Smelting between FeOmelt and graphite was observed above 1100 °C, evidenced by the generation of CO and CO2 gas and the formation of Fe-Si metal alloys, which were found in contact with residual graphite grains. Experiments with transition metal oxide-free starting compositions did not produce metal alloys and showed no significant gas production. In all runs that produced gas, C-O-H±S species dominated the degassing vapor. Our results suggest that the consideration of graphite smelting processes can significantly increase calculated eruption velocities and that gas produced by smelting alone can account for >75% of the pyroclastic deposits identified on Mercury. A combination of S-H-degassing and CO-CO2 production from smelting can explain all but the single largest pyroclastic deposit on Mercury.
... Boujibar et al. (2019) also showed that if Mercury's core was formed at fO 2 higher than IW-4, Mercury would not have enough Si in its core to yield a chondritic Fe/Si ratio, although this may be reconcilable for a CB-like Mercury bulk composition (Vander Kaaden et al., 2020). Finally, Anzures et al. (2020) suggested that Mercury's mantle has log fO 2 between IW-4 and IW-2, based on the correlation of Ca and S concentrations observed at the surface of the planet and on the formation of CaS complexes in silicate melts at this range of fO 2 values. Our results, based on the surface Cr abundance, suggest a broad range of fO 2 conditions (IW-6 to IW-2), which overlap and are consistent with previously suggested ranges, including those based on surface Fe, S, and Si abundances. ...
Mercury, the innermost planet, formed under highly reduced conditions, based mainly on surface Fe, S, and Si abundances determined from MESSENGER mission data. The minor element Cr may serve as an independent oxybarometer but only very limited Cr data have been previously reported for Mercury. We report Cr/Si abundances across Mercury's surface based on MESSENGER X‐Ray Spectrometer data throughout the spacecraft's orbital mission. The heterogeneous Cr/Si ratio ranges from 3.6 × 10⁻⁵ in the Caloris Basin to 0.0012 within the high‐magnesium region, with an average southern hemisphere value of 0.0008 (corresponding to about 200 ppm Cr). Absolute Cr/Si values have systematic uncertainty of at least 30%, but relative variations are more robust. By combining experimental Cr partitioning data along with planetary differentiation modeling, we find that if Mercury formed with bulk chondritic Cr/Al, Cr must be present in the planet's core and differentiation must have occurred at log fO2 in the range of IW‐6.5 to IW‐2.5 in the absence of sulfides in its interior and a range of IW‐5.5 to IW‐2 with an FeS layer at the core‐mantle boundary. Models with large fractions of Mg‐Ca‐rich sulfides in Mercury's interior are more compatible with moderately reducing conditions (IW‐5.5 to IW‐4) owing to the instability of Mg‐Ca‐rich sulfides at elevated fO2. These results indicate that if Mercury differentiated at a log fO2 lower than IW‐5.5, the presence of sulfides whether in the form of a FeS layer at the top of the core or Mg‐Ca‐rich sulfides within the mantle would be unlikely.
... (IW = iron-wüstite redox buffer, in lgfo 2 ) is necessary for the stable existence of oldhamite in the solar nebula model [8][9][10]. However, our experiment and calculation results show that the oldhamite can be stable below IW+3.3 (Fig. 2), which is 6 orders of magnitude higher than the limit in the solar nebula mechanism. ...
In the solar system, oldhamite (CaS) is generally considered to be formed by the condensation of solar nebula gas. Enstatite chondrites, one of the most important repositories of oldhamite, are believed to be the representative of the material which formed Earth. Thus, the formation mechanism and the evolution process of oldhamite are of great significance to the deep understanding of the solar nebula, meteorites, the origin of Earth, and the C-O-S-Ca cycles of Earth. Until now, oldhamite has not been reported to occur in mantle rock. However, here we show the formation of oldhamite through the reaction between sulfide-bearing orthopyroxenite and molten CaCO3 at 1.5 GPa/1510 K, 0.5 GPa/1320 K, and 0.3 GPa/1273 K. Importantly, this reaction occurs at oxygen fugacities within the range of upper mantle conditions, 6 orders of magnitude higher than that of the solar nebula mechanism. Oldhamite is easily oxidized to CaSO4 or hydrolyzed to produce calcium hydroxide. Low oxygen fugacity of magma, extremely low oxygen content of the atmosphere, and the lack of a large amount of liquid water on the planet's surface are necessary for the widespread existence of oldhamite on the surface of a planet; otherwise, anhydrite or gypsum will exist in large quantities. Oldhamites may exist in the upper mantle beneath mid-ocean ridges. Additionally, oldhamites may have been a contributing factor to the early Earth's atmospheric hypoxia environment, and the transient existence of oldhamites during the interaction between reducing sulfur-bearing magma and carbonate could have had an impact on the changes in atmospheric composition during the Permian-Triassic Boundary.
... The reduced nature of the aubrite parent bodies and Mercury has raised many questions regarding the geochemical behavior of typically lithophile elements in magmas at low fO 2 . Interpretations of magmatic processes on reduced rocky bodies are difficult at present because the number of experimental studies relevant to the extremely low fO 2 and/or high sulfur content of the aubrite parent bodies and Mercury is limited (e.g., Anzures et al., 2020;Boujibar et al., 2019;Chabot et al., 2014;Dickinson & McCoy, 1997;Malavergne et al., 2004Malavergne et al., , 2007Malavergne et al., , 2014McCoy et al., 1999;Mouser et al., 2021;Ricolleau et al., 2011;Rose-Weston et al., 2009;Steenstra, Seegers, et al., 2020;Steenstra, Trautner, et al. 2020;Vander Kaaden & McCubbin, 2016;Vander Kaaden et al., 2020;Vogel et al., 2018), and our only natural examples of what to expect with respect to element behavior in such oxygen-starved systems are found in the highly reduced enstatite chondrites and aubrites (Burbine et al., 2012;Ingrao et al., 2019;Keil, 2010;Udry et al., 2019). At such reduced conditions, elements will deviate from their typical geochemical behavior displayed at higher fO 2 , which will impact the thermochemical evolution of that body (e.g., Cartier et al., 2014;Kaufman et al., 2016 ;McCoy et al., 1999;McCubbin et al., 2012;Vander Kaaden & McCubbin, 2016). ...
Enstatite‐rich meteorites, including the aubrites, formed under conditions of very low oxygen fugacity (ƒO2: iron‐wüstite buffer −2 to −6) and thus offer the ability to study reduced magmatism present on multiple bodies in our solar system. Elemental partitioning among metals, sulfides, and silicates is poorly constrained at low ƒO2; however, studies of enstatite‐rich meteorites may yield empirical evidence of the effects of low ƒO2 on elemental behavior. This work presents comprehensive petrologic and oxygen isotopic studies of 14 aubrites, including four meteorites that have not been previously investigated in detail. The aubrites exhibit a variety of textures and mineralogy, and their elemental zoning patterns point to slow cooling histories for all 14 samples. Oxygen isotope analyses suggest that the aubrite parent bodies may be more heterogeneous than originally reported or may have experienced incomplete magmatic differentiation. Contrary to the other classified aubrites and based on textural and mineralogical observations, we suggest that the Northwest Africa 8396 meteorite shows an affinity for an enstatite chondrite parentage. By measuring major elemental compositions of silicates, sulfides, and metals, we calculate new metal–silicate, sulfide–silicate, and sulfide–metal partition coefficients for aubrites that are applicable to igneous systems at low ƒO2. The geochemical behavior of elements in aubrites, as determined using partition coefficients, is similar to the geochemical behavior of elements determined experimentally for magmatic systems on Mercury. Enstatite‐rich meteorites, including aubrites, represent valuable natural petrologic analogues to Mercury and their study could further our understanding of reduced magmatism in our solar system.
... 21 Alternatively to the use of sulfate, S could be added as a sulfide, requiring the glass synthesis under highly reducing conditions below the iron-wustite buffer (IW), where S has a high solubility in silicate melts. 24 Similarly, Se and Te could be dissolved in silicate melts under reducing conditions <IW, added as selenide and telluride compounds. Finally, the addition of FeO to the melt increases the solubility of chalcophile elements in silicate melts. ...
The evaporation of volatile and moderately volatile elements from silicate glasses is an important topic in geosciences, environmental, and materials sciences. Glasses that contain volatile elements are used in a wide range of experimental studies, but the synthesis of volatile-bearing glasses at high temperatures as well as the choice of starting materials is challenging. Here, we present a new method for the synthesis of 15–20 g of moderately volatile- and volatile element-bearing boron-aluminosilicate glasses using a two-stage melting process. Results show that the glasses contain between 7000 and 10,000 μg/g Zn, Cu, or Te and ∼3000 μg/g S. In situ analyses with scanning electron microscopy and electron microprobe analysis confirm that all glasses are homogeneous for major and trace elements within the analytical uncertainties.
... The presence of unusual sulfides containing normally lithophile elements (Mg, Ca, Mn, Ti, Na), Si-bearing kamacite, nitrides, phosphides and carbides reflects highly reducing environment of EC formation. Under these conditions, elements including the rare-earth elements (REE) and the actinides that are lithophile in terrestrial conditions and in other types of chondrites tend to be siderophile and/or chalcophile (e.g., Barrat et al. 2014;Crozaz and Lundberg1995;Gannoun et al. 2011;Piani et al. 2016;Cartier and Wood 2019;Anzures et al. 2020). ...
