Log fO2 Research Articles

Experimental modeling of the origin of the Moon’s metallic core at partial melting

Geochemical criteria of the Moon’s composition as deficient in Fe and depleted in volatile components and the distribution of siderophile elements in the planet offer the possibility of correlating, under certain conditions, the origin of the Moon and its core from an initial material of composition close to CI carbonaceous chondrites. In order to verify the model of the percolation of liquid metallic Fe through a silicate matrix of chondritic composition at low degrees of melting, we have experimentally modeled Fe movement and deposition in the course of high-temperature centrifugation. The starting experimental mixture had the composition 85% Ol, 10% ferropicrite, and 5% Fe-S (95% Fe and 5% S); the experimental conditions were 4000 g “gravity”, T = 1440°C, Δ log fO2(IW) ∼ −5.5. In our experiments, Fe was segregated in systems with Fe sulfide and silicate melts at partial melting under reduced conditions and the deformation of the silicate framework. Our results indicate that the Moon could be produced from a material of composition close to CI carbonaceous chondrite.

Thermal reduction of molybdite and hematite in water and hydrogen peroxide-bearing solutions: Insights on redox conditions in Hydrothermal Diamond Anvil Cell (HDAC) experiments

The oxygen fugacity in Hydrothermal Diamond Anvil Cell (HDAC) experiments was determined by means of direct observation of the thermal reduction of molybdite or hematite in water and water–H2O2 solutions. HDAC experiments were conducted with and without rhenium gasket in order to evaluate the effect of rhenium on oxygen fugacity of the contained sample. In experiments where pure water and no Re gasket were used, i.e., when the sample charge is loaded into a laser-drilled recess in the lower anvil, the Log(fO2) was shown to be −19.55±0.13 over a wide range of temperature and pressure conditions (335–500°C, 15.0–136.3MPa — pressure determined using EOS of water by Wagner and Pruss, 2002), i.e., HM+8 to HM−0.5, at the limit temperatures. In contrast, in experiments where a Re gasket was used to contain the sample, the Log(fO2) is slightly more reducing (−20.60±0.04), e.g., equals to HM+6 at 350°C, but is significantly higher than that predicted at a given temperature using the Re–ReO2 buffer. Experiments with hydrogen peroxide showed a non-linear increase of oxygen fugacity with increasing molarity of H2O2 from 0.10 to 0.25M. Results indicate that the oxygen fugacity in water dominated HDAC runs is imposed by the fluid medium, and that hydrogen peroxide may be used to increase oxygen fugacity of water-dominated fluid media in HDAC experiments.

Petrology and monazite dating of the Fe-rich gneisses from Kokava (Veporic Unit, Western Carpathians, Slovakia): Devonian sediments supplied from Gondwanan-sources metamorphosed in the Variscan times

*Corresponding author The Fe-rich gneisses from vicinity of Kokava nad Rimavicou (Central Slovakia) were studied by electron microprobe to resolve the origin of the iron ore bodies. Whole-rock chemistry of the ironstones shows the lack of CaO, MgO and partly elevated P 2 O 5 contents. This, together with the mineral assemblage of almandine + magnetite + grunerite + ilmenite + quartz + apatite + allanite + zircon + monazite suggests an origin by metamorphism of sedimentary protolith and disproves the older idea about the formation as a skarn. The Fe–Ti oxides thermometry suggests temperatures of 360–420 °C, and the average oxygen fugacity of ∆log fO 2 normalized to the FMαQ of +0.13. The reconstructed oxygen fugacity for temperatures 500–600 °C (amphibolite facies) yields an interval of c. +1 to +3 ∆log fO 2 (FMαQ). The presence of detrital zircons, and monazites dated in this study, reveals a participation of lithologies of the pan-African orogen. The source rocks of the Devonian ironstones sedimented probably as oolitic chamosite in lagoons. The dated detrital monazite cores (612–400 Ma) show an affinity to source rocks formed in northern peri-Gondwana due to rifting and opening of the Rheic Ocean and separation of Avalonia and Armorica microcontinents. The majority of monazite data from the rims of detrital grains and from unzoned metamorphic grains bear an evidence of Meso-Variscan metamorphism with ages clustering between 360 and 320 Ma (342 ± 4Ma – weighted mean ± 2σ). The younger population with elevated Th contents (18.0–20.5 wt. %) giving the ages of 310–240 Ma (279 ± 2 Ma – weighted mean) is connected with the collapse of Variscan orogen and/or the onset of the early Alpine continental rifting. The Alpine ages are very rare, but a few monazites and one Th–U–Si inclusion in quartz yielded Cretaceous ages (115–85 Ma).

