橄榄岩-卤水反应实验生成富烷氢流体

<正>1研究目的(Objective)利用地球化学动力学实验室流动反应装置(国家发明专利)探索高温高压下水(卤水)与橄榄岩相互作用产出氢气和甲烷的生成机制。实验发现:400℃反应时释放很高浓度氢气和甲烷等可燃气,同时橄榄岩发生蛇纹石化。实验表明氢和甲烷可能来自地下深处,这是蛇绿岩与油气有关的实验依据。近年来发现:大西洋中脊的一些水热活动地区,热水与超镁铁岩石反应。喷口流体富含甲烷、氢和     

The solubility of TiO2 in olivine: implications for the mantle wedge environment

One characteristic of many subduction-zone garnet peridotites is that they contain titanium-bearing phases not otherwise found in mantle rocks. In particular, titanoclinohumite and/or its breakdown assemblage consisting of symplectic intergrowths of olivine and ilmenite is common in many of these bodies. The Alpe Arami garnet lherzolite of the Swiss Alps, while lacking titanoclinohumite, displays instead large numbers of FeTiO₃ rod-shaped precipitates in the oldest generation of olivine, amounting to approximately 1% by volume, indicating that at some time in its past, the peridotite experienced conditions under which the solubility of TiO₂ in olivine was >0.6 wt.%. In order to test the hypothesis that the environment of very high solubility of TiO₂ in olivine is to be found at very high pressures, we have conducted experiments on lherzolite compositions with added ilmenite at pressures between 5 and 12 GPa and temperatures of 1350–1700 K. Our results on anhydrous compositions show that whereas solubility of TiO₂ was not detected in olivine at 5 GPa, 1400 K where it coexists with rutile, when rutile disappeared from the paragenesis, the solubility climbed to 0.4 wt.% at 8 GPa, 0.5 wt.% at 10 GPa and to >1.0 wt.% at 12 GPa, 1700 K. These results support our previous interpretations from titanate morphology and abundance that the Alpe Arami massif has surfaced from P=10 GPa but remove the need to suggest a deeper origin and possible precursor phase such as wadsleyite. They also support the hypothesis that garnet peridotites with unusual Ti-bearing phases reflect a unique mantle environment occurring in the mantle wedge overlying subduction zones.     

不同类型湿地CO2:CH4比例及其影响因素: 整合分析

湿地是温室气体二氧化碳(CO₂)和甲烷(CH₄)的主要来源之一,在全球碳循环中发挥着重要作用。由于CH₄在百年尺度上的全球增温潜势是CO₂的45倍,因此深入研究湿地CO₂:CH₄的排放比例及其影响因素对准确理解和预测湿地碳循环过程及其对未来全球变化的响应具有重要意义。本文采用文献整合分析方法,对比了不同类型湿地中CO₂:CH₄排放比例的特征及其影响因素。结果表明,藓类泥炭沼泽、滨海湿地和稻田中CO₂:CH₄排放比例显著高于草本沼泽、河流湿地和湖泊湿地等其他类型湿地;相关性分析研究发现,湿地CO₂:CH₄排放比例与pH和水位显著负相关,与盐度显著正相关。可见,藓类泥炭沼泽低水位和低pH抑制CH₄排放是导致其CO₂:CH₄排放比例较高的重要原因,而滨海湿地高盐分抑制CH₄排放是其CO₂:CH₄排放比例高的重要原因。与自然湿地相比,稻田CO₂:CH₄排放比例高与其人为施肥和稻草还田抑制CH₄排放有关。此外,大气温度、土壤温度、降水量、土壤含水率等因子也对湿地CO₂:CH₄排放比例具有重要影响,尽管它们之间的线性相关关系不显著。目前,湿地CO₂:CH₄排放比例和影响因素仍存在很大的不确定性,未来亟待加强不同类型湿地CO₂:CH₄排放比例及其关键影响因素研究。