We have determined rare-earth element (REE) abundances in oldhamites (CaS) from 13 unequilibrated and equili-brated enstatite chondrites (5 EH and 8 EL) and in a few enstatites by in situ, laser ablation ICP-MS. In EH chondrites, oldhamite REE patterns vary from the most primitive petrographic types (EH3) to the most metamorphosed types (EH5). In EH3, CI-normalized REE patterns are convex downward with strong positive Eu and Yb anomalies, whereas EH5 display flat patterns with enrichments reaching about 80 times CI abundances. The positive anomalies of Eu and Yb found in oldhamites of primitive EH chondrites indicate that they represent the condensation of a residual gas fraction, in a manner similar to fine-grained CAIs of carbonaceous chondrites. The early condensate may have been preserved in the matrix of unequilibrated EH. Equilibrated EH oldhamite patterns may result from metamorphic evolution and REE redistribution on the EH parent body. On the contrary, all the oldhamites from EL chondrites (EL3 to EL6) display a single kind of patterns, which is convex upward and is about 100 times enriched relative to CI, with a negative Eu anomaly. In addition, the EL pattern is similar to that of oldhamites from aubrites (enstatite achondrites). The latter observation suggests that oldhamites of all EL metamorphic types (including primitive ones) bear the signature of a magmatic event accompanied by FeS loss as vapor, prior to the assembly of the EL parent body. Given the difficulty of obtaining precise ages on enstatite chondrites, it is not possible to discuss the chronology of the events recorded by the oldhamite REE patterns.
... PSD has not been considered for other species such as S 0 . Although the MgS used in these experiments may not represent the exact mineralogical makeup of Mercury's surface, MgS, along with CaS and FeS, is thought to be a good representative of the prominent sulfide component of the crust (Anzures et al., 2020). ...
Mercury has a relatively high sulfur content on its surface, and a signal consistent with ionized atomic sulfur (S⁺) was observed by the fast ion plasma spectrometer (FIPS) instrument on the MESSENGER spacecraft. To help confirm this assignment and to better constrain the sources of exospheric sulfur at Mercury, 193 nm photon‐stimulated desorption (PSD) of neutral sulfur atoms (S⁰) from MgS substrates was studied using resonance‐enhanced multiphoton ionization (REMPI) spectroscopy and time‐of‐flight (TOF) mass spectrometry. Though the PSD process is inherently nonthermal, the measured velocities of ejected S⁰ were fit using flux‐weighted Maxwellian distributions with translation energies expressed as translational “temperatures” Ttrans = /μkB. A bimodal distribution consisting of both thermal (Ttrans = 300 K) and suprathermal (Ttrans > 1,000 K) components in roughly a 2:1 ratio was found to best fit the data. The PSD cross‐section was measured to be approximately 4 × 10⁻²² cm² and, together with the velocity distributions, was used to calculate the PSD source rate of S⁰ into the exosphere of Mercury. Exosphere simulations using the calculated rates demonstrate that PSD is likely the primary source of S⁰ in Mercury's exosphere at low (<1,000 km) altitudes.
Plain Language Summary
Mineral assemblages that constitute the mantles of reduced exoplanets, such as those formed in the inner regions of stellar nebulae, have been scarcely investigated. We propose that these exoplanets share several physical and chemical properties with planet Mercury in our Solar System, as they formed in a similar geochemical environment. In order to outline the minerals constituting these mantles, we take in account all the chemical compositions previously proposed to match Mercury's observed characteristics. These compositions are extrapolated from several classes of meteorites, among the most pristine materials currently known and best candidates as “building blocks” of planetary objects. We, then use the well‐established thermodynamic code Perple_X to predict the stable minerals at pressure and temperature ranges assumed for these exoplanets' interiors. We find that silicate mantles of exoplanets located in inner regions of nebulae are dominated by pyroxene group minerals, rather than olivine, as in the Earth's mantle. This results in different rheological (e.g., viscosity) and physical properties (e.g., melting behavior) for these reduced exoplanets, with significant implications for their mantle dynamics and evolution over time.
Abundant carbon was identified on Mercury by MESSENGER, which is interpreted as the remnant of a primordial graphite flotation crust, suggesting that the magma ocean and core were saturated in carbon. We re-evaluate carbon speciation in Mercury’s interior in light of the high pressure-temperature experiments, thermodynamic models and the most recent geophysical models of the internal structure of the planet. Although a sulfur-free melt would have been in the stability field of graphite, sulfur dissolution in the melt under the unique reduced conditions depressed the sulfur-rich liquidus to temperatures spanning the graphite-diamond transition. Here we show it is possible, though statistically unlikely, that diamond was stable in the magma ocean. However, the formation of a solid inner core caused diamond to crystallize from the cooling molten core and formation of a diamond layer becoming thicker with time.
Mercury, the innermost planet, formed under highly reduced conditions, based mainly on surface Fe, S, and Si abundances determined from MESSENGER mission data. The minor element Cr may serve as an independent oxybarometer, but only very limited Cr data have been previously reported for Mercury. We report Cr/Si abundances across Mercury's surface based on MESSENGER X-Ray Spectrometer data throughout the spacecraft's orbital mission. The heterogeneous Cr/Si ratio ranges from 0.0015 in the Caloris Basin to 0.0054 within the high-magnesium region, with an average southern hemisphere value of 0.0008 (corresponding to about 200 ppm Cr). Absolute Cr/Si values have systematic uncertainty of at least 30%, but relative variations are more robust. By combining experimental Cr partitioning data along with planetary differentiation modeling, we find that if Mercury formed with bulk chondritic Cr/Al, Cr must be present in the planet's core and differentiation must have occurred at log fO2 in the range of IW-6.5 to IW-2.5 in the absence of sulfides in its interior, and a range of IW-5.5 to IW-2 with an FeS layer at the core-mantle boundary. Models with large fractions of Mg-Ca-rich sulfides in Mercury's interior are more compatible with moderately reducing conditions (IW-5.5 to IW-4) owing to the instability of Mg-Ca-rich sulfides at elevated fO2. These results indicate that if Mercury differentiated at a log fO2 lower than IW-5.5, the presence of sulfides whether in the form of a FeS layer at the top of the core or Mg-Ca-rich sulfides within the mantle would be unlikely.
Various electrode materials are considered for sodium-ion batteries (SIBs) and one important prerequisite for developments of SIBs is a detailed understanding about charge storage mechanisms. Herein, we present a rigorous study about Na storage properties of ultra-small Fe3S4 nanoparticles, synthesized applying a solvothermal route, which exhibit a very good electrochemical performance as anode material for SIBs. A closer look into electrochemical reaction pathways on the nanoscale, utilizing synchrotron-based X-ray diffraction and X-ray absorption techniques, reveals a complicated conversion mechanism. Initially, separation of Fe3S4 into nanocrystalline intermediates occurs accompanied by reduction of Fe3+ to Fe2+ cations. Discharge to 0.1 V leads to formation of strongly disordered Fe0 finely dispersed in a nanosized Na2S matrix. The resulting volume expansion leads to a worse long-term stability in the voltage range 3.0-0.1 V. Adjusting the lower cut-off potential to 0.5 V, crystallization of Na2S is prevented and a completely amorphous intermediate stage is formed. Thus, the smaller voltage window is favorable for long-term stability, yielding highly reversible capacity retention, e.g., 486 mAh g-1 after 300 cycles applying 0.5 A g-1 and superior coulombic efficiencies >99.9%. During charge to 3.0 V, Fe3S4 with smaller domains are reversibly generated in the 1st cycle, but further cycling results in loss of structural long-range order, whereas the local environment resembles that of Fe3S4 in subsequent charged states. Electrokinetic analyses reveal high capacitive contributions to the charge storage, indicating shortened diffusion lengths and thus, redox reactions occur predominantly at surfaces of nanosized conversion products.
The surface of Mercury is dominated by extensive, widespread lava plains that formed early in its history. The emplacement of these lavas was accompanied by the release of magmatic volatiles, the bulk of which were lost to space via thermal escape and/or photodissociation. Here we consider the fate of these erupted volatiles by quantifying the volumes of erupted volcanic plains and estimating the associated masses of erupted volatiles. The concentrations and speciation of volatiles in Mercury's magmas are not known with certainty at this time, so we model a wide range of cases, based on existing experimental data and speciation models, at 3–7 log fO2 units below conditions determined by the iron-wüstite buffer. Cases range from relatively low gas content scenarios (total exsolved gas mass of 9×1015 kg) to high gas content scenarios (total exsolved gas = 5 × 10¹⁹ kg). We estimate that the average duration of a transient volcanic atmosphere resulting from a single eruption would be between ∼250 and ∼210,000 years, depending on the volume, degassed volatile content, and eruption rate of an individual eruption, as well as the fO2 conditions of the planet's interior. If a dense transient atmosphere was ever surface-bound long enough for the released volatiles to be transported to and cold-trapped at Mercury's polar regions, those trapped volatiles are predicted to be well-mixed with the regolith, and at least 16 m beneath the surface given regolith gardening rates. These volatiles would have a composition and age distinctly different from those of the H2O-ice deposits observed at the poles of Mercury today.
A striking feature of Mercury's volcanic surface is its high S and low FeO contents, which is thought to be produced by very reducing conditions compared to other terrestrial bodies. Experiments show that S solubility in silicate melts increases to % wt levels for oxygen fugacities lower than three log units below the iron‐wustite (IW) buffer. During magma ocean solidification, large amounts of sulfide could potentially precipitate. This work investigates the effects of primordial sulfide layering on the first 750 Myr of Mercury's mantle dynamics. It is proposed that sulfide layering could have been produced by fractional solidification in the highly reduced Mercury magma ocean (MMO). Such chemical layering implies mantle sources with variable sulfur contents that might have played an important role in early Mercurian magmatism. Our models investigate the production of sulfide‐rich layers and their preservation during post‐MO solid‐state mantle dynamics. An intriguing question is the role of these sulfide‐rich layers on mantle dynamics as they are expected to incorporate a substantial amount of heat‐producing elements (U, Th, and K). We use experimentally determined sulfur solubility in silicate melts to predict the depth at which sulfides precipitate in the MMO. The model produces primordial sulfide layers whose thickness and locations depend upon the oxygen fugacity (fO2) and initial sulfur content (Sinit) of the MMO. Several geodynamic regimes have been identified in the fO2‐Sinit space. This study shows that oxygen fugacity, bulk sulfur content, and sulfide segregation are key for the early thermochemical evolution of Mercury.
