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The diffusive exchange of major elements in Na-series tephrite-phonolite diffusion couples with compositions relevant to the Canary Islands magmatism was determined at 300 MPa and variable H.sub.2 O concentrations (0.3 wt % to 3.3 wt %), temperatures (1150 to 1300 °C), and fO.sub.2 levels (NNO-1.5 to NNO+1.7). Composition-dependent effective binary diffusion coefficients were determined from concentration-distance profiles. Results show a wide range of diffusivities for different cations, consistently following the sequence Na â« Al â« K ⥠Mg = Fe = Ca Si Ti, with a mild diffusivity contrast (0.2-0.8 log units) between tephritic and phonolitic melts. Na is the fastest component, with diffusivities falling â¼1.0 log units above those of Si for any given condition. An anomalously fast Al diffusion is observed, with D.sub.Al falling â¼0.4 log units above Si and â¼0.6 log units below Na, suggesting a prevalence of Al-alkali coupling across our range of run conditions. The relationships between log D and H.sub.2 O content in melt for all cations in an intermediate composition are strongly nonlinear and can be fitted using an exponential function with a convergence in diffusion coefficients for different temperatures with increasing H.sub.2 O content. Thus, Arrhenius analyses result in a decrease in activation energies from 222-293 kJ mol.sup.-1 at 1.7 wt % H.sub.2 O to 48-112 kJ mol.sup.-1 at 3.0 wt % H.sub.2 O. These results provide new data on chemical interdiffusion in highly alkaline Na-rich melts and suggest that H.sub.2 O content plays a key role in increasing the chemical efficiency of magma mixing at low temperatures. The obtained dataset is used to test chemical controls of magma mixing in the El Abrigo ignimbrite, Tenerife, where banded pumices involving basanitic-tephritic to phonolitic magmas are common in several compositionally bimodal ignimbrite units.

The oxygen fugacity of the deepest rock samples from Earth’s mantle is found to be more oxidized than previously thought, with the result that carbon in the asthenospheric mantle will be hosted as graphite or diamond but will be oxidized to produce carbonate melt through the reduction of Fe 3+ in silicate minerals during upwelling. Graphite and diamond in the upper mantle Vincenzo Stagno and colleagues report experiments on mantle xenoliths, and find that the oxygen fugacity of the deepest rocks they analyse is at least one order of magnitude more oxidized than previous estimates. They conclude from this that carbon in the asthenospheric mantle will be hosted as graphite or diamond, but it will be oxidized to produce carbonate melt during upwelling. This 'redox melting' relationship has important implications for the extraction of CO 2 from the mantle through decompressive melting. Determining the oxygen fugacity of Earth’s silicate mantle is of prime importance because it affects the speciation and mobility of volatile elements in the interior and has controlled the character of degassing species from the Earth since the planet’s formation 1 . Oxygen fugacities recorded by garnet-bearing peridotite xenoliths from Archaean lithosphere are of particular interest, because they provide constraints on the nature of volatile-bearing metasomatic fluids and melts active in the oldest mantle samples, including those in which diamonds are found 2 , 3 . Here we report the results of experiments to test garnet oxythermobarometry equilibria 4 , 5 under high-pressure conditions relevant to the deepest mantle xenoliths. We present a formulation for the most successful equilibrium and use it to determine an accurate picture of the oxygen fugacity through cratonic lithosphere. The oxygen fugacity of the deepest rocks is found to be at least one order of magnitude more oxidized than previously estimated. At depths where diamonds can form, the oxygen fugacity is not compatible with the stability of either carbonate- or methane-rich liquid but is instead compatible with a metasomatic liquid poor in carbonate and dominated by either water or silicate melt. The equilibrium also indicates that the relative oxygen fugacity of garnet-bearing rocks will increase with decreasing depth during adiabatic decompression. This implies that carbon in the asthenospheric mantle will be hosted as graphite or diamond but will be oxidized to produce carbonate melt through the reduction of Fe 3+ in silicate minerals during upwelling. The depth of carbonate melt formation will depend on the ratio of Fe 3+ to total iron in the bulk rock. This ‘redox melting’ relationship has important implications for the onset of geophysically detectable incipient melting and for the extraction of carbon dioxide from the mantle through decompressive melting.