     

Chemical and isotopic equilibrium between CO2 and CH4 in fumarolic gas discharges: Generation of CH4 in arc magmatic-hydrothermal systems

The chemical and isotopic composition of fumarolic gases emitted from Nisyros Volcano, Greece, and of a single gas sample from Vesuvio, Italy, was investigated in order to determine the origin of methane (CH₄) within two subduction-related magmatic-hydrothermal environments.Apparent temperatures derived from carbon isotope partitioning between CH₄ and CO₂ of around 340°C for Nisyros and 470°C for Vesuvio correlate well with aquifer temperatures as measured directly and/or inferred from compositional data using the H₂O-H₂-CO₂-CO-CH₄ geothermometer. Thermodynamic modeling reveals chemical equilibrium between CH₄, CO₂ and H₂O implying that carbon isotope partitioning between CO₂ and CH₄ in both systems is controlled by aquifer temperature.N₂/³He and CH₄/³He ratios of Nisyros fumarolic gases are unusually low for subduction zone gases and correspond to those of midoceanic ridge environments. Accordingly, CH₄ may have been primarily generated through the reduction of CO₂ by H₂ in the absence of any organic matter following a Fischer-Tropsch-type reaction. However, primary occurrence of minor amounts of thermogenic CH₄ and subsequent re-equilibration with co-existing CO₂ cannot be ruled out entirely. CO₂/³He ratios and δ¹³C_CO2 values imply that the evolved CO₂ either derives from a metasomatized mantle or is a mixture between two components, one outgassing from an unaltered mantle and the other released by thermal breakdown of marine carbonates. The latter may contain traces of organic matter possibly decomposing to CH₄ during thermometamorphism.     

Carbon-13 exchange between CO2 and CH4 under geothermal conditions

On the basis of recently reported data on the kinetics of carbon-13 exchange between CO₂ and CH₄ at temperatures above 500°C, first order rate constants $\text{log k = 11.16?10,190/T}$ were derived allowing variations in Δ, the difference in the isotopic composition of coexisting CO₂ and CH₄, to be evaluated as a function of initial composition and cooling rate of the rising geothermal fluid. Observed Δ-values in geothermal discharges are likely to represent frozen in compositions attained after minimum residence times of 20 ka at 400°C or 10 Ma at 300°C. The carbon-13 contents of any biogenic gases are unlikely to have been affected by thermal re-equilibration at temperatures below 200°C. The chemical equilibrium involving CO₂ and CH₄ can be expected to proceed about a hundred times faster than isotopic equilibration.     

Natural evidence for rapid abiogenic hydrothermal generation of CH4

Although recent hydrothermal experiments imply that abiogenic methane (CH₄) generation from hydrothermal reduction of CO₂ can occur, evidence from natural systems was still lacking. Based on the chemical and isotopic equilibrium signatures of low-temperature fumarolic gas discharges, we are able to provide hard evidence for its natural occurrence, namely in three subduction-related bi-phase hydrothermal systems of the Mediterranean, whose temperatures range from 260 to 470 °C. The attainment of equilibrium and the time spans of recent volcanic dormancy allowed us to calculate minimum rates for chemical and isotopic equilibration. These are significantly higher than those previously reported and might be due to the presence of a saturated water vapor phase in the investigated systems. The fact that nature provides conditions enabling relatively fast production of hydrocarbons from CO₂ strongly supports the concerns that were recently raised from laboratory experiments. These address the use of the carbon isotope composition of reduced carbon in Archean sediments as a tracer of early life and the occurrence of CH₄ on extraterrestrial planets as a bioindicator. In view of the potential role of abiogenic CH₄ as a precursor of life, we also present an estimate of abiogenic hydrothermal CH₄ fluxes throughout the Archean. It is not expected that these fluxes exceeded 80 Mt/yr during the past 4.0 Ga. This, however, would have been enough to facilitate HCN production on the prebiotic Earth.     