The dissolution mechanism of sulphur in hydrous silicate melts. II: Solubility and speciation of sulphur in hydrous silicate melts as a function of fO2

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 200MPa, 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).The systematic correlation of features in Raman and XANES spectra allows the identification of at least four different S-species in the glasses depending on fO2 and Fe/S of the system. In XANES spectra of Fe-free glasses SH−, H2S and SO42− are visible as peaks at 2466, 2471.8 and 2482eV, respectively. In Raman spectra peaks at 2574 and 990cm−1 indicate the presence of HS bonds and SO42−, respectively, but SH− and H2S can not be distinguished using a Raman spectroscopy. In Fe-bearing glasses FeS bonding is identified at 2469eV in the case of XANES and at 298, 372 and 420cm−1 in the case of Raman spectra. The intensities of peaks related to SH bonding systematically decrease and the intensities of peaks related to FeS bonding systematically increase with increasing Fe/S in both the XANES and the Raman spectra indicating that in the presence of Fe, FeS bonding is preferred over SH bonding. The total S solubility at sulphur saturation in the Fe-free melts is a function of the degree of melt polymerisation and it increases with increasing NBO/T (from 0.03 to 1.91wt.% S). The S2− species are more soluble than the S6+ species in contrast to previously studied Fe-bearing “natural” compositions.The change from S2− to S6+ is observed at log fO2=QFM−1 to QFM+1 which is ~1.5 log unit lower than the range of fO2 previously reported for Fe-rich compositions indicating that Fe influences not only the speciation but also the oxidation state of S in silicate melts at given redox conditions. The natural implications are that S6+ in Fe-poor magmas can be stable at lower fO2 than previously predicted and, hence, S6+ may act as an oxidising agent in the mantle wedge by successively oxidising Fe2+ to Fe3+ via the reaction H2SO4+9FeO=FeS+4Fe2O3+H2O. For the silicate melt generated in the mantle wedge and containing about 10wt.% total FeO, the change in the Fe3+/ΣFe ratio from 0.1 to 0.2 will correspond to an increase in the log fO2 from QFM−0.5 to QFM+1.5 and will require only 1000–3000ppm S extracted from subducted slab.

Influence of oxygen fugacity and temperature on the redox state of iron in natural silicic aluminosilicate melts

The influence of oxygen fugacity (fO2) and temperature on the valence and structural state of iron was experimentally studied in glasses quenched from natural aluminosilicate melts of granite and pantellerite compositions exposed to various T-fO2 conditions (1100–1420°C and 10−12–10−0.68 bar) at a total pressure of 1 atm. The quenched glasses were investigated by Mossbauer spectroscopy. It was shown that the effect of oxygen fugacity on the redox state of iron at 1320–1420°C can be described by the equation log(Fe3+/Fe2+) = k log(fO2) + q, where k and q are constants depending on melt composition and temperature. The Fe3+/Fe2+ ratio decreases with decreasing fO2 (T = const) and increasing temperature (fO2 = const). The structural state of Fe3+ depends on the degree of iron oxidation. With increasing Fe3+/Fe2+ ≥ 1, the dominant coordination of Fe3+ changes from octahedral to tetrahedral. Ferrous iron ions occur in octahedral (and/or five-coordinated) sites independent of Fe3+/Fe2+.

Comment on “Valence state of titanium in the Wark–Lovering rim of a Leoville CAI as a record of progressive oxidation in the early Solar Nebula” by K.A. Dyl, J.I. Simon and E.D. Young

Dyl et al. (2011) state that their results confirm the conclusion of J. Simon et al. (2005) that the pyroxene in Wark–Lovering rims (Wark and Lovering, 1977) found on Ca-, Al-rich refractory inclusions has lower Ti3+/Titot ratios than the primary pyroxene in the interiors of inclusions. While true, the claim is misleading because J. Simon et al. (2005) concluded that there was no Ti3+ in the rims, whereas Dyl et al. (2011) found Ti3+ in 41 of 42 new rim analyses. In addition, J. Simon et al. (2005) concluded that rims formed under much more oxidizing conditions, log fO2⩾IW-1, or ⩾6–7logunits higher, than inclusion interiors. The conclusions of J. Simon et al. (2005) were disputed by S. Simon et al. (2007) and are not supported by the new data of Dyl et al. (2011). The present work is intended for clarification of this and other issues.