Extremely reducing conditions, such as those that prevailed during the accretion and differentiation of Mercury, change the “normal” pattern of behaviour of many chemical elements. Lithophile elements can become chalcophile, siderophile elements can become lithophile, and volatile elements can become refractory. In this context, unexpected elements, such as Si, are extracted to the core, while others (S, C) concentrate in the silicate portion of the planet, eventually leading to an exotic surface mineralogy. In this article, experimental, theoretical and cosmochemical arguments are applied to the understanding of how reducing conditions influenced Mercury, from the nature of its building blocks to the dynamics of its volcanism.
The chemical composition of a planetary body reflects its starting conditions modified by numerous processes during its formation and geological evolution. Measurements by X-ray, gamma-ray, and neutron spectrometers on the MESSENGER spacecraft revealed Mercury's surface to have surprisingly high abundances of the moderately volatile elements sodium, sulfur, potassium, chlorine, and thorium, and a low abundance of iron. This composition rules out some formation models for which high temperatures are expected to have strongly depleted volatiles and indicates that Mercury formed under conditions much more reducing than the other rocky planets of our Solar System. Through geochemical modeling and petrologic experiments, the planet's mantle and core compositions can be estimated from the surface composition and geophysical constraints. The bulk silicate composition of Mercury is likely similar to that of enstatite or metal-rich chondrite meteorites, and the planet's unusually large core is most likely Si rich, implying that in bulk Mercury is enriched in Fe and Si (and possibly S) relative to the other inner planets. The compositional data for Mercury acquired by MESSENGER will be crucial for quantitatively testing future models of the formation of Mercury and the Solar System as a whole, as well as for constraining the geological evolution of the innermost planet.
We present new viscosity measurements of a synthetic silicate system considered an analogue for the lava erupted on the surface of Mercury. In particular, we focus on the northern volcanic plains (NVP), which correspond to the largest lava flows on Mercury and possibly in the Solar System. High-temperature viscosity measurements were performed at both superliquidus (up to 1736 K) and subliquidus conditions (1569–1502 K) to constrain the viscosity variations as a function of crystallinity (from 0 to 28%) and shear rate (from 0.1 to 5 s-1). Melt viscosity shows moderate variations (4 –16 Pa s) in the temperature range 1736–1600 K. Experiments performed below the liquidus temperature show an increase in viscosity as shear rate increases from 0.1 to 5 s-1, resulting in a shear thinning behaviour, with a decrease in viscosity of ca. 1 log unit. The low viscosity of the studied composition may explain the ability of NVP lavas to cover long distances, on the order of hundreds of kilometres in a turbulent flow regime. Using our experimental data we estimate that lava flows with thickness of 1, 5 and 10 m are likely to have velocities of 4.8, 6.5 and 7.2 m/s respectively, on a 5° ground slope. Numerical modelling incorporating both the heat loss of the lavas and its possible crystallization during emplacement allows us to infer that high effusion rates (> 10000 m3/s) are necessary to cover the large distances indicated by satellite data from the MESSENGER spacecraft.
NASA’s MESSENGER spacecraft has revealed geochemical diversity across Mercury’s volcanic crust. Near-infrared to ultraviolet spectra and images have provided evidence for the Fe2+-poor nature of silicate minerals, magnesium sulfide minerals in hollows and a darkening component attributed to graphite, but existing spectral data is insufficient to build a mineralogical map for the planet. Here we investigate the mineralogical variability of silicates in Mercury’s crust using crystallization experiments on magmas with compositions and under reducing conditions expected for Mercury. We find a common crystallization sequence consisting of olivine, plagioclase, pyroxenes and tridymite for all magmas tested. Depending on the cooling rate, we suggest that lavas on Mercury are either fully crystallized or made of a glassy matrix with phenocrysts. Combining the experimental results with geochemical mapping, we can identify several mineralogical provinces: the Northern Volcanic Plains and Smooth Plains, dominated by plagioclase, the High-Mg province, strongly dominated by forsterite, and the Intermediate Plains, comprised of forsterite, plagioclase and enstatite. This implies a temporal evolution of the mineralogy from the oldest lavas, dominated by mafic minerals, to the youngest lavas, dominated by plagioclase, consistent with progressive shallowing and decreasing degree of mantle melting over time.
The geochemical behavior of sulfur in magmas depends strongly on the oxidation state of sulfur, but this is not easily determined by standard analytical methods. We have measured XANES absorption spectra at the sulfur K-edge and have found that such measurements are useful to characterize the oxidation state and speciation of sulfur in silicate glasses of geological relevance. Measured spectra of a set of reference minerals show the effects of different oxidation states and coordination numbers of sulfur: there is a large shift in energy (similar to 10-12 eV) of the sulfur K-edge between S2- and S6+. This large and easily detectable difference makes possible the measurement of the valence of sulfur in unknown samples by measuring the shift in energy of the absorption edge. This approach is applicable to both crystalline and glassy materials, and useful results have been obtained on samples with as little as 450 ppm S. We have used XANES measurements to characterize oxidation state and speciation of sulfur in a set of natural and synthetic sulfur-bearing glasses. The samples cover a range of composition from basaltic to almost rhyolitic, and some were synthesized over a range of pressure, temperature and oxygen fugacity; glass S content varies between 450 and 3000 ppm. XANES analyses, carried out in fluorescence mode at LURE, allowed determination of the sulfur oxidation state in all of the samples and clearly show that some samples contain a mixture of S2- and S6+; no other sulfur species were observed. Quantitative determination of the abundance of sulfide and sulfate shows good agreement with independent measurements based on electron-microprobe determination of the wavelength shift of sulfur K alpha X-rays.
Recent (142)Nd isotope data indicate that the silicate Earth (its crust plus the mantle) has a samarium to neodymium elemental ratio (Sm/Nd) that is greater than that of the supposed chondritic building blocks of the planet. This elevated Sm/Nd has been ascribed either to a 'hidden' reservoir in the Earth or to loss of an early-formed terrestrial crust by impact ablation. Since removal of crust by ablation would also remove the heat-producing elements--potassium, uranium and thorium--such removal would make it extremely difficult to balance terrestrial heat production with the observed heat flow. In the 'hidden' reservoir alternative, a complementary low-Sm/Nd layer is usually considered to reside unobserved in the silicate lower mantle. We have previously shown, however, that the core is a likely reservoir for some lithophile elements such as niobium. We therefore address the question of whether core formation could have fractionated Nd from Sm and also acted as a sink for heat-producing elements. We show here that addition of a reduced Mercury-like body (or, alternatively, an enstatite-chondrite-like body) rich in sulfur to the early Earth would generate a superchondritic Sm/Nd in the mantle and an (142)Nd/(144)Nd anomaly of approximately +14 parts per million relative to chondrite. In addition, the sulfur-rich core would partition uranium strongly and thorium slightly, supplying a substantial part of the 'missing' heat source for the geodynamo.
In order to investigate the influence of very reducing conditions, we report enstatite-melt trace element partition coefficients (D) obtained on enstatite chondrite material at 5 GPa and under oxygen fugacities (fO2) ranging between 0.8 and 8.2 log units below the iron-wustite (IW) buffer. Experiments were conducted in a multianvil apparatus between 1580 and 1850°C, using doped (Sc, V, REE, HFSE, U, Th) starting materials. We used a two-site lattice strain model and a Monte-Carlo-type approach to model experimentally determined partition coefficient data. The model can fit our partitioning data i.e. trace elements repartition in enstatite, which provides evidence for the attainment of equilibrium in our experiments. The precision on the lattice strain model parameters obtained from modelling does not enable determination of the influence of intensive parameters on crystal chemical partitioning, within our range of conditions (fO2, P, T, composition). We document the effect of variable oxygen fugacity on the partitioning of multivalent elements. Cr and V, which are trivalent in the pyroxene at around IW-1 are reduced to 2+ state with increasingly reducing conditions, thus affecting their partition coefficients. In our range of redox conditions Ti is always present as a mixture between 4+ and 3+ states. However the Ti3+/Ti4+ ratio increases strongly with increasingly reducing conditions. Moreover in highly reducing conditions, Nb and Ta, that usually are pentavalent in magmatic systems, appear to be reduced to lower valence species, which may be Nb2+ and Ta3+. We propose a new proxy for fO2 based on D(Cr)/D(V) ratio. Our new data extend the redox range covered by previous studies and allows this proxy to be used in the whole range of redox conditions of the solar system objects. We selected trace-element literature data of six chondrules on the criterion of their equilibrium. Applying the proxy to opx-matrix systems, we estimated that three type I chondrules have equilibrated at IW-7±1, one type I chondrule at IW-4±1, and two type II chondrules at IW+3±1. This first accurate estimation of enstatite-melt fo2 for type I chondrules is very close to CAI values.
Recent images of the surface of Mercury have revealed an unusual and intriguing landform: sub-kilometre scale, shallow, flat-floored, steep-sided rimless depressions typically surrounded by bright deposits and generally occurring in impact craters. These ‘hollows’ appear to form by the loss of a moderately-volatile substance from the planet’s surface and their fresh morphology and lack of superposed craters suggest that this process has continued until relatively recently (and may be on-going). Hypotheses to explain the volatile-loss have included sublimation and space weathering, and it has been suggested that hollow-forming volatiles are endogenic and are exposed at the surface during impact cratering. However, detailed verification of these hypotheses has hitherto been lacking.