Effects of key geological factors in the long-term transport of CH4 and the CH4-hydrate formation behavior with formation dip

Many locations with concentrated hydrates at vents have confirmed the presence of abundant thermogenic gas in the middle of the Qiongdongnan Basin (QDNB). However, the impact of deep structures on gas-bearing fluids migration and gas hydrates distribution in tectonically inactive regions is still unclear. In this study, the authors apply high-resolution 3D seismic and logging while drilling (LWD) data from the middle of the QDNB to investigate the influence of deep-large faults on gas chimneys and preferred gas-escape pipes. The findings reveal the following: (1) Two significant deep-large faults, F1 and F2, developed on the edge of the Songnan Low Uplift, control the dominant migration of thermogenic hydrocarbons and determine the initial locations of gas chimneys. (2) The formation of gas chimneys is likely related to fault activation and reactivation. Gas chimney 1 is primarily arises from convergent fluid migration resulting from the intersection of the two faults, while the gas chimney 2 benefits from a steeper fault plane and shorter migration distance of fault F2. (3) Most gas-escape pipes are situated near the apex of the two faults. Their reactivations facilitate free gas flow into the GHSZ and contribute to the formation of fracture‐filling hydrates.     

Evidence for crustal degassing of CF4 and SF6 in Mojave Desert groundwaters

Dissolved tetrafluoromethane (CF₄) and sulfur hexafluoride (SF₆) concentrations were measured in groundwater samples from the Eastern Morongo Basin (EMB) and Mojave River Basin (MRB) located in the southern Mojave Desert, California. Both CF₄ and SF₆ are supersaturated with respect to equilibrium with the preindustrial atmosphere at the recharge temperatures and elevations of the Mojave Desert. These observations provide the first in situ evidence for a flux of CF₄ from the lithosphere. A gradual basin-wide enhancement in dissolved CF₄ and SF₆ concentrations with groundwater age is consistent with release of these gases during weathering of the surrounding granitic alluvium. Dissolved CF₄ and SF₆ concentrations in these groundwaters also contain a deeper crustal component associated with a lithospheric flux entering the EMB and MRB through the underlying basement. The crustal flux of CF₄, but not of SF₆, is enhanced in the vicinity of local active fault systems due to release of crustal fluids during episodic fracture events driven by local tectonic activity. When fluxes of CF₄ and SF₆ into Mojave Desert groundwaters are extrapolated to the global scale they are consistent, within large uncertainties, with the fluxes required to sustain the preindustrial atmospheric abundances of CF₄ and SF₆.     

Selection of coals of different maturities for CO2 Storage by modelling of CH4 and CO2 adsorption isotherms

CO₂ injection in unmineable coal seams could be one interesting option for both storage and methane recovery processes. The objective of this study is to compare and model pure gas sorption isotherms (CO₂ and CH₄) for well-characterised coals of different maturities to determine the most suitable coal for CO₂ storage. Carbon dioxide and methane adsorption on several coals have been investigated using a gravimetric adsorption method. The experiments were carried out using both CO₂ and CH₄ pure gases at 25 °C from 0.1 to 5 MPa (1 to 50 bar). The experimental results were fitted using Temkin's approach but also with the corrected Langmuir's and the corrected Tóth's equations. The two last approaches are more accurate from a thermodynamical point of view, and have the advantage of taking into account the fact that experimental data (isotherms) correspond to excess adsorption capacities. These approaches allow better quantification of the adsorbed gas. Determined CO₂ adsorption capacities are from 0.5 to 2 mmol/g of dry coal. Modelling provides also the affinity parameters of the two gases for the different coals. We have shown these parameters determined with adsorption models could be used for classification and first selection of coals for CO₂ storage. The affinity ratio ranges from a value close to 1 for immature coals to 41 for high rank coals like anthracites. This ratio allows selecting coals having high CO₂ adsorption capacities. In our case, the modelling study of a significant number of coals from various ranks shows that anthracites seem to have the highest CO₂ storage capacities. Our study provides high quality affinity parameters and values of CO₂ and CH₄ adsorption capacities on various coals for the future modelling of CO₂ injection in coal seams.     

