首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 46 毫秒
1.
Small, discoid globules and networks of magnesium-iron-calcium carbonates occur within impact-produced fracture zones in the ALH84001 Martian meteorite. Because these carbonates contain or are associated with the hydrocarbons, single-domain magnetite and iron-sulfide grains, and purported microfossils that collectively have been cited as evidence for ancient Martian life, it is critically important to understand their formation. Previous hypotheses for the origin of the carbonates involve either alteration of the rock by hydrothermal fluids at relatively low temperatures, or formation from a CO2-rich vapor at high temperatures. This paper explores an alternative mechanism–direct precipitation from a ponded evaporating brine infiltrating into fractures in the floor of an impact crater. Such a model can be reconciled with the observed carbonate compositional zoning and extreme stable-isotopic fractionations. If the carbonates formed in this manner, this removes a possible obstacle to the proposed existence of microbial remains in ALH84001; however, the cited evidence for life can be better explained by inorganic processes expected from brines in an evaporating alkaline lake, with an overprint of shock metamorphism and subsequent contamination by organic matter after falling to Earth.  相似文献   

2.
The carbonates in martian meteorite ALH84001 preserve a record of aqueous processes on Mars at 3.9 Ga, and have been suggested to contain signatures of ancient martian life. The conditions of the carbonate formation environment are critical for understanding possible evidence for life on Mars, the history of water on Mars, and the evolution of the martian atmosphere. Despite numerous studies of petrographic relationships, microscale oxygen isotope compositions, microscale chemical compositions, and other minerals associated with the carbonates, formation models remain relatively unconstrained. Microscale carbon isotope analyses of ALH84001 carbonates reveal variable δ13C values ranging from +27 to +64‰. The isotopic compositions are correlated with chemical composition and extent of crystallization such that the Mg-poor, early-formed carbonates are relatively 13C depleted and the Mg-rich, later forming carbonates, are 13C enriched. These data are inconsistent with many of the previously proposed environments for carbonate formation, and a new set of hypotheses are proposed. Specifically, two new models that account for the data involve low temperature (<100°C) aqueous processes: (1) the carbonates formed during mixing of two fluids derived from separate chemical and isotopic reservoirs; or (2) the carbonates formed from high pH fluids that are exposed to a CO2-rich atmosphere and precipitate carbonate, similar to high pH springs on Earth.  相似文献   

3.
The thermal histories of Martian meteorite are important for the interpretation of petrologic, geochemical, geochronological, and paleomagnetic constraints that they provide on the evolution of Mars. In this paper, we quantify 40Ar/39Ar ages and Ar diffusion kinetics of Martian meteorites Allan Hills (ALH) 84001, Nakhla, and Miller Range (MIL) 03346. We constrain the thermal history of each meteorite and discuss the resulting implications for their petrology, paleomagnetism, and geochronology. Maskelynite in ALH 84001 yields a 40Ar/39Ar isochron age of 4163 ± 35 Ma, which is indistinguishable from recent Pb-Pb (Bouvier et al., 2009a) and Lu-Hf ages (Lapen et al., 2010). The high precision of this result arises from clear resolution of a reproducible trapped 40Ar/36Ar component in maskelynite in ALH 84001 (40Ar/36Ar = 632 ± 90). The maskelynite 40Ar/39Ar age predates the Late Heavy Bombardment and likely represents the time at which the original natural remanent magnetization (NRM) component observed in ALH 84001 was acquired. Nakhla and MIL 03346 yield 40Ar/39Ar isochron ages of 1332 ± 24 and 1339 ± 8 Ma, respectively, which we interpret to date crystallization. Multi-phase, multi-domain diffusion models constrained by the observed Ar diffusion kinetics and 40Ar/39Ar age spectra suggest that localized regions within both ALH 84001 and Nakhla were intensely heated for brief durations during shock events at 1158 ± 110 and 913 ± 9 Ma, respectively. These ages may date the marginal melting of pyroxene in each rock, mobilization of carbonates and maskelynite in ALH 84001, and NRM overprints observed in ALH 84001. The inferred peak temperatures of the shock heating events (>1400 °C) are sufficient to mobilize Ar, Sr, and Pb in constituent minerals, which may explain some of the dispersion observed in 40Ar/39Ar, Rb-Sr, and U-Th-Pb data toward ages younger than ∼4.1 Ga. The data also place conservative upper bounds on the long-duration residence temperatures of the ALH 84001 and Nakhla protolith to be  °C and  °C over the last ∼4.16 Ga and ∼1.35 Ga, respectively. MIL 03346 has apparently not experienced significant shock-heating since it crystallized, consistent with the fact that various chronometers yield concordant ages.  相似文献   