Geothermobarometry of Askaoun Pluton in Ouzellarh-Sirwa Promontory (Central Anti-Atlas; Morocco)

Rocks of the late Neoproterozoic Askaoun pluton (558 ± 2 Ma) located in the Ouzellarh-Sirwa promontory (Anti-Atlas) include magmatic microgranular enclaves (MMEs) ranging from rounded to ovoid in shape, dark and fine grained and generally 5 to 10 cm in size, some reaching a size of 50 cm. They are composed of microdiorite, quartz microdiorite and micromonzodiorite, whereas the felsic host rocks comprise mainly quartz-diorite and amphibole-biotite granodiorite based on mineralogical compositions. The mineral assemblage is similar to those described in their hosting granitoids but with different proportions. In this study composition of minerals is used to describe the nature of the magma and estimate the pressure, temperature and oxygen fugacity at which Askaoun pluton is emplaced. Based on chemistry of biotite Askaoun pluton formed from calc-alkaline magma. Compositions of plagioclase (An5 - An29); hornblende (Mg ≠= 0.59 - 0.65) and biotite (Mg ≠= 0.49 - 0.55) of MMEs are slightly distinct or similar to those of host rocks (An7-40; hbl Mg ≠= 0.64 - 0.69; Bi Mg ≠= 0.49 - 0.50) which suggest partial to complete equilibration during mafic-felsic magma interaction. The coexisting hornblende and plagioclase (hornblende-plagioclase thermometry), Al content in hornblende (aluminum-in-hornblende barometry) and the assemblage titanite-magnetite-quartz were used to constrain the P, T and fO2 during the crystallization of the parent magmas. The Askaoun pluton was emplaced at temperature ca. 504°C - 633°C and at pressure ca. 0.9 - 4.66 ± 0.6 Kbars (average depth = 6.5 km) from a highly oxidized magma (log fO2 = ?24.8 to ?19.2).

Syn-eruptive desulfidation of pyrrhotite in the pumice of the Sakurajima 1914-15 eruption: Implication for potential magma ascent rate meter

Pyrrhotite (Po) occurs as inclusions and as isolated crystals in pumice from the 1914-15 eruption of the Sakurajima volcano, Kyushu, Japan. The Po crystals have partly reacted to form spongy Fe oxides. A similar texture has been reported in some previous studies (Hattori, 1993), but the mineral phases and formation processes of the spongy Fe oxides have not been clarified. Our quantitative and compositional map analyses with electron probe microanalysis (EPMA) reveal that the spongy Fe oxides are mostly magnetite (Mt), with a thin rim (<3 μm) of hematite on rare occasions. The spongy texture includes unreacted regions of Po, mesh-like pores, and S-rich spots, showing that it was formed by desulfidation of Po. No Ti was detected, even in the outermost rim; this indicates that the reaction occurred syn-eruptively. According to diffusion calculations, the spongy Mt was formed during the 4 h preceding quenching. Thermodynamic calculations showed that Po is stable at log fO2 < NNO + 2 at a pressure of 1 bar and magmatic temperature, which is 1-2 log units higher than the usual magmatic fO2. These constraints on the timing and oxidation condition of desulfidation lead to the conclusion that the reaction was caused by oxidation of the magma in a shallow volcanic conduit, not in magma chamber processes. The pumice groundmass consists mostly of glass, indicating that the rate of the desulfidation reaction is faster than the decompression-induced crystallization of microlites in the andesitic magma. Therefore, the desulfidation reaction of Po has the potential to be used as a geospeedometer for very fast magma ascent in vigorous explosive eruptions.