N2CH4z.sbnd;CO2 fluids during formation of the Dôme de l'Agout,France

The Dôme de l'Agout consists of an ellipsoidal mass of gneisses and migmatites emerging from a large area of chlorite-sericite schists. Fluid inclusions in syn-metamorphic quartz segregations are typically one-phase at room temperature. Analyses by gas chromatography indicate that their main constituents are N₂, CH₄ and CO₂; such compositions are confirmed by freezing studies on individual inclusions. Nitrogen contents of the inclusions range from 2 to 72 mol% and tend to increase with increasing degree of metamorphism. CH₄ and CO₂ show an erratic distribution with the exception of the lowest grade samples, which invariably have very high CO₂ contents. The origin of the nitrogen remains unsolved; however, it seems likely that the nitrogen was not produced by mineral reactions in the presently exposed rocks but came from an external source and moved from the centre of the dome outwards.     

N2-CH4（CO2）混合气体在线标样制备及其拉曼定量因子测定

利用混合气体的标准样品对激光拉曼探针进行标定,可以快速准确地对包裹体中的无机及有机气相组分进行定量分析。而常用的商用钢瓶装混合气体标样,存在费用高、气体组成单一固定等缺点。本文设计了一套在线标样制备装置,提出一种在线配置不同浓度和压力条件下混合气体标样的方法。利用高纯度(纯度99.999%)的N2、CH4以及CO2钢瓶气,经过在线混合增压,在5 MPa和10 MPa条件下制备了N2摩尔分数为30%、50%和70%的N2-CH4以及N2-CO2混合气体在线标样。该方法制备的标样与70%N2+30%CO2的商用钢瓶气标样对比表明,CO2与N2的拉曼相对峰高以及相对峰面积值的误差在4%以内,具有较高的准确度和重现性。通过不同压力和浓度条件下CH4以及CO2的拉曼相对定量因子测定表明,气体的相对定量因子在5~10 MPa压力条件下与压力及组成无关。地质样品应用结果表明,本方法可以方便、灵活、准确地按任意比例将两瓶及两瓶以上纯气体钢瓶样品进行混合及增压,为激光拉曼标定、气体组成原位测量等提供了一种新的技术思路。     

Strain response and re-equilibration of CH4-rich synthetic aqueous fluid inclusions in calcite during pressure drops