4.
The Martian meteorite ALH84001 preserves evidence of interaction with aqueous fluids while on Mars in the form of microscopic carbonate disks. These carbonate disks are believed to have precipitated 3.9 Ga ago at beginning of the Noachian epoch on Mars during which both the oldest extant Martian surfaces were formed, and perhaps the earliest global oceans. Intimately associated within and throughout these carbonate disks are nanocrystal magnetites (Fe3O4) with unusual chemical and physical properties, whose origins have become the source of considerable debate. One group of hypotheses argues that these magnetites are the product of partial thermal decomposition of the host carbonate. Alternatively, the origins of magnetite and carbonate may be unrelated; that is, from the perspective of the carbonate the magnetite is allochthonous. For example, the magnetites might have already been present in the aqueous fluids from which the carbonates were believed to have been deposited. We have sought to resolve between these hypotheses through the detailed characterization of the compositional and structural relationships of the carbonate disks and associated magnetites with the orthopyroxene matrix in which they are embedded. Extensive use of focused ion beam milling techniques has been utilized for sample preparation. We then compared our observations with those from experimental thermal decomposition studies of sideritic carbonates under a range of plausible geological heating scenarios. We conclude that the vast majority of the nanocrystal magnetites present in the carbonate disks could not have formed by any of the currently proposed thermal decomposition scenarios. Instead, we find there is considerable evidence in support of an alternative allochthonous origin for the magnetite unrelated to any shock or thermal processing of the carbonates.  相似文献   

5.
The magnetites and sulfides located in the rims of carbonate globules in the Martian meteorite ALH84001 have been claimed as evidence of past life on Mars. Here, we consider the possibility that the rims were formed by dissolution and reprecipitation of the primary carbonate by the action of water. To estimate the rate of these solution-precipitation reactions, a kinetic model of magnesite-siderite carbonate dissolution was applied and used to examine the physicochemical conditions under which these rims might have formed. The results indicate that the formation of the rims could have taken place in < 50 yr of exposure to small amounts of aqueous fluids at ambient temperatures. Plausible conditions pertaining to reactions under a hypothetical ancient Martian atmosphere (1 bar CO2), the modern Martian atmosphere (8 mbar CO2), and the present terrestrial atmosphere (0.35 mbar CO2) were explored to constrain the site of the process. The results indicated that such reactions likely occurred under the latter two conditions. The possibility of Antarctic weathering must be entertained, which, if correct, would imply that the plausibly biogenic minerals (single-domain magnetite of characteristic morphology and sulfide) reported from the rims may be the products of terrestrial microbial activity. This model is discussed in terms of the available isotope data and found to be compatible with the formation of ALH84001 rims. Particularly, anticorrelated variations of radiocarbon with δ13C indicate that carbonate in ALH84001 was affected by solution-precipitation reactions immediately after its initial fall (∼13,000 yr ago) and then again during its recent exposure prior to collection.  相似文献   