Origin of ultramafic xenoliths in high-Mg diorites from east-central China based on their oxidation state and abundance of platinum group elements

Ultramafic xenoliths are abundant in late Mesozoic high-Mg diorites at Laiwu, western Shandong Province; they are composed of dunite (>80 vol.%) with minor harzburgite (<10 vol.%) and olivine (Ol)-pyroxenite (<10 vol.%). We determined the abundance of siderophile and chalcophile elements of representative samples of xenoliths, and calculated oxidation conditions based on mineral chemistry. Bulk compositions of harzburgite show high Cr (2640–3430 ppm), Co (103–111 ppm), Ni (2210–2400 ppm), and high Ir-type platinum group elements (PGE) (IPGE, 14.0–17.8 ppb), with high ratios of IPGE to Pt-type PGE (PPGE, 3.1–6.3). Spinel contains moderate Cr (Cr# = 0.41–0.61). Our data suggest that the harzburgite is the residue of partial melting. The occurrence of orthopyroxene replacing Ol suggests that harzburgite was metasomatized by a Si-rich melt. Dunite contains high concentrations of Cr (> 3800 ppm), Co (>110 ppm), and Ni (>1680 ppm), and low concentrations of IPGE (<4.8 ppb), with low ratios of IPGE/PPGE (as low as 0.05), suggesting that dunite is the cumulate of a mafic melt. High Cr in chromite (Cr# > 0.7) and high Mg in Ol suggest that the parental melt was boninitic. Some Ol grains show variable, locally high Mg, up to Fo 94.3. We attribute these high values to the interaction of the Ol with abundant chromite in the cumulate. Both dunite and harzburgite indicate high log fO2 values, ranging from fayalite-magnetite-quartz (FMQ) +1.4 to +2.4. The values contrast with fO2 below that of the graphite–CO2 buffer for xenoliths in early Palaeozoic diamondiferous kimberlite pipes in the area. The data indicate a sharp increase in fO2 during Mesozoic time, likely caused by subduction of the Tethyan oceanic plate before collision with the Yangtze continent below the eastern margin of the North China craton.

Bulk rock and mineral chemistries and ascent rates of high-K calc-alkalic epidote-bearing magmas, Northeastern Brazil

A manifestation of the Pan-African-Brasiliano orogeny (700–550Ma) in northeastern Brazil was the emplacement of widespread Neoproterozoic granitoids in diverse tectonic terranes. Among these plutons are the magmatic epidote-bearing Conceição das Creoulas, Caldeirão Encantado, Murici, and Boqueirão plutons, located close to the boundary between the Alto Pajeú and Cachoeirinha–Salgueiro terranes. The plutons are high-K calc-alkalic granodiorites to monzogranites, with tabular K-feldspar megacrysts.Pistacite [atomic Fe+3/(Fe3++Al)] in epidote in these granitoids ranges from 21 to 27%. High oxygen fugacity (log fO2 −19 to −13) and the preservation of epidote suggest that the magma was oxidized. Al-in-hornblende barometry indicates hornblende solidification between 6 and 8kbar, at 620 to 780°C according to the hornblende–plagioclase thermometer. Zircon saturation thermometry attests to a near-liquidus temperature range from 794 to 853°C.Partial corrosion of magmatic epidote in these four plutons occurred during an interval of no more than 10–30years, which corresponds to maximum magma ascent rates of 650–1000m/year. Diking, associated with regional shearing, probably facilitated rapid transport of granitic magma through hot continental crust at peak metamorphism, and permitted survival of epidote that was out of equilibrium at the low pressure of final emplacement.Similarities between mineralogical composition, chemistry, and isotopic compositions (εNd(0.60Ga) between −2 and −5,TDM from 1.2 to 1.3Ga, δ18O values>10‰, V-SMOW) of these four plutons and Neoproterozoic magmatic epidote-bearing plutons elsewhere in northeastern Brazil, argue for similar metabasaltic/mafic sources that had previously experienced low-temperature alteration.

Temperature dependence of sulfide and sulfate solubility in olivine-saturated basaltic magmas