Aqueous fluids in sedimentary basins often contain dissolved methane, particularly in petroleum environments. PVTX (Pressure-Volume-Temperature-Composition) reconstructions performed using fluid inclusion data are largely based on the assumption that inclusions do not change from the time of trapping until the present. Many authors, however, consider that fluid inclusions can re-equilibrate, particularly in fragile minerals like calcite. In order to understand this re-equilibration phenomenon in the metamorphic domain, previous experiments have been performed under high PT conditions, but few have been performed at low to medium PT conditions such as those associated with sedimentary burial diagenesis, and no previous studies have examined CH₄-bearing aqueous inclusions in calcite.An experimental study of the preservation/modification of CH₄-rich synthetic fluid inclusions in calcite during isothermal decompression was conducted. An autoclave was used for accurate PTX control allowing equilibrium between liquid and vapour in the CH₄-H₂O system. PTX conditions were maintained at four stages of decreasing pressure, with each stage held for 7 days to simulate an isothermal pressure drop. In order of decreasing pressure, the pressure-temperature conditions monitored were 276 ± 10 bar at 180 ± 7 °C, 176 ± 10 bar at 180 ± 7 °C, 76 ± 10 bar at 180 ± 7 °C and 10 ± 3 bar at 180 ± 15 °C. At the end of the experiment, the calcite was recovered and analyzed by microthermometry and Raman microspectroscopy for PTX reconstruction. A careful procedure was adopted to limit re-equilibration of inclusions during analytical procedures. Four types of inclusion shapes and four types of strain patterns were differentiated. Classification of the petrographic strain patterns was carried out. These strain patterns were associated with inclusion stretching and/or leakage regarding CH₄, T_h and P_h compared to experimental conditions. Factors controlling the preservation or acquisition of strain patterns included the initial shape and size of the inclusion, and the pressure differential (ΔP) between the confining pressure (P_cf) and the internal pressure (P_i) within the inclusion. Most fluid inclusions seemed to be trapped during the first 7 days of the experiment, although few (4%) of these preserved the initial PT conditions of 276 ± 10 bar, whereas 8% preserved the second and third run of PT conditions. Overall, the majority of inclusions (88%) did not reflect accurately the PTX trapping conditions. A petrographic guide to the inclusions is presented here that allows strain identification for PVT reconstructions. Re-equilibration patterns and evidence for preferential methane leakage from aqueous inclusions in calcite are important findings revealed by this study, and may be useful for the reconstruction of post-trapping events in investigations of natural samples, and in other experiments using synthetic inclusions in calcite.     

Fluid inclusions in upper mantle xenoliths from Northern Patagonia,Argentina: Evidence for an upper mantle diapir

Summary Three generations of fluid inclusions can be recognized in upper mantle xenoliths from alkali basalts of the Somoncura Massif, Northern Patagonia, Argentina. The first (early, primary) one consists of dense CO₂ inclusions which were trapped in the mantle-crust boundary zone (22–36 km minimum trapping depth). Their co-genetic relationship with silicate melt inclusions enables us to constrain their minimum trapping temperature at 1200°C, indicating a high temperature event in a cooler environment. The late (pseudosecondary and secondary) generations of fluid inclusions were classified in accordance with their homogenization temperature to liquid CO₂ (L1) and vapor CO₂ (L2) phase. The minimum trapping depth for the first of the late inclusions (L1) is about 16 km. In spite of the uncertainties related to this value, L1 inclusions indicate that the upper mantle rocks, of which samples were delivered by the basalts, had some residence time in the middle crust where they experienced a metasomatic event. The fact that this event did not destroy the earlier inclusions, places severe constraints on its duration. The second late inclusions (L2) are low-pressure CO₂ inclusions with a minimum trapping depth of only 2 km, presumably a shallow magma chamber of the host basalts. The succession of fluid inclusions strongly points toward a fairly fast uprising upper mantle underneath Northern Patagonia. The petrology and mineral chemistry of the peridotitic xenoliths support this view. Extensive partial melting and loss of these melts is indicated by the preponderance of harzburgites in the upper mantle underneath Northern Patagonia, a fairly unusual feature for a continental upper mantle. That depletion event as well as several metasomatic events — including those which left traces of fluid inclusions — are possibly related to a high-speed diapiric uprise of the upper mantle in this area. The path can be traced from the garnet peridotite stability field into the middle crust, a journey which must have been unusually fast. Differences in rock, mineral, and fluid inclusion properties between geographic locations suggest a diffuse and differential type of diapirism. Future studies will hopefully help to map the full extent and the highs and lows of this diapir and elucidate questions related to its origin and future.