We present the results of B.sub.2 O.sub.3 evaporation experiments from Ca- and Mg-bearing aluminoborosilicate melts. Our experiments were conducted at 1245 to 1249 .sup.\" C and 1350 to 1361 .sup.\" C for different run times (60-1020 min), and at oxygen fugacities (logfO.sub.2) relative to the fayalite-magnetite-quartz (FMQ) buffer of FMQ-6 to FMQ+1.5, and in air. Our results show that with increasing fO.sub.2, evaporation of B from the melt increases by a factor of 5 compared to reducing conditions. Using Gibbs free energy minimization calculations, we suggest two possible evaporation reactions for B.sub.2 O.sub.3 which constrain its speciation in the gas phase to be either 3+ or 4+ (B.sub.2 O.sub.3(g) and BO.sub.2(g)). The measured B.sub.2 O.sub.3 contents of the B evaporated residual glasses were used to calculate evaporation rate constants (k.sub.i) for B.sub.2 O.sub.3 in oxidizing conditions (air, ki=2.09x10-4 cm min.sup.-1 at 1350 .sup.\" C) and reducing conditions (FMQ-4, ki=4.46x10-5 cm min.sup.-1 at 1350 .sup.\" C). The absence of diffusion profiles in the experimental glasses suggests that the evaporation rates are slower than B.sub.2 O.sub.3 diffusion rates and therefore the rate-limiting process. Overall, the rate of B evaporation in air is approximately a factor of 5 higher compared to reducing conditions at FMQ-4.

Newly analysed titanomagnetite–ilmenite (Tim–Ilm) intergrowths from Martian nakhlite meteorite Northwest Africa (NWA) 5790 yielded crystallisation temperature up to 1032 °C and oxygen fugacity (fO2) up to ΔQFM + 1.6, notably higher than previous estimates for nakhlite magmas (temperature < 950 °C, fO2 = ΔQFM − 0.5 to ΔQFM + 1). To interpret how the magma was reduced from ΔQFM − 0.5 to ΔQFM + 1.6, we used D-Compress to model the sulphur degassing process within a single thick lava pile. For fO2 to significantly decrease in this extended range, a sulphur-rich (S content 4000–7000 ppm) Martian lava flow had to degas all the sulphur species at a certain final degassing pressure, which was 2–4 bar for NWA 988 and Lafayette and < 0.7 bar for Y-000593 and Nakhla. These final degassing pressure data are in good agreement with the Martian nakhlite burial depth estimated by other petrological and geochemical methods. These estimates are also comparable with the excavation depth of ~ 40 m based on the small (6.5 km in diameter) impact crater over the Elysium lava plain. The fO2-controlled sulphur degassing pressure may constitute a method for estimating the burial depth of sulphur-rich lava flows on Mars.

Oxygen fugacity (fO2) is a fundamental variable affecting phase equilibrium in magmas, and in externally heated pressure vessel experiments it is typically controlled by using redox buffer assemblages. However, these do not allow fine enough resolution; for example, most arc magmas fall between the fO2 imposed by the neighboring Ni–NiO and Re–ReO2 buffers and so does the transition of S2− to S6+ in magmas. Here we propose a new method to quantitatively impose fO2 in hydrous high-P–T experiments in molybdenum hafnium carbide (MHC) pressure vessels by admixing small amounts of hydrogen into the Ar pressure medium. The thermodynamic calculation procedure used to determine the initial amount of hydrogen to be loaded to constrain desired fO2 values was verified by CoPd alloy redox sensor experiments to be accurate within ±0.3 log units for the pressure (P) – temperature (T) range of 940–2060 bar and 800–1100 ∘C. As hydrogen can be slowly lost from the pressure medium due to diffusion through the vessel walls at high T, we also determined the hydrogen permeability of the MHC alloy as a function of T. The such-obtained hydrogen permeability equation for the MHC alloy can be used to determine the rate of fO2 increase for any MHC pressure vessel configuration. As the rate of fO2 increase is slow (e.g., 0.36 log units per day in our setup at T= 1000 ∘C), we propose that H2 addition to the Ar pressure medium is an effective way to accurately impose fO2 in many types of experiments conducted in MHC vessels allowing experimentation up to T= 1200 ∘C and P= 300 MPa.

We compute the canonical partition functions and the Lee–Yang zeros in Nf=2 lattice QCD at temperature T=1.20Tc lying above the Roberge–Weiss phase transition temperature TRW. The phase transition is characterized by the discontinuities in the baryon number density at specific values of imaginary baryon chemical potential. We further develop our method to compute the canonical partition functions using the asymptotic expression for respective integral. Then, we compute the Lee–Yang zeros and study their behavior in the limit of high baryon density.