The sulfur concentration at pyrrhotite- and anhydrite-saturation in primitive hydrous basaltic melt of the 2001–2002 eruption of Mt. Etna was determined at 200 MPa, T = 1050–1250 °C and at log fO 2 from FMQ to FMQ+2.2 (FMQ is Fayalite–Magnetite–Quartz oxygen buffer). At 1050 °C Au sample containers were used. A double-capsule technique, using a single crystal olivine sample container closed with an olivine piston, embedded in a sealed Au 80Pd 20 capsule, was developed to perform experiments in S-bearing hydrous basaltic systems at T > 1050 °C. Pyrrhotite is found to be a stable phase coexisting with melt at FMQ–FMQ+0.3, whereas anhydrite is stable at FMQ+1.4–FMQ+2.2. The S concentration in the melt increases almost linearly from 0.12 ± 0.01 to 0.39 ± 0.02 wt.% S at FeS-saturation and from 0.74 ± 0.01 to 1.08 ± 0.04 wt.% S at anhydrite-saturation with T ranging from 1050–1250 °C. The relationships between S concentration at pyrrhotite and/or anhydrite saturation, MgO content of the olivine-saturated melt, T, and log fO 2 observed in this study and from previous data are used to develop an empirical model for estimating the magmatic T and fO 2 from the S and MgO concentrations of H 2O-bearing olivine-saturated basaltic melts. The model can also be used to determine maximum S concentrations, if fO 2 and MgO content of the melt are known. The application of the model to compositions of melt inclusions in olivines from Mt. Etna indicates that the most primitive magmas trapped in inclusions might have been stored at log fO 2 slightly higher than FMQ+1 and at T = 1100–1150 °C, whereas more evolved melts could have been trapped at T ⩽ 1100 °C. These values are in a good agreement with the estimates obtained by other independent methods reported in the literature.

Late-magmatic mineral assemblages with siderite and zirconian pyroxene and amphibole in the anorogenic Mt Gibraltar microsyenite, New South Wales, Australia, and their petrological implications

The Mt Gibraltar intrusion near Mittagong and Bowral in New South Wales, Australia (lat. 34°27′54″S, long. 150°25′44″) is a small intrusive body of hypersolvus microsyenite emplaced into the Triassic Hawkesbury Sandstone of the Sydney Basin in Jurassic time, possibly related to extensional faulting. The rock itself consists of intermediate alkali feldspar with minor titanomagnetite and interstitial pyroxene ranging from nearly pure hedenbergite to ≈Hd34Aeg65 in composition. It is crosscut by an irregular system of late-magmatic veins consisting of homogeneous alkali feldspar (≈Ab50Or50), clinopyroxene evolving from sodic hedenbergite to zirconium-rich aegirine, arfvedsonite and siderite. During postmagmatic evolution of the veins, microcrystalline or amorphous silica precipitated together with calcite filling miarolitic cavities. The late-magmatic mineral assemblage of the veins indicate crystallisation (at assumed 700bar pressure) at T=650–670°C, log fO2=−22. This corresponds to conditions very close to the magnetite–wustite curve. Zirconium-bearing pyroxene has a restricted stability field in the system SiO2–ZrO2–FeO–FeO1.5–NaO0.5–HO0.5, at moderately elevated peralkalinity and intermediate silica activity. Under such conditions, pyroxene and amphibole will act as effective sinks for Zr, preventing crystallisation of magmatic zircon or more exotic Zr silicates. The Mt Gibraltar microsyenite is therefore a rare example of an igneous rock in which zirconium is camouflaged in pyroxene and amphibole rather than forming its own minerals.

The effect of fO2 on the partitioning and valence of V and Cr in garnet/melt pairs and the relation to terrestrial mantle V and Cr content

Chromium and vanadium are stable in multiple valence states in natural systems, and their distribution between garnet and silicate melt is not well understood. Here, the partitioning and valence state of V and Cr in experimental garnet/melt pairs have been studied at 1.8-3.0 GPa, with variable oxygen fugacity between IW-1.66 and the Ru-RuO2 (IW+9.36) buffer. In addition, the valence state of V and Cr has been measured in several high-pressure (majoritic garnet up to 20 GPa) experimental garnets, some natural megacrystic garnets from the western United States, and a suite of mantle garnets from South Africa. The results show that Cr remains in trivalent in garnet across a wide range of oxygen fugacities. Vanadium, on the other hand, exhibits variable valence state from 2.5 to 3.7 in the garnets and from 3.0 to 4.0 in the glasses. The valence state of V is always greater in the glass than in the garnet. Moreover, the garnet/melt partition coefficient, D(V), is highest when V is trivalent, at the most reduced conditions investigated (IW-1.66 to FMQ). The V2.5+ measured in high P-T experimental garnets is consistent with the reduced nature of those metal-bearing systems. The low V valence state measured in natural megacrystic garnets is consistent with fO₂ close to the IW buffer, overlapping the range of fo₂ measured independently by Fe2+/Fe3+ techniques on similar samples. However, the valence state of V measured in a suite of mantle garnets from South Africa is constant across a 3 log fo₂ unit range (FMQ-1.8 to FMQ-4.5), suggesting that the valence state of V is controlled by the crystal chemistry of the garnets rather than fo₂ variations. The compatibility of V and Cr in garnets and other deep mantle silicates indicates that the depletion of these elements in the Earth’s primitive upper mantle could be due to partitioning into lower mantle phases as well as into metal.