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Fluid-Einschlüsse in Erdmantel-Xenolithen von Nord-Patagonien: Evidenz für einen Diapir im oberen Erdmantel

Zusammenfassung Erdmantel - Xenolithe in Alkali-Basalten des Somoncure Massivs, Nord-Patagonien, Argentinien, führen drei Generationen von Fluid-Einschlüssen. Die erste (frühe, primäre) Generation besteht aus dichten CO₂-Einschlüssen, welche offenbar in der Mantel-Kruste Grenzzone (22–36 km Minimum-Tiefe) eingeschlossen wurden. CO₂-Einschlüsse sind kogenetisch mit Silikat-Schmelzeinschlüssen. Dies erlaubt die Abschätzung der Einschließ-Temperatur mit minimal 1200°C, was auf ein Hochtemperatur-Ereignis in einer deutlich kühleren Umgebung hinweist. Die späten (pseudosekundäre und sekundäre) CO₂- Fluid-Einschlüsse bilden zwei Generationen von denen die eine in die flüssige (L1), die andere in die Dampfphase (L2) homogenisieren. Die minimale Einschließ-Tiefe für die L1 Generation ist etwa 16 km. Dies bedeutet - auch bei Berücksichtigung der mit diesem Wert verbundenen Ungenauigkeit - daß diese Erdmantel-Gesteine einige Zeit in der mittleren Erdkruste verbrachten und ein metasomatisches Ereignis erlebten, bevor sie von den Basalten zur Erdoberfläche gebracht wurden. Die Tatsache, daß dieses Ereignis die frühen Einschlüsse nicht zerstörte, kann nur bedeuten, daß es von kurzer Dauer war. Die L2-Generation besteht aus Niedrigdruck CO₂-Einschlüssen mit einer Minimum-Einschließtiefe von nur 2 km. Dies könnte in einer seichten Magmakammer des Wirt Basaltes geschehen sein.Die Abfolge von Fluid-Einschlüssen deutet auf einen relativ schnell aufsteigenden oberen Erdmantel unterhalb von Patagonien hin. Die Petrologie und Mineralchemie der peridotitischen Xenolithe unterstützen das. Die Vorherrschaft von Harzburgiten im Erdmantel unterhalb von Nord-Patagonien deutet auf umfangreiche Bildung partieller Schmelzen und deren Abfuhr hin — eine für einen kontinentalen Mantel ungewöhnliche Situation. Sowohl die Verarmungsereignisse, als auch die metasomatischen Veränderungen (einschließlich jene, welche Spuren in Form von Fluid Einschlüssen hinterließen) machen das Vorhandensein eines schnell aufsteigenden Daipirs im oberen Erdmantel dieser Gegend wahrscheinlich. Der Aufstieg kann vom Stabilitätsbereich der Granat-Peridotite bis in die mittlere Kruste verfolgt werden und muß daher relativ schnell erfolgt sein. Unterschiede in Gesteins-, Mineral und Fluid-Eigenschaften zwischen verschiedenen Lokalitäten legen einen diffusen und differenziellen Diapirismus nahe. Zukünftige Studien sollten es ermöglichen, das Gesamtausmaß und die unterschiedlichen Aufstiegshöhen des Diapirs zu kartieren und Hinweise auf seine Entstehung und zukünftige Entwicklung zu erhalten.

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With 5 Figures     

Molecular dynamics simulation of the CH4 and CH4-H2O systems up to 10 GPa and 2573 K

Based on our previous study of the intermolecular potential for pure H₂O and the strict evaluation of the competitive potential models for pure CH₄ and the ab initio fitting potential surface across CH₄-H₂O molecules in this study, we carried out more than two thousand molecular dynamics simulations for the PVTx properties of pure CH₄ and the CH₄-H₂O mixtures up to 2573 K and 10 GPa. Comparison of 1941 simulations with experimental PVT data for pure CH₄ shows an average deviation of 0.96% and a maximum deviation of 2.82%. The comparison of the results of 519 simulations of the mixtures with the experimental measurements reveals that the PVTx properties of the CH₄-H₂O mixtures generally agree with the extensive experimental data with an average deviation of 0.83% and 4% in maximum, which is equivalent to the experimental uncertainty. Moreover, the maximum deviation between the experimental data and the simulation results decreases to about 2% as temperature and pressure increase, indicating that the high accuracy of the simulation is well retained in the high temperature and pressure region.After the validation of the simulation method and the intermolecular potential models, we systematically simulated the PVTx properties of this binary system from 673 K and 0.05 GPa to 2573 K and 10 GPa. In order to integrate all the simulation results and the experimental data for the calculation of thermodynamic properties, an equation of state (EOS) is developed for the CH₄-H₂O system covering 673-2573 K and 0.01-10 GPa. Isochores for compositions <4 mol% CH₄ up to 773 K and 600 MPa are also determined in this paper. The program for the EOS can be downloaded from www.geochem-model.org/programs.htm.     