6.
Carbon isotopic composition was measured for products of the Fischer-Tropsch synthesis: catalytic reaction between CO and H2 to produce CO, CO2, light hydrocarbons C1-C4 and “oil” fraction. Hydrogen isotopes were also measured in the oil fraction and the produced water. Experimental runs were conducted in the flow-through reactor at 260-310 °C and 30 bar using the synthesis gas composed of 5N2 + 3H2 + 2CO, on Fe-catalyst mixed with ZSM-5 synthetic zeolite. In the two of seven runs a Fe + Co-catalyst was used that gives a lower yield of unsaturated hydrocarbons in reaction products. The isotopic effects depended on the conversion of the carbon monoxide. Under steady-state conditions (CO conversion more than 90%) a strong kinetic fractionation was observed between CO and CO2 (∼−10‰) and CO and hydrocarbons (∼+38‰). At low conversion a clear “inverse” isotopic trend of the depletion in 13C of longer hydrocarbon chains was observed. On average, Δ12 = δ13C(CH4) − δ13C(C2H6) correlates well with the CO conversion: the C2H6 is ∼6‰ isotopically lighter than CH4 at low conversion and ∼2‰ heavier at steady-state regime. Under steady-state conditions there almost no difference was observed in the isotopic composition of methane and ethane and higher hydrocarbons. The chemical composition of light hydrocarbons in the products of flow-through, dynamic FTS is different from that found in the static FTS-type experiments with Fe-catalyst, but isotopic effects are similar. Our results suggest that the isotopic distribution of carbon found in so-called “abiogenic” hydrocarbons from some natural gases (δ13C1 > δ13C2 > δ13C3  >?) is somewhat similar to that at low conversion of CO, but do not resemble the distribution characteristic for the high conversion products, at least, on Fe-catalyst. Other processes (a simple mixing of two or more endmembers) or other P-T conditions of the carbon reduction could be responsible for the “inverse” isotopic trend found in meteorites and some natural gases.  相似文献   

7.
Nanometer-sized magnetite crystals associated with carbonates in fracture zones within Martian meteorite ALH84001 have been examined using analytical transmission electron microscopy. Some of the the crystals exhibit distinctive morphologies: filamentary rods and ribbons, and platelets. The rods and ribbons are elongated along the crystallographic [100] and [111] directions. Some of the rods contain microstructural defects indicating that they grew by spiral growth about screw dislocations. Platelets are flattened along the [100] and [110] directions. These unique morphologies and microstructures constrain the growth conditions of magnetite. The whiskers and platelets most likely formed in the temperature range 500–800°C by direct condensation from a vapor or precipitation from a supercritical fluid, and their properties are inconsistent with a biogenic origin.  相似文献   

8.
Chemical weathering in the Three Rivers region of Eastern Tibet   总被引：2，自引：0，他引：2  
Three large rivers - the Chang Jiang (Yangtze), Mekong (Lancang Jiang) and Salween (Nu Jiang) - originate in eastern Tibet and run in close parallel over 300 km near the eastern Himalayan syntaxis. Seventy-four river water samples were collected mostly during the summer season from 1999 to 2004. Their major element compositions vary widely, with total dissolved solids (TDS) ranging from 31 to 3037 mg/l, reflecting the complex geologic makeup of the vast drainage basins. The major ion distribution of the main channel samples primarily reflects the weathering of carbonates. Evaporite dissolution prevails in the headwater samples of the Chang Jiang in the Tibetan Plateau interior, as evidenced by the high TDS (928 and 3037 mg/l) and the Na-Cl dominant major element composition. Local tributary samples of the Mekong and Salween, draining the Lincang Batholith and the Tengchong Volcano, show distinctive silicate weathering signatures. We used five reservoirs - rain, halite, sulfate, carbonate, and silicate - in a forward model to calculate the contribution from silicate weathering to the total dissolved load and to estimate the consumption rate of atmospheric CO2 by silicate weathering. Carbonate weathering accounts for about 50% of the total cationic charge (TZ+) in the samples of the Mekong and the Salween exiting the Tibetan Plateau. In the “exit” sample of the Chang Jiang, 45% of TZ+ is from halite dissolution inherited from the extreme headwater tributaries in the interior of the plateau, and carbonates contribute only 26% to the TZ+. The net rate of CO2 consumption by silicate weathering is (103-121) × 103 mol km−2 year−1, lower than the rivers draining the Himalayan front. GIS-based analyses indicate that runoff and relief can explain 52% of the spread in the rate of atmospheric CO2 drawdown by silicate weathering, but other climatic (temperature, precipitation, potential evapotranspiration) and geomorphic (elevation, slope) factors also show collinearity. Only qualitative conclusions can be drawn for the significance of lithology due to lack of digitized lithologic information. The effect of the peculiar drainage pattern due to tectonic forcing is not readily apparent in the major element composition or in increased chemical weathering rates. The 87Sr/86Sr ratios and the silicate weathering rates are in general lower in the Three Rivers than in the rivers draining the Himalayan front.  相似文献   