High-grade contact metamorphism in the Reykjanes geothermal system: Implications for fluid-rock interactions at mid-oceanic ridge spreading centers

Granoblastic hornfels identified in cuttings from the Reykjanes seawater-dominated hydrothermal system contains secondary pyroxene, anorthite, and hornblendic amphibole in locally equilibrated assemblages. Granoblastic assemblages containing secondary orthopyroxene, olivine, and, locally, cordierite and spinel occur within groups of cuttings that show dominantly greenschist facies hydrothermal alteration. Granoblastic plagioclase ranges continuously in composition from An54 to An96, in contrast with relict igneous plagioclase that ranges from An42 to An80. Typical hydrothermal clinopyroxene compositions range from Wo49En3Fs48 to Wo53En30Fo17; clinopyroxene from the granoblastic grains is less calcic with an average composition of Wo48En27Fs25. The hornfels is interpreted to form during contact metamorphism in response to dike emplacement, resulting in local recrystallization of previously hydrothermally altered basalts. Temperatures of granoblastic recrystallization estimated from the 2-pyroxene geothermometer range from 927°C to 967°C. Redox estimates based on the 2-oxide oxybarometer range from log fO2 of −13.4 to −15.9. Granoblastic hornfels comprised of clinopyroxene, orthopyroxene, and calcic plagioclase have been described in a number of ancient hydrothermal systems from the conductive boundary layer between the hydrothermal system and the underlying magma source, most notably in Integrated Ocean Drilling Program Hole 1256D, Ocean Drilling Program Hole 504B, and in the Troodos and Oman ophiolites. To our knowledge, this is the first evidence of high-grade contact metamorphism from an active geothermal system and the first description of equilibrated amphibole-absent pyroxene hornfels facies contact metamorphism in any mid-ocean ridge (MOR) hydrothermal system. This contribution describes how these assemblages develop through metamorphic reactions and allows us to predict that higher-temperature assemblages may also be present in MOR systems.

C–O–H–S fluids and granitic magma: how S partitions and modifies CO2 concentrations of fluid-saturated felsic melt at 200 MPa

Hydrothermal volatile-solubility and partitioning experiments were conducted with fluid-saturated haplogranitic melt, H2O, CO2, and S in an internally heated pressure vessel at 900°C and 200 MPa; three additional experiments were conducted with iron-bearing melt. The run-product glasses were analyzed by electron microprobe, FTIR, and SIMS; and they contain ≤0.12 wt% S, ≤0.097 wt% CO2, and ≤6.4 wt% H2O. Apparent values of log fO2 for the experiments at run conditions were computed from the [(S6+)/(S6++S2−)] ratio of the glasses, and they range from NNO −0.4 to NNO + 1.4. The C–O–H–S fluid compositions at run conditions were computed by mass balance, and they contained 22–99 mol% H2O, 0–78 mol% CO2, 0–12 mol% S, and <3 wt% alkalis. Eight S-free experiments were conducted to determine the H2O and CO2 concentrations of melt and fluid compositions and to compare them with prior experimental results for C–O–H fluid-saturated rhyolite melt, and the agreement is excellent. Sulfur partitions very strongly in favor of fluid in all experiments, and the presence of S modifies the fluid compositions, and hence, the CO2 solubilities in coexisting felsic melt. The square of the mole fraction of H2O in melt increases in a linear fashion, from 0.05 to 0.25, with the H2O concentration of the fluid. The mole fraction of CO2 in melt increases linearly, from 0.0003 to 0.0045, with the CO2 concentration of C–O–H–S fluids. Interestingly, the CO2 concentration in melts, involving relatively reduced runs (log fO2 ≤ NNO + 0.3) that contain 2.5–7 mol% S in the fluid, decreases significantly with increasing S in the system. This response to the changing fluid composition causes the H2O and CO2 solubility curve for C–O–H–S fluid-saturated haplogranitic melts at 200 MPa to shift to values near that modeled for C–O–H fluid-saturated, S-free rhyolite melt at 150 MPa. The concentration of S in haplogranitic melt increases in a linear fashion with increasing S in C–O–H–S fluids, but these data show significant dispersion that likely reflects the strong influence of fO2 on S speciation in melt and fluid. Importantly, the partitioning of S between fluid and melt does not vary with the (H2O/H2O + CO2) ratio of the fluid. The fluid-melt partition coefficients for H2O, CO2, and S and the atomic (C/S) ratios of the run-product fluids are virtually identical to thermodynamic constraints on volatile partitioning and the H, S, and C contents of pre-eruptive magmatic fluids and volcanic gases for subduction-related magmatic systems thus confirming our experiments are relevant to natural eruptive systems.