It is well established through field observations, experiments, and chemical models that oxidation (redox) state and pH exert a strong influence on the speciation of dissolved components and the solubility of minerals in hydrothermal fluids. log -pH diagrams were used to depict the influence of oxygen fugacity and pH on monocarboxylate- and dicarboxylate-transport of Pb and Zn in low-temperature (100°C) hydrothermal ore fluids that are related to diagenetic processes in deep sedimentary basins, and allow a first-order comparison of Pb and Zn transport among proposed model fluids for Mississippi Valley-type (MVT) and red-bed related base metal (RBRBM) deposits in terms of their approximate pH and conditions. To construct these diagrams, total Pb and Zn concentrations and Pb and Zn speciation were calculated as a function of log and pH for a composite ore-brine with concentrations of major elements, total sulfur, and total carbonate that approximate the composition of MVT and RBRBM model ore fluids and modern basinal brines. In addition to acetate and malonate complexation, complexes involving the ligands Cl , HS , H S, and OH were included in the model of calculated total metal concentration and metal speciation. Also, in the model, Zn and Pb are competing with the common-rock forming metals Ca, Mg, Na, Fe, and Al for the same ligands. Calculated total Pb concentration and calculated total Zn concentration are constrained by galena and sphalerite solubility, respectively. Isopleths, in log -pH space, of the concentration of Pb and concentration of Zn in carboxylate (acetate + malonate) complexes illustrate that the oxidized model fluids of T. H. Giordano (in , ed. E. D. Pittman and M. D. Lewan, Springer-Verlag, New York, 1994, pp. 319-354) and G. M. Anderson ( ., 1975, , 937-942) are capable of transporting sufficient amounts of Pb (up to 10 ppm) and Zn (up to 100 ppm) in the form of carboxylate complexes to form economic deposits of these metals. On the other hand, the reduced ore fluid models of D. A. Sverjensky ( ., 1984, , 23-37) and T. H. Giordano and H. L. Barnes ., 1981, , 2200-2211) can at best transport amounts of Pb and Zn, as carboxylate complexes, that are many orders of magnitude below the 1 to 10 ppm minimum required to form economic deposits. Lead and zinc speciation (mol% of total Pb or Zn) in the model ore fluid was calculated at specific log -pH conditions along the 100, 0.01, and 0.001 ppm total Pb and total Zn isopleths. Along the 100 ppm isopleth conditions are oxidized (∑SO >> ∑H S) with Pb and Zn predominantly in the form of chloride complexes under acid to mildly alkaline conditions (pH from 3 to approximately 7.5), while hydroxide complexes dominate Pb and Zn speciation under more alkaline conditions. Sulfide complexes are insignificant under these oxidized conditions. For more reduced conditions along the 0.01 and 0.001 ppm isopleths chloride complexes dominate Pb and Zn speciation in the SO field and near the SO -reduced sulfur boundary from pH = 4 to approximately 7.5, while hydroxide complexes dominate Pb and Zn speciation under alkaline conditions above pH = 7.5 in the SO field. In the most reduced fluids (∑H S >> ∑SO ) along the 0.01 and 0.001 isopleths, sulfide complexes account for almost 100% of the Pb and Zn in the model fluid. Acetate (monocarboxylate) complexation is significant only under conditions of chloride and hydroxide complex dominance and its effect is maximized in the pH range 5 to 7, where it complexes 2 to 2.6% of the total Pb and 1 to 1.25% of the total Zn. Malonate (dicarboxylate) complexes are insignificant along all isopleths. The speciation results from this study show that deep formation waters characterized by temperatures near 100°C, high oxidation states and ∑H S < 0.03 mg L ( < 10 ), high chlorinities (~ 100000 mg L ), and high but reasonable concentrations of carboxylate anions can mobilize up to 3% of the total Pb and up to 1.3% of the total Zn as carboxylate complexes. Furthermore, these percentages, under the most favorable conditions, correspond to approximately 1 to 100 ppm of these metals in solution; concentrations that are adequate to form economic deposits of these metals. However, the field evidence suggests that all of these optimum conditions for carboxylate complexation are rarely met at the same time. A comparison of the composite ore fluid compositions from this study and modern brine data shows that the ore brines, corresponding to log -pH conditions based on the Anderson (1975) and Giordano (1994) model fluids, are similar in many respects to modern, high trace-metal petroleum-field brines. The principal differences between modern high trace-metal brines and the composite ore fluids of Anderson (1975) and Giordano (1994) relate to their carboxylate anion content. The reported concentrations of monocarboxylate anions (∑monocbx) and dicarboxylate anions (Edicbx) in high trace-metal petroleum-field brines (< 1 to 300 mg L and < 1 mg L , respectively) are significantly lower than the concentrations assumed in the modelled brines of this study (∑monocbx = 7 700 mg L and ∑dicbx = 300 mg L ). There are also major differences in the corresponding total chloride to carboxylate ratio (∑ /∑ ) and monocarboxylate to dicarboxylate ratio (∑ /∑ ). Modern high trace-metal brines have much higher ∑ /∑ values and, therefore, the contribution of carboxylate complexes to the total Pb and Zn content in these modern brines is likely to be significantly less than the 1 to 3 percent for the composite ore fluids of Anderson (1975) and Giordano (1994). The composite ore-brine based on the Giordano and Barnes (1981) MVT ore fluid is comparable to the high salinity (> 170 000 mg L TDS) subset of modern brines characterized by low trace-metal content and high total reduced sulfur (∑H S). A comparison of the Sverjensky (1984) composite ore-brine with modern petroleum-field brines in terms of ∑H S and Zn content, reveals that this ore fluid corresponds to a \"border-type\" brine, between modern high trace-metal brines and those with low trace-metal content and high ∑H S. A brine of this type is characterized by values of ∑H S, ∑Zn, and/or ∑Pb within or near the 1 to 10 mg L range. Based on brine-composition data from numerous references cited in this paper, border-type brines do exist but are rare. The model results and field evidence presented in this study are consistent with other chemical simulation studies of carboxylate complexation in modern petroleum-field brines. Thus, it appears that carboxylate complexation plays a minor, if not insignificant, role as a transport mechanism for Pb and Zn in high salinity Na-Cl and Na-Ca-Cl basinal brines and related ore fluids.