峨眉山大火成岩省：地幔柱活动的证据及其熔融条件

对苦橄岩中橄榄石斑晶及其中熔体包裹体的电子探针分析表明，峨眉山大火山岩省的原始岩浆具高镁（ MgO > 16％）特征。玄武岩的 REE反演计算揭示，参与峨眉山玄武岩岩浆作用的地幔具有异常高的潜能温度（ 1 550℃）。这些特征以及峨眉山玄武岩的大面积分布和一些熔岩所显示的类似于洋岛玄武岩 (OIB)的微量元素和 Sr- Nd同位素特征均为地幔热柱在能量和物质上参与峨眉山溢流玄武岩的形成提供了确凿证据。峨眉山两个主要岩类（高钛和低钛玄武岩）可能是不同地幔源区物质在不同条件下的熔融产物。低钛玄武岩形成于温度最高、岩石圈最薄的地幔柱轴部。地幔（ ISr≈ 0.705,ε Nd(t)≈＋ 2）熔融始于 140 km，并一直延续到较浅的深度（ 60 km,尖晶石稳定区 ),部分熔融程度为 16％，这类岩石可能代表了峨眉山玄武岩的主体。而高钛玄武岩的母岩浆的形成基本局限在石榴子石稳定区（ > 70 km），其源区特征为 : ISr≈ 0.704,ε Nd(t)≈＋ 5，可能代表了热柱边部或消亡期地幔小程度部分熔融（ 1.5％）的产物。     

Prediction of CH4 and CO2 hydrate phase equilibrium and cage occupancy from ab initio intermolecular potentials

Presented is an improved model for the prediction of phase equilibria and cage occupancy of CH₄ and CO₂ hydrate in aqueous systems. Different from most hydrate models that employ Kihara potential or Lennard-Jones potential with parameters derived from experimental phase equilibrium data of hydrates, we use atomic site-site potentials to account for the angle-dependent molecular interactions with parameters directly from ab initio calculation results. Because of this treatment, our model can predict the phase equilibria of CH₄ hydrate and CO₂ hydrate in binary systems over a wide temperature-pressure range (from 243-318 K, and from 10-3000 bar for CH₄ hydrate; from 253-293 K, and from 5-2000 bar for CO₂ hydrate) with accuracy close to experiment. The average deviation of this model from experimental data is less than 3% in pressures for a given temperature. This accuracy is similar to previous models for pressures below 500 bar, but is more accurate than previous models at higher pressures. This model is also capable of predicting the cage occupancy and hydration number for CH₄ hydrate and CO₂ hydrate without fitting any experimental data. The success of this study validates the predictability of ab initio intermolecular potentials for thermodynamic properties.     

Natural CaTi2O4-structured FeCr2O4 polymorph in the Suizhou meteorite and its significance in mantle mineralogy

The first natural occurrence of a high-pressure polymorph of chromite has been discovered in the shock-metamorphosed Suizhou meteorite. The composition of this high-pressure polymorph is identical to that of the precursor chromite. The Raman spectrum of this polymorph is distinct from that of chromite. Synchrotron X-ray diffraction analysis revealed that this polymorph has an orthorhombic CaTi₂O₄- type structure. The cell parameters are: a = 9.462(6) Å, b = 9.562(9) Å, c = 2.916(1) Å, V = 263.8(4) ų (Z=4), space group = Bbmm, and the density = 5.63 g/cm³ (the numbers in parentheses are standard deviations on the last significant digits). This polymorph is 11.5% denser than chromite. The P-T conditions for the phase transformation from chromite to the CaTi₂O₄-structured polymorph are estimated at 20-23 GPa and 1800 to 2000 °C, respectively. This dense CaTi₂O₄-structured FeCr₂O₄ phase could be a host phase for Cr, Al, Fe, Mg and Mn and other metallic elements in the deep Earth.     