9.
CM chondrites contain carbonates and other secondary minerals such as phyllosilicates, sulfides, sulfates, oxides and hydroxides that are believed to have formed by aqueous alteration reactions on their parent asteroid. We report in situ Mn-Cr isotope measurements in the highly aqueously altered CM2.1 chondrites QUE 93005 and ALH 83100 using secondary ion mass spectrometry (Cameca ims-1270 ion microprobe). The 53Cr excesses are correlated with the 53Mn/55Mn ratio and result from the in situ decay of 53Mn, a short-lived radioisotope with a half-life of 3.7 Ma. If we assume that carbonate grains in samples QUE 93005 and ALH 83100 are cogenetic, then the excesses define initial 53Mn/55Mn ratios ((53Mn/55Mn)0) of (4.1 ± 1.2) × 10−6 and (5.1 ± 1.7) × 10−6, respectively. These values are comparable to those in carbonates from other CM chondrites as reported in the literature. Initial 53Mn/55Mn ratios for calculated model isochrones for individual carbonate grains range from (3.8 ± 1.4) × 10−6 to (4.8 ± 2.1) × 10−6 for QUE 93005 and from (3.1 ± 1.6) × 10−6 to (1.3 ± 0.5) × 10−5 for ALH 83100. A possible interpretation for the ranges in (53Mn/55Mn)0 could be that alteration in individual CM chondrites was episodic and occurred over an extended period of time. However, isochrones based on the entire set of carbonate grains in each of the CM chondrites imply that the degree of aqueous alteration is roughly correlated with the age of carbonate formation in CM chondrites of different subtypes and that alteration on the CM parent asteroid started contemporaneously with or shortly after CAI formation and lasted at least 4 Ma.  相似文献   

10.
We report the mineral compositions and micro-texture of the isotopically anomalous (δ17,18OSMOW ∼ +180‰) Fe-S-Ni-O material recently discovered in matrix of the primitive carbonaceous chondrite Acfer 094 [Sakamoto N., Seto Y., Itoh S., Kuramoto K., Fujino K., Nagashima K., Krot A. N. and Yurimoto H. (2007) Oxygen isotope evidence for remnants of the early solar system primordial water. Science317, 231-233]. Synchrotron radiation X-ray diffraction and transmission electron microscopy studies indicate that this material consists of the symplectitically intergrown magnetite (Fe3O4) and pentlandite (Fe5.7Ni3.3S8) with magnetite/pentlandite volume ratio of ∼2.3. Magnetite forms column-shaped grains (10-30 nm in diameter and 100-200 nm in length); pentlandite occurs as worm-shaped grains or aggregates of grains 100-300 nm in size between magnetite crystals. Although both the X-ray diffraction and electron energy loss spectra support identification of iron oxide as magnetite, the electron diffraction patterns show that magnetite has a weak 3-fold superstructure, possibly due to ordering of vacancies. We infer that the isotopically anomalous symplectite formed by sulfurization and oxidization of metal grains either in the solar nebula or on an icy planetesimal. The intersite cation distribution of pentlandite suggests that timescale of oxidation was no longer than 1000 years.  相似文献   