Titanomagnetite–melt partitioning of Mg, Mn, Al, Ti, Sc, V, Co, Ni, Cu, Zn, Ga, Zr, Nb, Mo, Hf and Ta was investigated experimentally as a function of oxygen fugacity ( f O 2 ) and temperature ( T ) in an andesitic–dacitic bulk-chemical compositional range. In these bulk systems, at constant T, there are strong increases in the titanomagnetite–melt partitioning of the divalent cations (Mg 2+ , Mn 2+ , Co 2+ , Ni 2+ , Zn 2+ ) and Cu 2+ /Cu + with increasing f O 2 between 0.2 and 3.7 log units above the fayalite–magnetite–quartz buffer. This is attributed to a coupling between magnetite crystallisation and melt composition. Although melt structure has been invoked to explain the patterns of mineral–melt partitioning of divalent cations, a more rigorous justification of magnetite–melt partitioning can be derived from thermodynamic principles, which accounts for much of the supposed influence ascribed to melt structure. The presence of magnetite-rich spinel in equilibrium with melt over a range of f O 2 implies a reciprocal relationship between a (Fe 2+ O) and a (Fe 3+ O 1.5 ) in the melt. We show that this relationship accounts for the observed dependence of titanomagnetite–melt partitioning of divalent cations with f O 2 in magnetite-rich spinel. As a result of this, titanomagnetite–melt partitioning of divalent cations is indirectly sensitive to changes in f O 2 in silicic, but less so in mafic bulk systems.