A new thermodynamic analysis of the intergrowth of hedenbergite and magnetite with Ca-Fe-rich olivine

Some errors and metastable phase relations are found in a recently reported oxygen fugacity-silica activity [logf O₂ -loga(SiO 2 )] diagram of the fayalite-kirschsteinite-hedenbergite-magnetite-wollastonite (Fa-Kst-Hd-Mt-Wo) system at 400 °C and 1 kbar (Markl et al. 2001). Systematic thermodynamic analysis reveals that the composite exsolution lamellae of Mt and Hd in the olivine from the Ilimaussaq Intrusion, Greenland, should form when the reaction 3Fa+3Kst+O 2 = 3Hd+2Mt is overstepped on rapid cooling. This process is also accompanied by favorable f O₂ and a(SiO 2 ) conditions. It is also found that the reaction mechanisms from Ca-Fe-rich olivine (Fa+Kst) to Mt+Hd are very different above and below the equilibrium temperature of the Mt-absent reaction Hd+Kst = Fa+2Wo. The mechanisms above the equilibrium temperature are obviously simpler than those below the equilibrium temperature. Despite this difference, the two types of mechanisms have many features in common: (1) Mt and Hd can form in Ca-Fe-rich olivine on continuous cooling; (2) If a(SiO 2 ) is not buffered, an increase in f O₂ alone is enough to transform Ca-Fe-rich olivine into Mt and Hd; (3) At a fixed f O₂ or a(SiO 2 ), the formation of Mt and/or Hd on cooling requires appropriate a(SiO 2 ) or f O₂ ; and (4) The increase in a(SiO 2 ) is favorable to the formation of Hd, but it is not essential for the intergrown exsolution of Mt and Hd. These common features are closely related to the fact that the process from Fa+Kst to Mt+Hd is an entropy decreasing oxidation reaction.

Solubility of chromium oxide in binary soda-silicate melts

The total chromium oxide (Cr 2O 3) solubility is measured in the liquids belonging to the Na 2O–SiO 2 system using a thermochemical reactor which allows the independent control of temperature (1125 °C < T < 1250 °C), glass composition (1.5 < x < 3) and oxygen fugacity (−15 < log fO 2 < −0.61). The contribution of each chromium species (Cr(II), Cr(III) and Cr(VI)) and the total Cr content dissolved in the melt are determined using a model describing the chromium chemistry in melts involving dissolution, redox and oxo-complex reactions. A manipulation of the obtained results allows the determination of the physicochemical properties of chromium species in melts (chromium complex formulas) as well as thermodynamic properties (entropies and enthalpies) related to the dissolution, reduction and oxidation processes of chromium oxide in binary melts.

Petrology and trace element geochemistry of Robert Massif 04261 and 04262 meteorites, the first examples of geochemically enriched lherzolitic shergottites