11.
The Martian meteorite ALH84001 contains ∼1% by weight of carbonate formed by secondary processes on the Martian surface or in the shallow subsurface. The major form of this carbonate is chemically and isotopically zoned rosettes which have been well documented elsewhere. This study concentrates upon carbonate regions ∼200 μm across which possess previously unobserved magnesium rich inner cores, interpreted here as rosette fragments, surrounded by a later stage cement containing rare Ca-rich carbonates (up to Ca81Mg07Fe04Mn07) intimately associated with feldspar. High spatial resolution ion probe analyses of Ca-rich carbonate surrounding rosette fragments have δ18OV-SMOW values as low as −10‰. These values are not compatible with deposition from a global Martian atmosphere invoked to explain ALH84001 rosettes. The range of δ18O values are also incompatible with a fluid that has equilibrated with the Martian crust at high temperature or from remobilisation of carbonate of rosette isotopic composition. At Martian atmospheric temperatures, the small CO2(gas)-CO2(ice) fractionation makes meteoric CO2 an unlikely source for −10‰ carbonates. In contrast, closed system Rayleigh fractionation of H2O can generate δ18OH2O −30‰, as observed at high latitudes on Earth. We suggest that atmospheric transport and precipitation of H2O in a similar fashion to that on Earth provides a source of suitably 18O depleted water for generation of carbonate with δ18OV-SMOW = −10‰.  相似文献   

12.
The aim of this experimental study was to evaluate and compare the geochemical impact of pure and impure CO2 on rock forming minerals of possible CO2 storage reservoirs. This geochemical approach takes into account the incomplete purification of industrial captured CO2 and the related effects during injection, and provides relevant data for long-term storage simulations of this specific greenhouse gas. Batch experiments were conducted to investigate the interactions of supercritical CO2, brine and rock-forming mineral concentrates (albite, microcline, kaolinite, biotite, muscovite, calcite, dolomite and anhydrite) using a newly developed experimental setup. After up to 42 day (1000 h) experiments using pure and impure supercritical CO2 the dissolution and solution characteristics were examined by XRD, XRF, SEM and EDS for the solid, and ICP–MS and IC for the fluid reactants, respectively. Experiments with mixtures of supercritical CO2 (99.5 vol.%) and SO2 or NO2 impurities (0.5 vol.%) suggest the formation of H2SO4 and HNO3, reflected in pH values between 1 and 4 for experiments with silicates and anhydrite and between 5 and 6 for experiments with carbonates. These acids should be responsible for the general larger amount of cations dissolved from the mineral phases compared to experiments using pure CO2. For pure CO2 a pH of around 4 was obtained using silicates and anhydrite, and 7–8 for carbonates. Dissolution of carbonates was observed after both pure and impure CO2 experiments. Anhydrite was corroded by approximately 50 wt.% and gypsum precipitated during experiments with supercritical CO2 + NO2. Silicates do not exhibit visible alterations during all experiments but released an increasing amount of cations in the reaction fluid during experiments with impure CO2. Nonetheless, precipitated secondary carbonates could not be identified.  相似文献   