Stable carbon isotope composition and concentrations of CO2 and CH4 in the deep catotelm of a peat bog

Vertical profiles of concentration and C-isotopic composition of dissolved methane and carbon dioxide were observed over 26 months in the catotelm of a deep (6.5 m) peat bog in Switzerland. The dissolved concentrations of these gases increase with depth while CO₂ predominates over CH₄ (CO₂ ca. 5 times CH₄). This pattern can be reproduced by a reaction-advection-ebullition model, where CO₂ and CH₄ are formed in a ratio of 1:1. The less soluble methane is preferentially lost via outgassing (bubbles). The isotopic fractionation between CO₂ and CH₄ also increases with depth, with α_C values ranging from 1.045 to 1.075. The isotopic composition of the gases traces the passage of respiration-derived CO₂ (from the near surface) through a shallow zone with methanogenesis of low isotopic fractionation (splitting of fermentation-derived acetate). This solution then moves through the catotelm, where methanogenesis occurs by CO₂ reduction (large isotopic fractionation). In the upper part of the catotelm the C-13-depleted respiration-derived CO₂ pool buffers the isotopic composition of CO₂; the δ¹³C of CO₂ increases only slowly. At the same time strongly depleted CH₄ is formed as CO₂ reduction consumes the depleted CO₂. In the lower part of the catotelm, the respiration-derived CO₂ and shallow CH₄ become less important and CO₂ reduction is the dominant source of CO₂ and CH₄. Now, the δ¹³C values of both gases increase until equilibrium is reached with respect to the isotopic composition of the substrate. Thus, the δ¹³C values of methane reach a minimum at intermediate depth, and the deep methane has δ¹³C values comparable to shallow methane. A simple mixing model for the isotopic evolution is suggested. Only minor changes of the observed patterns of methanogenesis (in terms of concentration and isotopic composition) occur over the seasons. The most pronounced of these is a slightly higher rate of acetate splitting in spring.     

汶川M_s 8.0地震破裂带CO_2、CH_4、Rn和Hg脱气强度

地震活动断裂带能够向大气释放大量的温室气体、放射性气体和有毒气体(CO_2、CH_4、Rn和Hg),并对大气环境的影响产生复杂的影响。利用静态暗箱法,对汶川M_s8.0地震破裂带CO_2、Rn和Hg脱气强度进行实地测量,并计算了CO_2和Hg脱气对大气的年贡献量。结果表明:(1)破裂带土壤气中CO_2、CH_4、Rn和Hg异常浓度最大值分别可以达到7.98%、2.38%、524.30k Bq/m~3和161.00ng/m~3;破裂带CO_2、Rn和Hg脱气平均通量是34.95g·m~(-2)d~(-1)、36.11m Bq·m~(-2)s~(-1)和26.56ng·m~(-2)h~(-1),最大值分别达到259.23g·m~(-2)d~(-1)、580.35m Bq·m~(-2)s~(-1)和387.67ng·m~(-2)h~(-1);(2)汶川Ms8.0地震破裂带向大气脱气的CO_2年贡献量是0.95Mt,Hg的年贡献量是15.94kg。汶川Ms8.0地震破裂带破裂CO_2、CH_4、Rn和Hg等的脱气强度,不仅与破裂带渗透率有关,还与断裂带浅部存在的气藏、煤层以及磷矿层等气体源有重要的联系。