MErcury Surface, Space ENvironment, GEochemistry, and Ranging
(MESSENGER) orbital observations of Mercury have revealed elevated S
abundances, Ca-S and Mg-S correlations, and a low upper limit for
ferrous iron in surface silicates. These data indicate the presence of
Ca and/or Mg sulfides in volcanic rocks and a low oxygen fugacity
(fO2) in their parental magmas. We have evaluated coupled
fO2 and fS2 values and FeO contents in Mercury's
magmas from silicate-sulfide equilibria and empirical models for
silicate melts and metallurgical slags. The evaluated fO2 at
1700-1800 K is 4.5 to 7.3 log10 units below the
iron-wüstite buffer. These values correspond to 0.028-0.79 wt %
FeO, implying that Fe must be also present in sulfides and metal and are
also consistent with the composition of the partial melt of an enstatite
chondrite. This derived upper limit for FeO is substantially lower than
the limits obtained from reflectance measurements of Mercury's surface
materials. The low fO2 and FeO values provide new constraints
for igneous processes on Mercury as well as the formation, evolution,
and internal structure of the innermost planet.
X-ray absorption fine structure (XAFS) spectroscopy has proven to be a valuable tool in defining valence states of multivalent elements in minerals and glasses that can then be used as oxybarometry proxies. First row transition multivalent elements Ti, V, Cr, and Fe are common targets. Micro-XAFS provides wide coverage of oxygen fugacity on all minerals and glasses, with high spatial resolution, trace level sensitivity, and no stoichiometry constraints. This method has been applied to an extensive array of geochemical problems including heterogeneity of terrestrial mantle sources, effects of volatile degassing of magmas, evolution of lunar melts, metamorphism of chondrites, and relationships between achondrites. Valence states alone can be insightful indicators of oxidation, such as the presence or absence of Ti³⁺ and the effects of metamorphism on asteroidal parent bodies. Importantly, XAFS can be extended to infer oxygen fugacity of parent melts by calibrating with laboratory experiment products under controlled conditions. When valence measurements or oxygen fugacity determinations are undertaken on non-cubic minerals, careful calibration data using oriented samples as well as knowledge of valence-specific partition coefficients are needed. XAFS offers the ability to apply multiple oxybarometers (e.g., Ti, Cr, and V valence proxies) to individual, potentially zoned, mineral grains. Challenges include corrections for orientation effects, X-ray beam-induced modifications, and the sparcity of valence-specific partition coefficient measurements. In some cases, application of statistical and machine-learning methods based on linear algebra such as principal component analysis (PCA), partial least-squares (PLS) analysis, and least absolute shrinkage and selection operator (Lasso) regressions can help to identify and compensate for some of external factors complicating XAFS analysis.
X-ray absorption near-edge structure (XANES) spectroscopy is a powerful technique to quantitatively investigate sulfur speciation in geologically complex materials such as minerals, glasses, soils, organic compounds, industrial slags, and extraterrestrial materials. This technique allows non-destructive investigation of the coordination chemistry and oxidation state of sulfur species ranging from sulfide (2- oxidation state) to sulfate (6+ oxidation state). Each sulfur species has a unique spectral shape with a characteristic K-edge representing the s → p and d hybridization photoelectron transitions. As such, sulfur speciation is used to measure the oxidation state of samples by comparing the overall XANES spectra to that of reference compounds. Although many S XANES spectral standards exist for terrestrial applications under oxidized conditions, new sulfide standards are needed to investigate reduced (oxygen fugacity, fO2, below IW) silicate systems relevant for studies of extraterrestrial materials and systems. Sulfides found in certain meteorites (e.g., enstatite chondrites and aubrites) and predicted to exist on Mercury, such as CaS (oldhamite), MgS (niningerite), and FeCr2S4 (daubréelite), are stable at fO2 below IW-3 but rapidly oxidize to sulfate and/or produce sulfurous gases under terrestrial surface conditions. XANES spectra of these compounds collected to date have been of variable quality, possibly due to the unstable nature of certain sulfides under typical (e.g., oxidizing) laboratory conditions. A new set of compounds were prepared for this study and their XANES spectra are analyzed for comparison with potential extraterrestrial analogs. S K-edge XANES spectra were collected at Argonne National Lab for FeS (troilite), MnS (alabandite), CaS(oldhamite), MgS (niningerite), Ni1-xS, NiS2, CaSO4 (anhydrite), MgSO4, FeSO4, Fe2(SO4)3, FeCr2S4 (daubréelite), Na2S, Al2S3, Ni7S6, and Ni3S2; the latter five were analyzed for the first time using XANES. These standards expand upon the existing S XANES endmember libraries at a higher spectral resolution (0.25 eV steps) near the S K-edge. Processed spectra, those that have been normalized and ‘flattened’, are compared to quantify uncertainties due to data processing methods. Future investigations that require well-characterized sulfide standards such as the ones presented here may have important implications for understanding sulfur speciation in reduced silicate glasses and minerals with applications for the early Earth, Moon, Mercury, and enstatite chondrites.
The distribution of heat-producing elements (HPE) potassium (K), uranium (U) and thorium (Th) within planetary interiors has major implications for the thermal evolution of the terrestrial planets and for the inventory of volatile elements in the inner Solar System. To investigate the abundances of HPE in Mercury's interior, we conducted experiments at high pressure and temperature (up to 5 GPa and 1900 °C) and reduced conditions (IW-1.8 to IW-6.5) to determine U, Th and K partitioning between metal, silicate and sulfide (D met/sil and D sulf/sil). Our experimental data combined with those from the literature show that partitioning into sulfide is more efficient than into metal and enhanced with decreasing FeO and increasing O contents of the silicate and sulfide melts respectively. Also, at low oxygen fugacity (log fO2 < IW-5), U and Th are more efficiently partitioned into liquid iron metal and sulfide than K. D met/sil for U, Th and K increases with decreasing oxygen fugacity, while DU met/sil and DK met/sil increase when the metal is enriched and depleted in O or Si respectively. We also used available data from the literature to constrain the concentrations of light elements (Si, S, O and C) in Fe metal and sulfide. We provided chemical compositions of Mercury's core after core segregation, for a range of fO2 conditions during its differentiation. For example, if Mercury differentiated at IW-5.5, its core would contain 49 wt% Si, 0.02 wt% S and negligible C. Also if core-mantle separation happened at a fO2 lower than IW-4, bulk Mercury Fe/Si ratio is likely to be chondritic. We calculated concentrations of U, Th and K in the Fe-rich core and possible sulfide layer of Mercury. Bulk Mercury K/U and K/Th were calculated taking all U, Th and K reservoirs into account. Without any sulfide layer or if Mercury's core segregated at a higher fO2 than IW-4, bulk K/U and K/Th would be similar to those measured on the surface, confirming more elevated volatile K concentration than previously expected for Mercury. However, Mercury could fall on an overall volatile depletion trend where K/U increases with the heliocentric distance, if core segregation Always consult and cite the final, published document. See http:/www.minsocam.org or GeoscienceWorld 5 occurred near IW-5.5 or more reduced conditions and with a sulfide layer of at least 130 km thickness. In these conditions, bulk Mercury K/Th ratio is close to Venus' and Earth's values. Since U and Th become more chalcophile with decreasing oxygen fugacity, to a higher extent than K, it is likely that at a fO2 close to or lower than IW-6, both K/U and K/Th become lower than values of the other terrestrial planets. Therefore, our results suggest that the elevated K/U and K/Th ratios of Mercury's surface should not be exclusively interpreted as the result of a volatile enrichment in Mercury, but could also indicate a sequestration of more U and Th than K in a hidden iron sulfide reservoir, possibly a layer present between the mantle and core. Hence, Mercury could be more depleted in volatiles than Mars with a K concentration similar or lower than the Earth's and Venus', suggesting a volatile depletion in the inner Solar System. In addition, we show that the presence of a sulfide layer formed between IW-4 and IW-5.5 decreases the total radioactive heat production of Mercury by up to 30%.
Unique physical and chemical characteristics of Mercury have been revealed by measurements from NASA’s MESSENGER spacecraft. The closest planet to our Sun is made up of a large metallic core that is partially liquid, a thin mantle thought to be formed by solidification of a silicate magma ocean, and a relatively thick secondary crust produced by partial melting of the mantle followed by volcanic eruptions. However, the origin of the large metal/silicate ratio of the bulk planet and the conditions of accretion remain elusive. Metal enrichment may originate from primordial processes in the solar nebula or from a giant impact that stripped most of the silicate portion of a larger planet leaving Mercury as we know it today.
This study describes microscale sulfur (S), vanadium (V), and iron (Fe) K-edge X-ray absorption near edge structure (µ-XANES) spectroscopy measurements on olivine-hosted melt inclusions (MI) preserved in tephras (1986 and 2006) and lavas (1938 and 1948) erupted from Nyamuragira volcano (D.R. Congo, Africa). The S, V, and Fe spectroscopic data are used to constrain the evolution of oxygen fugacity (fO2) and sulfur speciation for the entrapped melts. Melt inclusions from lavas show evidence of post-entrapment crystallization and were thus reheated prior to µ-XANES analysis. The MI from tephra show no evidence of post-entrapment crystallization and were, therefore, not reheated. Sulfur, V, and Fe µ-XANES results from 1938, 1948, and 2006 eruptive materials are all similar within analytical uncertainty and provide similar average calculated melt fO2's based on XANES oxybarometry. However, olivine-hosted MI from the 1986 tephras yield significantly different S, V, and Fe XANES spectra when compared to MI from the other eruptions, with disagreement between calculated fO2's from the three valence state oxybarometers beyond the uncertainty of the calibration models. Their V µ-XANES spectra are also significantly more ordered and yield more reduced average V valence. The S µ-XANES spectra display a significantly more intense low-energy spectral resonance, which indicates differences in Fe-S bonding character, and greater variability in their measured sulfate content. These V and S spectroscopic features are best explained by crystallization of sub-micrometer magnetite and sulfide crystallites within the 1986 inclusions. The sensitivity of XANES spectroscopy to short-range order allows these crystallites to be recognized even though they are not easily detected by imaging analysis. This shows that V and S µ-XANES are potentially highly sensitive tools for identifying the presence of volumetrically minor amounts of spinel and sulfide within inclusions extracted from rapidly-cooled samples of tephra. Additionally, the observation that rehomogenized 1938 and 1948 inclusions from lava yield similar S, V, and Fe XANES spectra to the 2006 inclusions from tephra may be an encouraging indication that rehomogenization appears to have enabled the successful recovery of their pre-eruptive fO2, despite their complex post-eruptive histories.