13.
The speciation of carbon in subseafloor hydrothermal systems has direct implications for the maintenance of life in present-day vent ecosystems and possibly the origin of life on early Earth. Carbon monoxide is of particular interest because it represents a key reactant during the abiotic synthesis of reduced carbon compounds via Fischer-Tropsch-type processes. Laboratory experiments were conducted to constrain reactions that regulate the speciation of aqueous single carbon species under hydrothermal conditions and determine kinetic parameters for the oxidation of CO according to the water water-gas shift reaction (CO2 + H2 = CO + H2O). Aqueous fluids containing added CO2, CO, HCOOH, NaHCO3, NaHCOO, and H2 were heated at 150, 200, and 300 °C and 350 bar in flexible-cell hydrothermal apparatus, and the abundances of carbon compounds was monitored as a function of time. Variations in fluid chemistry suggest that the reduction of CO2 to CH3OH under aqueous conditions occurs via a stepwise process that involves the formation of HCOOH, CO, and possibly CH2O, as reaction intermediaries. Kinetic barriers that inhibit the reduction of CH3OH to CH4 allow the accumulation of reaction intermediaries in solution at high concentrations regulated by metastable thermodynamic equilibrium. Reaction of CO2 to CO involves a two-step process in which CO2 initially undergoes a reduction step to HCOOH which subsequently dehydrates to form CO. Both reactions proceed readily in either direction. A preexponential factor of 1.35 × 106 s−1 and an activation energy of 102 kJ/mol were retrieved from the experimental results for the oxidation of CO to CO2. Reaction rates amongst single carbon compounds during the experiments suggest that ΣCO2 (CO2 + HCO3− + CO32−), CO, ΣHCOOH (HCOOH + HCOO−), and CH3OH may reach states of redox-dependent metastable thermodynamic equilibrium in subseafloor and other hydrothermal systems. The abundance of CO under equilibrium conditions is strongly dependent on temperature, the total carbon content of the fluid, and host-rock lithology. If crustal residence times following the mixing of high-temperature hydrothermal fluids with cool seawater are sufficiently long, reequilibration of aqueous carbon can result in the generation of additional reduced carbon species such as HCOOH and CH3OH, and the consumption of H2. The present study suggests that abiotic reactions involving aqueous carbon compounds in hydrothermal systems are sufficiently rapid to influence metabolic pathways utilized by organisms that inhabit vent environments.  相似文献   

14.
H isotope measurements of carbonate, phosphate, feldspathic and mafic glasses, and post-stishovite silica phase in the shergottites Zagami, Shergotty, SaU 005, DaG 476, ALHA 77005 and EETA 79001, as well as in Chassigny and ALH 84001, show that all these phases contain deuterium-enriched water of extraterrestrial origin. The minerals and glasses analyzed may contain an initial primary hydrogen component, but their isotopic composition was modified to varying degrees by three different processes: interaction with a fractionated exchangeable water reservoir on Mars, hydrogen devolatilization by impact melting, and terrestrial contamination. Positive correlations between δD and water abundance in feldspathic glass and post-stishovite silica in Zagami, Shergotty, and SaU 005 is indicative of mixing of a high δD component (3000-4000‰) and a less abundant, low δD component (∼0‰). The high δD component is primarily derived from the Martian exchangable reservoir, but may also have been influenced by isotopic fractionation associated with shock-induced hydrogen loss. The low δD component is either a terrestrial contaminant or a primary “magmatic” component. The negative correlation between δD and water abundances in mafic and feldspathic glasses in ALH 84001, ALHA 77005, and EETA 79001 is consistent with the addition of a low δD terrestrial contaminant to a less abundant high-deuterium Martian component. The low δD of magmatic glass in melt inclusions suggests that the δD of Martian parent magma was low and that the initial H isotope signature of Mars may be similar to that of Earth.  相似文献   