Decades of study have documented several orders of magnitude variation in the oxygen fugacity of terrestrial magmas and of mantle peridotites. This variability has commonly been attributed either to differences in the redox state of multivalent elements (e.g., Fe /Fe ) in mantle sources or to processes acting on melts after segregation from their sources (e.g., crystallization or degassing). We show here that the phase equilibria of plagioclase, spinel, and garnet lherzolites of constant bulk composition (including whole-rock Fe /Fe ) can also lead to systematic variations in in the shallowest ~100 km of the mantle. Two different thermodynamic models were used to calculate vs. pressure and temperature for a representative, slightly depleted peridotite of constant composition (including total oxygen). Under subsolidus conditions, increasing pressure in the plagioclase-lherzolite facies from 1 bar up to the disappearance of plagioclase at the lower pressure limit of the spinel-lherzolite facies leads to an decrease (normalized to a metastable plagioclase-free peridotite of the same composition at the same pressure and temperature) of ~1.25 orders of magnitude. The spinel-lherzolite facies defines a minimum in and increasing pressure in this facies has little influence on (normalized to a metastable spinel-free peridotite of the same composition at the same pressure and temperature) up to the appearance of garnet in the stable assemblage. Increasing pressure across the garnet-lherzolite facies leads to increases in (normalized to a metastable garnet-free peridotite of the same composition at the same pressure and temperature) of ~1 order of magnitude from the low values of the spinel-lherzolite facies. These changes in normalized reflect primarily the indirect effects of reactions involving aluminous phases in the peridotite that either produce or consume pyroxene with increasing pressure: Reactions that produce pyroxene with increasing pressure (e.g., forsterite + anorthite ⇆ Mg-Tschermak + diopside in plagioclase lherzolite) lead to dilution of Fe -bearing components in pyroxene and therefore to decreases in normalized whereas pyroxene-consuming reactions (e.g., in the garnet stability field) lead initially to enrichment of Fe -bearing components in pyroxene and to increases in normalized (although this is counteracted to some degree by progressive partitioning of Fe from the pyroxene into the garnet with increasing pressure). Thus, the variations in normalized inferred from thermodynamic modeling of upper mantle peridotite of constant composition are primarily passive consequences of the same phase changes that produce the transitions from plagioclase → spinel → garnet lherzolite and the variations in Al content in pyroxenes within each of these facies. Because these variations are largely driven by phase changes among Al-rich phases, they are predicted to diminish with the decrease in bulk Al content that results from melt extraction from peridotite, and this is consistent with our calculations. Observed variations in FMQ-normalized of primitive mantle-derived basalts and peridotites within and across different tectonic environments probably mostly reflect variations in the chemical compositions (e.g., Fe /Fe or bulk O content) of their sources (e.g., produced by subduction of oxidizing fluids, sediments, and altered oceanic crust or of reducing organic material; by equilibration with graphite- or diamond-saturated fluids; or by the effects of partial melting). However, we conclude that in nature the predicted effects of pressure- and temperature-dependent phase equilibria on the of peridotites of constant composition are likely to be superimposed on variations in that reflect differences in the whole-rock Fe /Fe ratios of peridotites and therefore that the effects of phase equilibria should also be considered in efforts to understand observed variations in the oxygen fugacities of magmas and their mantle sources.

To better understand the CO 2 sequestration and enhanced shale gas recovery, it is of great significance to study the adsorption characteristics of CO 2 and CH 4 in different types of shale. In this study, the mineral composition, pore structure and CH 4 and CO 2 adsorption isothermals of marine and continental shale samples were determined, an adsorption model was proposed to describe the adsorption behaviors of CH 4 and CO 2 , the thermodynamics parameter of adsorption was obtained, and then the influence of mineral composition and pore structure on the adsorption characteristics of CH 4 and CO 2 in shale was clarified. The results showed that the total organic carbon content (TOC), the specific surface area (SSA) and micropore volume of marine shale samples are larger than those of continental shale samples. Shale has a higher TOC and clay minerals contents corresponding to a higher adsorption capacity. Under the same conditions, the CO 2 adsorption capacity of shale is significantly higher than that of CH 4 . The proposed adsorption model considered the different adsorption mechanisms in different pores and the temperature effect, which can well describe the CH 4 and CO 2 adsorption behaviors of shale in various temperatures. Based on the adsorption model, considering the real gas conditions, the variation of the calculated isosteric heat (Δ H ) and entropy (Δ S ) of CH 4 and CO 2 adsorption with the increasing adsorption amount experienced three stages: slow decline, rapid decline, and gradual flattening. For a certain adsorption amount, the Δ H and Δ S of CO 2 adsorption in shale are higher than those of CH 4 , and with the increase in temperature, the Δ H and Δ S show a downward trend. Combining the proposed adsorption model with ideal adsorbed solution theory, the predicted selectivity factor ( α CO 2 / CH 4 ) of CO 2 over CH 4 of all shale samples at the CH 4 and CO 2 mixed gas environment is greater than 1. Shale has a lower TOC corresponding to a higher α CO 2 / CH 4 , and thus the α CO 2 / CH 4 of continental shale samples is higher than that of marine shale samples. The α CO 2 / CH 4 increased with the increase in fugacity and CO 2 mole fraction, while decreased with the increase in temperature, and the variation of α CO 2 / CH 4 can be well explained by thermodynamics analysis.