The MESSENGER mission sought to discover what physical processes determined Mercury's high metal to silicate ratio. Instead, the mission has discovered multiple anomalous characteristics about our innermost planet. The lack of FeO and the reduced oxidation state of Mercury's crust and mantle are more extreme than nearly all other known materials in the solar system. In contrast, moderately volatile elements are present in abundances comparable to the other terrestrial planets. No single process during Mercury's formation is able to explain all of these observations. Here, we review the current ideas for the origin of Mercury's unique features. Gaps in understanding the innermost regions of the solar nebula limit testing different hypotheses. Even so, all proposed models are incomplete and need further development in order to unravel Mercury's remaining secrets.
Data from the Gamma-Ray Spectrometer (GRS) that flew on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft indicate that the O/Si weight ratio of Mercury's surface is 1.2 ± 0.1. This value is lower than any other celestial surface that has been measured by GRS and suggests that 12-20% of the surface materials on Mercury are composed of Si-rich, Si-Fe alloys. The origin of the metal is best explained by a combination of space weathering and graphite-induced smelting. The smelting process would have been facilitated by interaction of graphite with boninitic and komatiitic parental liquids. Graphite entrained at depth would have reacted with FeO components dissolved in silicate melt, resulting in the production of up to 0.4-0.9 wt % CO from the reduction of FeO to Fe⁰-CO production that could have facilitated explosive volcanic processes on Mercury. Once the graphite-entrained magmas erupted, the tenuous atmosphere on Mercury prevented the buildup of CO over the lavas. The partial pressure of CO would have been sufficiently low to facilitate reaction between graphite and SiO2 components in silicate melts to produce CO and metallic Si. Although exotic, Si-rich metal as a primary smelting product is hypothesized on Mercury for three primary reasons: (1) low FeO abundances of parental magmas, (2) elevated abundances of graphite in the crust and regolith, and (3) the presence of only a tenuous atmosphere at the surface of the planet within the 3.5-4.1 Ga timespan over which the planet was resurfaced through volcanic processes.
In this study, we use micro-X-ray absorption near-edge structures (mu-XANES) spectroscopy at the S K-edge to investigate the oxidation state of S in natural magmatic-hydrothermal apatite (Durango, Mexico, and Mina Carmen, Chile) and experimental apatites crystallized from volatile-saturated lamproitic melts at 1000 degrees C and 300 MPa over a broad range of oxygen fugacities [(log(fO2) = FMQ, FMQ+ 1.2, FMQ+ 3; FMQ = fayalite-magnetite-quartz solid buffer]. The data are used to test the hypothesis that S oxidation states other than S6+ may substitute into the apatite structure. Peak energies corresponding to sulfate S6+ (similar to 2482 eV), sulfite S4+ (similar to 2478 eV), and sulfide S2-(similar to 2470 eV) were observed in apatite, and the integrated areas of the different sulfur peaks correspond to changes in fO2 and bulk S content. Here, multiple tests confirmed that the S oxidation state in apatite remains constant when exposed to the synchrotron beam, at least for up to 1 h exposure (i. e., no irradiation damages). To our knowledge, this observation makes apatite the first mineral to incorporate reduced (S2-), intermediate (S4+), and oxidized (S6+) S in variable proportions as a function of the prevailing fO2 of the system. Apatites crystallized under oxidizing conditions (FMQ+ 1.2 and FMQ+ 3), where the S6+/STotal peak area ratio in the coexisting glass (i. e., quenched melt) is similar to 1, are dominated by S6+ with a small contribution of S4+, whereas apatites crystallizing at reduced conditions (FMQ) contain predominantly S2-, lesser amounts of S6+, and possibly traces of S4+. A sulfur oxidation state vs. S concentration analytical line transect across hydrothermally altered apatite from the Mina Carmen iron oxide-apatite (IOA) deposit (Chile) demonstrates that apatite can become enriched in S4+ relative to S6+, indicating metasomatic overprinting via a SO2-bearing fluid or vapor phase. This XANES study demonstrates that as the fO2 increases from FQM to FMQ+ 1.2 to FMQ+ 3 the oxidation state of S in igneous apatite changes from S2-dominant to S6+ > S4+ to S6+ >> S4+. Furthermore, these results suggest that spectroscopic studies of igneous apatite have potential to trace the oxidation state of S in magmas. The presence of three S oxidations states in apatite may in part explain the non-Henrian partitioning of S between apatite and melt. Our study reveals the potential to use the S signature of apatite to elucidate both oxygen and sulfur fugacity in magmatic and hydrothermal systems.
Many igneous rocks contain mineral assemblages that are not appropriate for application of common mineral equilibria or oxybarometers to estimate oxygen fugacity. Spinel-structured oxides, common minerals in many igneous rocks, typically contain sufficient V for XANES measurements, allowing use of the correlation between oxygen fugacity and V K pre-edge peak intensity. Here we report V pre-edge peak intensities for a wide range of spinels from source rocks ranging from terrestrial basalt to achondrites to oxidized chondrites. The XANES measurements are used to calculate oxygen fugacity from experimentally produced spinels of known fo2. We obtain values, in order of increasing fo2, from IW-3 for lodranites and acapulcoites, to diogenites, brachinites (near IW), ALH 84001, terrestrial basalt, hornblende-bearing R chondrite LAP 04840 (IW+1.6), and finally ranging up to IW+3.1 for CK chondrites (where the ΔIW logfo2 of a sample relative to the logfo2 of the IW buffer at specific T). To place the significance of these new measurements into context we then review the range of oxygen fugacities recorded in major achondrite groups, chondritic and primitive materials, and planetary materials. This range extends from IW-8 to IW+2. Several chondrite groups associated with aqueous alteration exhibit values that are slightly higher than this range, suggesting that water and oxidation may be linked. The range in planetary materials is even wider than that defined by meteorite groups. Earth and Mars exhibit values higher than IW+2, due to a critical role played by pressure. Pressure allows dissolution of volatiles into magmas, which can later cause oxidation or reduction during fractionation, cooling, and degassing. Fluid mobility, either in the sub-arc mantle and crust, or in regions of metasomatism, can generate values >IW+2, again suggesting an important link between water and oxidation. At the very least, Earth exhibits a higher range of oxidation than other planets and astromaterials due to the presence of an O-rich atmosphere, liquid water, and hydrated interior. New analytical techniques and sample suites will revolutionize our understanding of oxygen fugacity variation in the inner solar system, and the origin of our solar system in general.
High resolution images from MESSENGER provide morphological information on the nature and origin of Mercury's hollows, small depressions that likely formed when a volatile constituent was lost from the surface. Because graphite may be a component of the low-reflectance material that hosts hollows, we suggest that loss of carbon by ion sputtering or conversion to methane by proton irradiation could contribute to hollows formation. Measurements of widespread hollows in 565 images with pixel scales <20 m indicate that the average depth of hollows is 24 ± 16 m. We propose that hollows cease to increase in depth when a volatile-depleted lag deposit becomes sufficiently thick to protect the underlying surface. The difficulty of developing a lag on steep topography may account for the common occurrence of hollows on crater central peaks and walls. Disruption of the lag, e.g., by secondary cratering, could restart growth of hollows in a location that had been dormant. Extremely high-resolution images (~3 m/pixel) show that the edges of hollows are straight, as expected if the margins formed by scarp retreat. These highest-resolution images reveal no superposed impact craters, implying that hollows are very young. The width of hollows within rayed crater Balanchine suggests that the maximum time for lateral growth by 1 cm is ~10,000 yr. A process other than entrainment of dust by gases evolved in a steady-state sublimation-like process is likely required to explain the high-reflectance haloes that surround many hollows.
Phase equilibrium experiments were conducted on a synthetic rock composition matching that of the northern volcanic plains of Mercury as measured by the MErcury Surface, Space ENvironment, GEochemistry and Ranging spacecraft (MESSENGER). The northern volcanic plains are smooth plains of suspected volcanic origin that cover more than 6% of the surface area of Mercury. The northern volcanic plains are less cratered than their surroundings and reported to be the product of flood volcanism, making them a prime candidate for experimental study. The bulk composition of the northern volcanic plains is that of an alkali-rich boninite and represents the first silica-enriched crustal terrane identified on an extraterrestrial planet from orbital data. Phase equilibrium experiments were conducted over the pressure range of the mercurian mantle (0.5-5 GPa) at very low oxygen fugacity (~δIW0 to -7) using a piston-cylinder apparatus ( P 0.5-1.7 GPa) and a Walker-style multi-anvil device ( P≥ 2.5 GPa). Our results indicate the origin of the northern volcanic plains lavas (boninites) are best explained by high degrees of partial melting of an olivine-dominant, pyroxene- and plagioclase-bearing mantle source at low pressure (≤1.4 GPa) and does not require hydrous melting to achieve the silica-enriched melt composition. The formation mechanism for boninites on Mercury contrasts substantially with terrestrial boninites, which typically occur in oxidized and hydrous arc environments associated with subduction zones. Instead, mercurian boninites form at exceptionally low oxygen fugacity and do not require melting of hydrated source materials. The NVP lavas represent a novel mechanism by which planetary bodies can form silica-enriched secondary crusts without the aid of water.