15.
In southwestern China, several large magmatic Fe–Ti–V oxide ore deposits are hosted by gabbroic intrusions associated with the Emeishan flood basalts. The Panzhihua gabbroic intrusion, a little deformed sill that contains a large titanomagnetite deposit at its base, concordantly intrudes late-Proterozoic dolostones. Mineralogical and chemical studies of the contact aureole in the footwall dolostones demonstrate that the metamorphism was largely isochemical but released large quantities of CO2 as the rocks were converted to marble and skarns during intrusion of the gabbroic magma. Petrological modelling of the crystallization of the intrusion, using H2O-poor Emeishan basalt as parent magma, shows that under normal conditions, Fe–Ti oxides crystallize at a late stage, after the crystallization of abundant olivine, clinopyroxene and plagioclase. In order for titanomagnetite to separate efficiently to form the ore deposit, this mineral must have crystallized earlier and close to the liquidus. We propose that CO2-rich fluids released during decarbonatization of sedimentary floor rocks passed up through the magma. Redox equilibria calculations show that when magma with the composition of Emeishan basalt is fluxed by a CO2-rich gas phase, its equilibrium oxygen fugacity (fO2) increases from the fayalite–magnetite–quartz buffer (FMQ) to FMQ + 1.5. From experimental constraints on magnetite saturation in basaltic magma under controlled fO2, such an oxidizing event would allow magnetite to crystallize near to the liquidus, leading to the formation of the deposit.  相似文献   

16.
Methane and CO2 emissions from the two most active mud volcanoes in central Japan, Murono and Kamou (Tokamachi City, Niigata Basin), were measured in from both craters or vents (macro-seepage) and invisible exhalation from the soil (mini- and microseepage). Molecular and isotopic compositions of the released gases were also determined. Gas is thermogenic (δ13CCH4 from −32.9‰ to −36.2‰), likely associated with oil, and enrichments of 13C in CO2 (δ13CCO2 up to +28.3‰) and propane (δ13CC3H8 up to −8.6‰) suggest subsurface petroleum biodegradation. Gas source and post-genetic alteration processes did not change from 2004 to 2010. Methane flux ranged within the orders of magnitude of 101-104 g m−2 d−1 in macro-seeps, and up to 446 g m−2 d−1 from diffuse seepage. Positive CH4 fluxes from dry soil were widespread throughout the investigated areas. Total CH4 emission from Murono and Kamou were estimated to be at least 20 and 3.7 ton a−1, respectively, of which more than half was from invisible seepage surrounding the mud volcano vents. At the macro-seeps, CO2 fluxes were directly proportional to CH4 fluxes, and the volumetric ratios between CH4 flux and CO2 flux were similar to the compositional CH4/CO2 volume ratio. Macro-seep flux data, in addition to those of other 13 mud volcanoes, supported the hypothesis that molecular fractionation (increase of the “Bernard ratio” C1/(C2 + C3)) is inversely proportional to gas migration fluxes. The CH4 “emission factor” (total measured output divided by investigated seepage area) was similar to that derived in other mud volcanoes of the same size and activity. The updated global “emission-factor” data-set, now including 27 mud volcanoes from different countries, suggests that previous estimates of global CH4 emission from mud volcanoes may be significantly underestimated.  相似文献   

17.
A simple synthesis of various forms of calcium carbonate with spherical and 'floral' morphologies is reported. Vaterite formation occurs at approximately 25 degrees C, aragonite at approximately 70 degrees C and calcite at about approximately 80 degrees C. These are produced when CO2 is reacted with an aqueous solution of calcium chloride in the presence of ammonia. These conditions may have existed at the surface of Mars in the past, leading us to conclude that such mineral formations may be common there. Although the initial phases are modified over time with changing temperature and pressure conditions, they still influence the final morphology of the carbonates observed. A comparison of these structures with those found in the Martian meteorite ALH84001 suggests, but does not confirm, a non-biogenic origin for the ALH84001 carbonates.  相似文献   