Shergottites sampled two distinct geochemical reservoirs on Mars. Basaltic and olivine-phyric shergottites individually sampled both geochemically enriched and depleted reservoirs, whereas lherzolitic shergottites are previously known only to exhibit a relatively limited intermediate geochemical signature that may have resulted from the mixing of the two geochemical end-member reservoirs. Here we show that recently discovered shergottites Robert Massif (RBT) 04261 and RBT 04262 are the first examples of lherzolitic shergottites originating from the enriched reservoir. RBT 04261 and RBT 04262, initially identified as olivine-phyric shergottites, are actually lherzolitic shergottites. Both meteorites exhibit nearly identical textures and mineral compositions, suggesting that they should be paired. Each consists of two distinct textures: poikilitic and non-poikilitic. The poikilitic areas are composed of pyroxene oikocrysts enclosing olivine grains; all pyroxene oikocrysts have pigeonite cores mantled by augite. The non-poikilitic areas are composed of olivine, pyroxene, maskelynite and minor amounts of merrillite, chromite and ilmenite. Olivine and pyroxene show the lowest Mg-number, and maskelynite has the lowest anorthite component among the lherzolitic shergottites. Moreover, the modal abundances of maskelynite in these two meteorites are distinctly higher than the other lherzolitic shergottites. The rare earth element (REE) budgets of RBT 04261 and RBT 04262 are dominated by merrillite. The slightly light rare earth element (LREE)-enriched pattern of this mineral is similar to that of merrillite in the geochemically enriched basaltic shergottites Shergotty and Zagami, and unlike the LREE-depleted pattern of merrillite in the other lherzolitic shergottites. The REE patterns of both high- and low-Ca pyroxenes are also similar to those in Shergotty and Zagami. The REE pattern of a melt calculated to be in equilibrium with the core of a pyroxene oikocryst is parallel to that of the RBT 04262 whole-rock as well as whole-rock compositions of other geochemically enriched basaltic shergottites. These observations imply that RBT 04262 sampled an enriched and oxidized reservoir similar to that sampled by some of the basaltic shergottites and are consistent with an oxidizing condition for the formation of RBT 04262 (log fO 2 = QFM-1.6). The petrographic and geochemical observations presented here suggest that RBT 04261 and RBT 04262 represent the most evolved magma among the lherzolitic shergottites and that this magma originated from a geochemically enriched reservoir on Mars. Based on an evaluation of the relationship between petrographic, geochemical and chronological signatures for shergottites including RBT 04261 and RBT 04262, we propose that both geochemically enriched and depleted shergottites were ejected from the same launch site on Mars.

Effect of oxygen fugacity on the H 2O storage capacity of forsterite in the carbon-saturated systems

High pressure experiments have been performed in the systems Mg 2SiO 4–C–O–H and Mg 2SiO 4–K 2CO 3–C at 6.3 GPa and 1200 to 1600 °C using a split-sphere multi-anvil apparatus. In the Mg 2SiO 4–C–O–H system the composition of fluid was modeled by adding different amounts of water and stearic acid. The fO 2 was controlled by the Mo–MoO 2 or Fe–FeO oxygen buffers. Several experiments in the Mg 2SiO 4–C–O–H system and all experiments in the Mg 2SiO 4–K 2CO 3–C system have been conducted without buffering the fO 2. Forsterite in the system Mg 2SiO 4–K 2CO 3–C does not reveal OH absorption bands in the IR spectra, while forsterite coexisting with carbon-bearing fluid and silicate melt at log fO 2 from FMQ-2 to FMQ-5 (from 2 to 5 log units below fayalite-magnetite-quartz oxygen buffer) contains 800–1850 wt. ppm H 2O. The maximum concentrations were detected at 1400 °C and FMQ-3.5. We observed an increase in the solidus temperature in the system Mg 2SiO 4–C–O–H from 1200 to above 1600 °C with log fO 2 decreasing from FMQ-2 to FMQ-5. The increase of the solidus temperature and the broadening of the stability field of the H 2O–H 2–CH 4 subsolidus fluid phase at 1400–1600 °C explain the high H 2O storage capacity of forsterite relative to that crystallized from carbon-free, oxidized, hydrous, silicic melt. At temperatures above 1400 °C liquidus forsterite precipitated along with diamond from oxidized (FMQ-1) carbonate–silicate melt and from silicate melt dissolving the moderately reduced C–O–H fluid (from FMQ-2 to FMQ-3.5). Formation of diamond was not detected under ultra-reduced conditions (FMQ-5) at 1200–1600 °C. Olivine co-precipitating with diamond from dry carbonate–silicate or hydrous–silicic fluid/melt can provide information on the H 2O contents and speciation of the diamond-forming media in the mantle. The conditions for minimum post-crystallization alteration of olivine and its hydrogen content are discussed.

Parameterization of the Fe = Mg exchange coefficient (Kd) between clinopyroxene and silicate melts

Compilations of experimental data allow the clinopyroxene Fe = Mg exchange equilibrium constant (Kd = (XFe/XMg) Clinopyroxene / (XFe/XMg) Melt) to be parameterized. Values of Kd increase with increasing pressure, temperature, Log fO 2, melt CaO, NBO/TO, LnMgO, MgO#; and with decreasing melt SiO 2, and Na 2O + K 2O. Multiple regression analysis yields improved fits, including some based solely on mineral compositional data. These parameterizations allow improved petrogenetic hypothesis testing for clinopyroxene-bearing assemblages.