The equilibrium phase relations of two volcanic rocks from the subduction-related Mexican Volcanic Belt have been determined with an argon-pressurized internally heated vessel. One rock is the trachyandesite erupted from El Chichón Volcano in 1982; the other is a primitive basalt erupted from Jorullo Volcano in 1759. A simplified synthetic equivalent to the trachyandesite was also investigated in lesser detail. All charges were saturated with hydrous vapor and a sulfur-bearing mineral. Temperature ranged from 800 to 1000°C, pressure from 1 to 4 kb, and f o2 was controlled by four different solid oxygen buffers in a double gold capsule configuration: fayalite-magnetite-quartz (FMQ), Ni-NiO (NNO), manganosite-hausmanite (MNH), and magnetite-hematite (MTH).Pyrrhotite was the only sulfur-bearing mineral observed in charges buffered under FMQ and NNO, whereas anhydrite crystallized under the more oxidizing MNH or MTH; both of these observations are consistent with those of earlier workers. With increasing temperature and pressure, SiO 2 and K 2O decreased in the experimental melts, whereas Al 2O 3 and CaO increased. Sulfur solubility in silicate melts was low (<0·1 wt% equivalent SO t3) for pyrrhotite-saturated charges, but significantly greater (to 1·3 wt.% SO t3) when anhydrite was present. Sulfur solubility in anhydrite-saturated melts showed strong positive dependence on both temperature and P vapor.Sulfur amounted to some 2·5 wt.% (SO t3) of the total ejecta during the 1982 El Chichón eruptions, and the original magmatic sulfur content was in the range 1·25-2-5 wt% SO t3. Extrapolations of experimental temperature and pressure dependences for sulfur solubility indicate that such concentrations could be contained in a hydrous, oxidized, basaltic parent melt generated under Benioff zone conditions. During ascent through the upper mantle and crust, the sulfur solubility limit of the melt would continuously decrease; in response, most of the sulfur would be transferred from the melt to anhydrite crystals and a separate gas phase. Trachyandesite pumices erupted from El Chichón in 1982 contained both pyrrhotite and anhydrite at a temperature of ∼800°C. The composition of the natural pyrrhotite yielded an f o2 estimate ∼1 log unit above the NNO buffer. Based on compositional variations in the experimental melts with temperature and pressure, the composition of the matrix glass in the 1982 pumices indicates equilibration of the magmatic liquid at about P total=P vapor=2 kb just before eruption. At that time, sulfur in El Chichón trachyandesite was about equally partitioned between anhydrite microphenocrysts and some 20 vol.% gas phase in which H 2S was probably the dominant sulfur-bearing species. The melt then contained only 0·05 wt.% SO t3, consistent with experimental solubility limits at 800°C and P vapor=2 kb.
The effect of sulfur dissolved in anorthite-diopside eutectic (AD) glasses on the glasstransition temperature (Tg) has been investigated via Differential Scanning Calorimetricmeasurements (DSC) and Thermogravimetric Analysis (TGA) under moderately reducing tooxidizing conditions.In a series of AD glasses, we have measured the change in Tg as a function of S contentpresent as SO42- (HS- is also identified to a lesser extent) and H2O content. The AD glassesinvestigated have S contents ranging from 0 to 7519 ppm and H2O contents ranging from 0 to5.3 wt.%. In agreement with previous studies, increasing H2O content induces a strongexponential decrease in Tg: volatile free AD glass has a Tg at 758±13C and AD glass with5.18±0.48 wt.% H2O has a Tg at 450±11C. The change in Tg as a function of H2O is wellreproducedwith a third-order polynomial function and has been used to constrain Tg at anyH2O content. The effect of S on Tg is almost inexistent or towards a decrease in Tg withincreasing S content. For instance, at ~2.4 wt.% H2O, the addition of S induces a change inTg from 585±10°C with 0 ppm S to 523±3C with 2365±138 ppm S; a further increase in Sup to 7239±90 ppm S does not induce a dramatic change in Tg measured at 529±2C.The limited effect of S on the glass transition temperature contrasts with recent spectroscopicmeasurements suggesting that S dissolution as SO42- groups provokes an increase in thepolymerization degree. We propose an alternative view which reconciles the spectroscopicevidence with the Tg measurements. The dissolution of S as SO42- does not induce theformation of Si-O-Si molecular bonding through consumption of available non-bridgingoxygens (NBO) but instead we suggest that Si-O-S molecular bonds are formed which are not detectable by DSC measurements but mimic the increase in glass polymerization. Therefore, spectroscopic measurements must be used with caution in order to extract melt physicalproperties.
The concentration of sulfur in basalt-like silicate melts as S2– is limited to the amount at which the melt becomes saturated with a sulfide phase, such as an immiscible sulfide melt. The limiting solubility is called the ‘sulfur content at sulfide saturation’ (SCSS). Thermodynamic modelling shows that the SCSS depends on the FeO content of the silicate melt from two terms, one with a negative dependence that comes from the activity of FeO in the silicate melt, and the other with a positive dependence that comes from the strong dependence of the sulfide capacity of the melt ( C S ) on FeO content. The interaction between these two terms should yield a net SCSS that has an asymmetric U-shaped dependence on the FeO content of the melt, if other variables are kept constant. We have tested this thermodynamic model in a series of experiments at 1400°C and 1·5 GPa to determine the sulfur contents at saturation with liquid FeS in melt compositions along the binary join between a haplobasaltic composition and FeO. The SCSS is confirmed to have the asymmetric U-shaped dependence, with a minimum at ∼5 wt % FeO. The effect of FeO on the selenide content at selenide saturation (SeCSeS) was investigated in an analogous fashion. SeCSeS shows a similar, though not identical, U-shaped dependence, implying that the solubility mechanism of selenide in basalt-like silicate melts is similar to that of sulfide. The observation of increasing SCSS with decreasing FeO in hydrous silicic melts was explored by inverse modelling of datasets from pyrrhotite-saturated hydrous silicic liquids, revealing that high SCSS at low FeO can be explained in terms of the low-FeO limb of the ‘U’, rather than dissolution of sulfur as hydrous species such as H 2 S or HS–. Recent measurements of the composition of the surface of Mercury prompted examination of the high-SCSS, low-FeO limb of the ‘U’ as a potential explanation for the sulfur-rich but Fe-poor surface of Mercury.
### Background
For the past 37 years the Mineralogical Society of America, and in conjunction with the Geochemical Society (since 2000), have sponsored and published 72 review volumes that communicate the results of significant advances in research in the Earth sciences. Several of these have either directly or indirectly addressed the fundamental importance, role, and behavior of volatile components on processes influencing magma rheology, crystallization, evolution, eruption, and related metasomatism and mineralization. Volume 30—which was published in 1994—focused on this topic broadly, and this volume has provided a lasting summary on the geochemical and physical behaviors of a wide variety of magmatic volatiles (Carroll and Holloway 1994). Since that year, continued research has brought important and new knowledge about the role of the volatile component sulfur in natural magmas, and significant progress was made simultaneously in understanding the role of sulfur in industrial or technical processes such as glass or steel production. Here, in volume 73, we have assembled in 15 chapters the current state of research concerning sulfur in melts based on the extensive experience of various authors practically working on these topics.
The behavior of sulfur in melts and its implications for natural and industrial processes are still insufficiently understood, and hence, are difficult to apply as a tool for interpreting problems of geological or industrial interest. In recent decades, various new investigations in the geosciences as well as in the engineering and material sciences have employed modern spectroscopic, analytical, theoretical, and experimental techniques to improve our understanding of the complex and volatile behavior of sulfur in a wide variety of molten systems. However, these different research initiatives (e.g., empirical vs. applied research and natural vs. technical applications) were rarely well integrated, and the scientific goals were usually approached with specific and relatively focused points of view. Consequently, bridging this …
The range in density and compressibility of Mercurian melt compositions was determined to better understand the products of a possible Mercurian magma ocean and subsequent volcanism. Our experiments indicate that the only mineral to remain buoyant with respect to melts of the Mercurian mantle is graphite; consequently, it is the only candidate mineral to have composed a primary floatation crust during a global magma ocean. This exotic result is further supported by Mercury's volatile-rich nature and inexplicably darkened surface. Additionally, our experiments illustrate that partial melts of the Mercurian mantle that compose the secondary crust were buoyant over the entire mantle depth and could have come from as deep as the core-mantle boundary. Furthermore, Mercury could have erupted higher percentages of its partial melts compared to other terrestrial planets because magmas would not have stalled during ascent due to gravitational forces. These findings stem from the FeO-poor composition and shallow depth of Mercury's mantle, which has resulted in both low-melt density and a very limited range in melt density responsible for Mercury's primary and secondary crusts. The enigmatically darkened, yet low-FeO surface, which is observed today, can be explained by secondary volcanism and impact processes that have since mixed the primary and secondary crustal materials.
The Moon is thought to have originated with a magma ocean that produced a plagioclase otation crust as solidication proceeded. Ages of anorthositic crust range over at least 200 million years. The model for solidication presented here integrates chemical and physical constraints of lunar magma ocean solidication to determine (1) the nal thickness of otation crust generated by a fractionally solidifying magma ocean, (2) the timescale of crystallization before plagioclase is a stable phase, (3) the timescale of solidication after the formation of the plagioclase otation crust, and (4) the post-overturn lunar mantle composition and structure. We nd that magma oceans of as much as 1000 km depth are consistent with creating an anorthositic crust 40 to 50 km in thickness. Solidication of the magma ocean prior to formation of the otation crust may occur on the order of 1000 years, and complete solidication would require additional ten to tens of millions of years. Reconciling these short model timescales with radiometric dates of crustal samples requires either a very late-forming Moon combined with nding older crustal ages to be incorrect, or calling on tidal heating of the crust by the early Earth to prolong solidication. Gravitationally driven overturn of cumulates during tidal heating provides a mechanism for creating the compositions and ages of the lunar Mg suite of crustal rocks. Further, we nd that upon crystallization, the Moon likely began with an azimuthally heterogeneous, gravitationally stable mantle, after magma ocean cumulate overturn. This result may help explain the experimentally determined origin of picritic glasses at similar depths but from different source materials.
The density and compressibility of four synthetic molten lunar picritic glasses was investigated from 0-10 GPa and 1748-2473 K. The picritic glasses were collected from the lunar surface during the Apollo missions, and they are hypothesized to have rapidly quenched as glass beads during pyroclastic fire fountain eruptions. The specific melt compositions investigated in the present study are the Apollo 15 green glass Type C (A15C, TiO2 = 0.26 wt%), the Apollo 14 yellow glass (A14Y, TiO2 = 4.58 wt%), the Apollo 17 orange glass 74220-type (A17O TiO2 = 9.12 wt%), and the Apollo 14 black glass (A14B, TiO2 = 16.40 wt%). These glasses are reported to represent primary unfractionated melts, making them a prime candidate for experimental studies into lunar basalt density and compressibility during partial melting of the lunar mantle. Sink-float experiments were conducted on the synthetic molten lunar glass compositions using a piston-cylinder apparatus (P < 2 GPa) and a Walker-style multi-anvil device (P > 2.5 GPa) in order to bracket the density of the melts. New sink-float data are reported for A15C, A14Y, and A17O, which are combined with previously published density and compressibility data on A15C, A17O, and A14B. Although the Ti-rich liquids are highly compressible at lower pressures, they become nearly incompressible at much higher pressures when compared to the molten low-Ti glasses. Consequently, the melts with the most TiO2 (A14B) are the least dense at higher pressures, a reversal of what is seen at lower pressures. This change in density and compressibility is attributed to changes in coordination of Ti and Fe in the silicate melt structure. As Ti4+ abundances in the silicate melt increase, predominantly [IV]Ti4+ and [IV]Fe2+ change to [VI]Ti4+ and [VI]Fe2+ in the melt structure. All of the data from the present study were used to calculate a Birch-Murnaghan equation-of-state (BM-EOS) for each melt composition. The BM-EOS model for each composition was then combined with previously published estimates for the residual mantle source mineralogy and depth of origin for each of the glasses to assess the density of the partial melt with respect to its point of origin. This information was used to determine whether or not the melt would rise or sink with respect to its source region. We determined that all melt compositions, with the exception of the A17O melt, should have been able to rise to the crust-mantle boundary as a result of buoyancy forces alone, although different mechanisms are likely required for magma ascent through the lunar crust. For the rise of A17O, other modes of ascent through the lunar mantle are required to extract this melt composition from the mantle, and volatiles are not a plausible solution on the grounds of melt density alone.
The sulfur concentrations and the relative proportions of S2− and S6+ were measured by electron microprobe in a series of melt inclusions trapped in phenocrysts from different subduction-related and within-plate volcanoes. The melt inclusions correspond to potassic and shoshonitic primary melts to tholeiitic and hawaiitic primitive melts. In the tholeiitic and the transitional basaltic melt inclusions, sulfur is mainly present as S2− (), and varies from 0.13 to 0.18 wt%. The occurrence of immiscible sulfides attests to their saturation. In shoshonitic and potassic primary melts, sulfur (S = 0.12 to 0.32 wt%) is dissolved as both S2− and S6+ (). Their oxygen fugacity, estimated from the ratios, ranges from NiNiO to NiNiO + 1 log unit. Hawaiitic melts may also dissolve up to 0.3 wt% sulfur possibly because of their oxidation state close to NiNiO, as illustrated by samples from Mt. Etna, Italy.Variations of sulfur, at constant ratio and temperature in both the potassic and shoshonitic primary melts, indicate that these melts are undersaturated, with respect to a S-rich condensed phase, in agreement with their relatively high oxidation state. It strongly suggests that sulfur behavior in relatively oxidized primary alkaline melts is controlled by the mantle source melting conditions.
We present measurements of Mercury’s surface composition from the analysis of MESSENGER X-Ray Spectrometer data acquired during 55 large solar flares, which each provide a statistically significant detection of Fe X-ray fluorescence. The Fe/Si data display a clear dependence on phase angle, for which the results are empirically corrected. Mercury’s surface has a low total abundance of Fe, with a mean Fe/Si ratio of ∼0.06 (equivalent to ∼1.5 wt% Fe). The absolute Fe/Si values are subject to a number of systematic uncertainties, including the phase-angle correction and possible mineral mixing effects. Individual Fe/Si measurements have an intrinsic error of ∼10%. Observed Fe/Si values display small variations (significant at two standard deviations) from the planetary average value across large regions in Mercury’s southern hemisphere. Larger differences are observed between measured Fe/Si values from more spatially resolved footprints on volcanic smooth plains deposits in the northern hemisphere and from those in surrounding terrains. Fe is most likely contained as a minor component in sulfide phases (e.g., troilite, niningerite, daubréelite) and as Fe metal, rather than within mafic silicates. Variations in surface reflectance (i.e., differences in overall reflectance and spectral slope) across Mercury are unlikely to be caused by variations in the abundance of Fe.
Mercury is notorious as the most reduced planet with the highest metal/silicate ratio, yet paradoxically data from the MESSENGER spacecraft show that its iron-poor crust is high in sulfur (up to ∼6 wt%, ∼80× Earth crust abundance) present mainly as Ca-rich sulfides on its surface. These particularities are simply impossible on the other terrestrial planets. In order to understand the role played by sulfur during the formation of Mercury, we investigated the phase relationships in Mercurian analogs of enstatite chondrite-like composition experimentally under conditions relevant to differentiation of Mercury (∼1 GPa and 1300–2000 °C). Our results show that Mg-rich and Ca-rich sulfides, which both contain Fe, crystallize successively from reduced silicate melts upon cooling below 1550 °C. As the iron concentration in the reduced silicates stays very low (≪1 wt%), these sulfides represent new host phases for both iron and sulfur in the run products. Extrapolated to Mercury, these results show that Mg-rich sulfide crystallization provides the first viable and fundamental means for retaining iron as well as sulfur in the mantle during differentiation, while sulfides richer in Ca would crystallize at shallower levels. The distribution of iron in the differentiating mantle of Mercury was mainly determined by its partitioning between metal (or troilite) and Mg–Fe–Ca-rich sulfides rather than by its partitioning between metal (or troilite) and silicates. Moreover, the primitive mantle might also be boosted in Fe by a reaction at the core mantle boundary (CMB) between Mg-rich sulfides of the mantle and FeS-rich outer core materials to produce (Fe, Mg)S. The stability of Mg–Fe–Ca-rich sulfides over a large range of depths up to the surface of Mercury would be consistent with sulfur, calcium and iron abundances measured by MESSENGER.
The recent discovery of high S concentrations on the surface of Mercury by spacecraft measurements from the MESSENGER mission provides the potential to place new constraints on the composition of Mercury's large metallic core. In this work, we conducted a set of systematic equilibrium metal-silicate experiments that determined the effect of different metallic compositions in the Fe-S-Si system on the S concentration in the coexisting silicate melt. We find that metallic melts with a range of S and Si combinations can be in equilibrium with silicate melts with S contents consistent with Mercury's surface, but that such silicate melts contain Fe contents lower than measured for Mercury's surface. If Mercury's surface S abundance is representative of the planet's bulk silicate composition and if the planet experienced metal-silicate equilibrium during planetary core formation, then these results place boundaries on the range of possible combinations of Si and S that could be present as the light elements in Mercury's core and suggest that Mercury's core likely contains Si. Except for core compositions with extreme abundances of Si, bulk Mercury compositions calculated by using the newly determined range of potential S and Si core compositions do not resemble primitive meteorite compositions.
The MESSENGER spacecraft’s Mercury Atmospheric and Surface Composition Spectrometer (MASCS) obtained more than 1.6 million reflectance spectra of Mercury’s surface from near-ultraviolet to near-infrared wavelengths during the first year of orbital operations. A global analysis of spectra in the wavelength range 300–1450 nm shows little regional variation in absolute reflectance or spectral slopes and a lack of mineralogically diagnostic absorptions. In particular, reflectance spectra show no clear evidence for an absorption band centered near 1 μm that would be associated with the presence of ferrous iron in silicates. There is, however, evidence for an ultraviolet absorption possibly consistent with a very low iron content (2–3 wt% FeO or less) in surface silicates and for the presence of small amounts of metallic iron or other opaque minerals in the form of nano- or micrometer-sized particles. These findings are consistent with MESSENGER X-ray and gamma-ray measurements of Mercury’s surface iron abundance. Although X-ray and gamma-ray observations indicate higher than expected quantities of sulfur on the surface, reflectance spectra show no absorption bands diagnostic of sulfide minerals. Whereas there is strong evidence of water ice in permanently shadowed craters near Mercury’s poles, MASCS spectra provide no evidence for hydroxylated materials near permanently shadowed craters.
MESSENGER Gamma-Ray Spectrometer measurements demonstrate that the abundance of Na varies across the surface of Mercury. The maximum Na/Si abundance ratio of 0.20 ± 0.03 by weight (∼5 wt% Na) is observed at high northern latitudes and is significantly larger than the equatorial Na/Si ratio of 0.11 ± 0.01 (∼2.6 wt% Na). Comparisons of forward-modeled surface distributions with the gamma-ray measurements suggest that the observed distribution of Na can be explained by differences in elemental composition between the volcanic smooth plains units and heavily cratered terrain. The comparison improves when thermally driven depletion of Na from areas near Mercury’s hot poles is included. When combined with other MESSENGER data sets, these results indicate that the smooth plains units include substantial abundances of alkali feldspars. Thermal depletion of Na from the hot poles without an assumed underlying compositional variability can also reproduce the measured Na/Si distribution, but that mechanism fails to account for other MESSENGER observations that support the presence of higher abundances of feldspars in the smooth plains units.
Orbital gamma-ray measurements obtained by the MESSENGER spacecraft have
been analyzed to determine the abundances of the major elements Al, Ca,
S, Fe, and Na on the surface of Mercury. The Si abundance was determined
and used to normalize those of the other reported elements. The Na
analysis provides the first abundance estimate of
2.9 ± 0.1 wt% for this element on Mercury's
surface. The other elemental results
(S/Si = 0.092 ± 0.015,
Ca/Si = 0.24 ± 0.05, and
Fe/Si = 0.077 ± 0.013) are consistent with
those previously obtained by the MESSENGER X-Ray Spectrometer, including
the high sulfur and low iron abundances. Because of different sampling
depths for the two techniques, this agreement indicates that Mercury's
regolith is, on average, homogenous to a depth of tens of centimeters.
The elemental results from gamma-ray and X-ray spectrometry are most
consistent with petrologic models suggesting that Mercury's surface is
dominated by Mg-rich silicates. We also compare the results with those
obtained during the MESSENGER flybys and with ground-based observations
of Mercury's surface and exosphere.
We present the analysis of 205 spatially resolved measurements of the
surface composition of Mercury from MESSENGER's X-Ray Spectrometer. The
surface footprints of these measurements are categorized according to
geological terrain. Northern smooth plains deposits and the plains
interior to the Caloris basin differ compositionally from older terrain
on Mercury. The older terrain generally has higher Mg/Si, S/Si, and
Ca/Si ratios, and a lower Al/Si ratio than the smooth plains. Mercury's
surface mineralogy is likely dominated by high-Mg mafic minerals (e.g.,
enstatite), plagioclase feldspar, and lesser amounts of Ca, Mg, and/or
Fe sulfides (e.g., oldhamite). The compositional difference between the
volcanic smooth plains and the older terrain reflects different
abundances of these minerals and points to the crystallization of the
smooth plains from a more chemically evolved magma source. High-degree
partial melts of enstatite chondrite material provide a generally good
compositional and mineralogical match for much of the surface of
Mercury. An exception is Fe, for which the low surface abundance on
Mercury is still higher than that of melts from enstatite chondrites and
may indicate an exogenous contribution from meteoroid impacts.
We have conducted petrologic modeling of MESSENGER-derived compositions
and analog compositions to gain a better understanding of the
petrogenesis of the crust of Mercury. Analog compositions included a
1425°C partial melt of the Indarch (EH4) meteorite and a range of
Mg-rich terrestrial rocks (magnesian basalt, basaltic komatiite, and
peridotitic komatiite). All models were held at the iron-wüstite
buffer to simulate the reducing conditions that likely existed during
Mercury's formation. We then compared modeled mineral compositions and
abundances, liquidus temperatures, and viscosities to better constrain
the characteristics of the lavas that erupted on Mercury's surface. Our
results show that the surface composition of Mercury is most similar to
that of a terrestrial magnesian basalt (with lowered FeO), composed
mainly of Mg-rich orthopyroxene and plagioclase. Because the model
magmas are Mg-rich, their counterparts on Mercury would have erupted at
high temperatures and displayed low viscosities. Producing melts of
these compositions would have required high temperatures at the mantle
source regions on Mercury. The inferred low-viscosity lavas would have
erupted as thin, laterally extensive flows (depending upon their
effusion rate) and would be expected to display surficial flow features
that might be preserved to the present.
The origin of Mercury's anomalous core and low FeO surface mineralogy are outstanding questions in planetary science. Mercury's composition may result from cosmochemical controls on the precursor solids that accreted to form Mercury. High temperatures and enrichment in solid condensates are likely conditions near the midplane of the inner solar protoplanetary disk. Silicate liquids similar to the liquids quenched in ferromagnesian chondrules are thermodynamically stable in oxygen-rich systems that are highly enriched in dust of CI-chondrite composition. In contrast, the solids surviving into the orbit of Mercury's accretion zone were probably similar to highly unequilibrated, anhydrous, interstellar organic- and presolar grain-bearing chondritic, porous interplanetary dust particles (C-IDPs). Chemical systems enriched in an assumed C-IDP composition dust produce condensates (solid+liquid assemblages in equilibrium with vapor) with super-chondritic atomic Fe/Si ratios at high temperatures, approaching 50% of that estimated for bulk Mercury. Sulfur behaves as a refractory element, but at lower temperatures, in these chemical systems. Stable minerals are FeO-poor, and include CaS and MgS, species found in enstatite chondrites. Disk gradients in volatile compositions of planetary and asteroidal precursors can explain Mercury's anomalous composition, as well as enstatite chondrite and aubrite parent body compositions. This model predicts high sulfur content, and very low FeO content of Mercury's surface rocks.
We employ a double capsule technique to determine the solubility of CH4 in haplobasaltic (Fe-free) liquid under conditions of constrained methane fugacity, fCH4fCH4, at pressures of 0.7–3 GPa at 1400–1450 °C. Dissolved C–O–H species are examined with FTIR and Raman spectroscopy, and CH4 and CH3− are the only C-bearing species detected. Carbon solubilities are quantified using SIMS, range from 70 to 480 ppm when calculated as CH4, and increase with pressure. Concentrations are parameterized with a thermodynamic model and are found to be related to fCH4fCH4 and pressure. Application of this thermodynamic model shows dissolved CH4 contents of graphite-saturated magmas are little-influenced by pressure for conditions of fixed fO2fO2 relative to metal–oxide buffers and fixed total H content. Because fCH4fCH4 of graphite-saturated systems increases with the square of hydrogen fugacity, dissolved fCH4fCH4 increases with decreasing fO2fO2 and increases exponentially with increasing total H content. The experimentally-observed increase with pressure is related to variations in fO2fO2 and H content. Dissolved CH4 contents of Martian magmas in their source regions are small, such that it is unlikely that magmatic CH4 is a principal contributor to greenhouse conditions early in Martian history. Concentrations of dissolved C–O–H volatiles in a magma ocean early in the history of a terrestrial planet may be diminished by reducing conditions, leading to development of a massive atmosphere and a greatly decreased inventory of volatiles stored in planetary interiors at the outset of planetary history. Dissolution of methane may enhance the retention of C in the silicate Earth during core formation, but experimental evaluation of its influence on metal/silicate partitioning of C requires careful matching of the magmatic H concentration between experiments and natural systems.
Raman and X-ray absorption spectroscopy (XANES) measurements on a series of experimentally synthesised, sulphur (S)-bearing, hydrous silicate glasses were used to determine the S-speciation and S-oxidation state as a function of glass composition and oxygen fugacity (fO2) and to decipher the dissolution mechanism of S in silicate melts. Synthesised glasses include soda-lime (SLG), K2Si4O9 (KSG), albite and trondhjemite (TROND) compositions. A series of SLG and KSG glasses, doped with small quantities of Fe, was also studied in order to determine the effect of Fe/S on the S solubility. The experiments were performed in internally heated (IHPV) and cold seal (CSPV) pressure vessels at 200 MPa, 1000 and 850 °C and a range of fO2 from log fO2 = QFM − 2.35 to QFM + 4 (QFM is quartz–fayalite–magnetite oxygen buffer).
S K-edge XANES, λ (SKα) wavelength shift, ³³S MAS NMR and Raman spectroscopy have been applied to a series of experimentally synthesised hydrous sulphur bearing silicate glasses to determine the oxidation state of sulphur dissolved in the glass. Glasses investigated include soda–lime glass (SLG), K2Si4O9 (KSG) and albitic (Albite) and trondhjemitic (TROND) glass compositions. The four spectroscopic techniques are compared with each other to investigate the applicability of each technique as a method to determine the sulphur oxidation state and structural aspects of sulphur dissolution in silicate melts.
Experiments have been performed that calibrate the stoichiometry and thermodynamics of the iron oxidation reaction in natural silicate melts. A series of experiments was carried out on six melt compositions covering a far larger range of oxygen fugacities than had been examined previously. Oxygen fugacities between air and 5.2 logââ units below those defined by the nickel-nickel oxide assemblage were investigated at 1360 C and 1460 C. Results of these experiments confirm that ln(X{sub FeâOâ}/X{sub FeO}) is a linear function of ln {line integral}{sub Oâ} over this entire range, and that this linear behavior is independent of composition over the range considered. These results are inconsistent with an ideal mixing between FeO and FeâOâ components. They are, however, entirely consistent with ideal mixing between FeO and FeO{sub 1.464{plus minus}0.003} (FeO 6FeâOâ) components. A second series of experiments was performed on a single mid-ocean ridge basalt composition (JDFD2) in order to better constrain the temperature dependence of the iron oxidation reaction in this simplified two-component subsystem. This series was carried out at temperatures between 1299 C and 1636 C in air, COâ, and 0.2 logââ units below the fayalite-magnetitie-quartz buffer assemblage. Results of both series of experiments were combined with the Sack et. al. (1980) and Kilinc et al. (1983) databases to estimate thermodynamic parameters for the iron oxidation reaction expressed in terms of FeO and FeO{sub 1.464} components. These coefficients offer the most precise method available for estimation of iron oxidation state in natural silicate melts as a function of ln {line integral}{sub Oâ}, temperature, and composition.