18.
Combining analytical data from hot spring samples with thermodynamic calculations permits a quantitative assessment of the availability and ranking of various potential sources of inorganic chemical energy that may support microbial life in hydrothermal ecosystems. Yellowstone hot springs of diverse geochemical composition, ranging in pH from <2 to >9 were chosen for this study, and dozens of samples were collected during three field seasons. Field measurements of dissolved oxygen, nitrate, nitrite, total ammonia, total sulfide, alkalinity, and ferrous iron were combined with laboratory analyses of sulfate and other major ions from water samples, and carbon dioxide, hydrogen, methane, and carbon monoxide in gas samples to evaluate activity products for ∼300 coupled oxidation-reduction reactions. Comparison of activity products and independently calculated equilibrium constants leads to values of the chemical affinity for each of the reactions, which quantifies how far each reaction is from equilibrium. Affinities, in turn, show systematic behavior that is independent of temperature but can be correlated with pH of the hot springs as a proxy for the full spectrum of geochemical variability. Many affinities are slightly to somewhat dependent on pH, while others are dramatically influenced by changes in chemical composition. All reactions involving dissolved oxygen as the electron acceptor are potential energy sources in all hot spring samples collected, but the ranking of dominant electron donors changes from ferrous iron, and sulfur at high pH to carbon monoxide, hydrogen, and magnetite as pH decreases. There is a general trend of decreasing energy yields depending on electron acceptors that follows the sequence: O2(aq) > NO3− ≈ NO2− ≈ S > pyrite ≈ SO4−2 ≈ CO(g) ≈ CO2(g) at high pH, and O2(aq) ≈ magnetite > hematite ≈ goethite > NO3− ≈ NO2− ≈ S ≈ pyrite ≈ SO4−2 at low pH. Many reactions that are favorable sources of chemical energy at one set of geochemical conditions fail to provide energy at other conditions, and vice versa. This results in energy profiles supplied by geochemical processes that provide fundamentally different foundations for chemotrophic microbial communities as composition changes.  相似文献   

19.
Mud volcanoes are important pathways for CH4 emission from deep buried sediments; however, the importance of gas fluxes have hitherto been neglected in atmospheric source budget considerations. In this study, gas fluxes have been monitored to examine the stability of their chemical compositions and fluxes spatially, and stable C isotopic ratios of CH4 were determined, for several mud volcanoes on land in Taiwan. The major gas components are CH4 (>90%), “air” (i.e. N2 + O2 + Ar, 1–5%) and CO2 (1–5%) and these associated gas fluxes varied slightly at different mud volcanoes in southwestern Taiwan. The Hsiao-kun-shui (HKS) mud volcano emits the highest CH4 concentration (CH4 > 97%). On the other hand, the Chung-lun mud volcano (CL) shows CO2 up to 85%, and much lower CH4 content (<37%). High CH4 content (>90%) with low CO2 (<0.2%) are detected in the mud volcano gases collected in eastern Taiwan. It is suggestive that these gases are mostly of thermogenic origin based on C1 (methane)/C2 (ethane) + C3 (propane) and δ13CCH4 results, with the exception of mud volcanoes situated along the Gu-ting-keng (GTK) anticline axis showing unique biogenic characteristics. Only small CH4 concentration variations, <2%, were detected in four on-site short term field-monitoring experiments, at Yue-shi-jie A, B, Kun-shui-ping and Lo-shan A. Preliminary estimation of CH4 emission fluxes for mud volcanoes on land in Taiwan fall in a range between 980 and 2010 tons annually. If soil diffusion were taken into account, the total amount of mud volcano CH4 could contribute up to 10% of total natural CH4 emissions in Taiwan.  相似文献   

20.
The R chondrite meteorite LaPaz Icefield (LAP) 04840 is unique among metamorphosed, non-carbonaceous chondrites in containing abundant OH-bearing silicate minerals: ∼13% ferri-magnesiohornblende and ∼0.4% phlogopite by volume. Other minerals include olivine (Fo62), orthopyroxene (En69Fs30Wo1), albite (An8Ab90Or2), magnetite, pyrrhotite, pentlandite, and apatite. Ferromagnesian minerals are rich in Fe3+, as determined by Mössbauer spectrometry and electron microprobe chemical analyses. Fe3+/Fetot values are olivine ?5%, amphibole 80%, phlogopite 65%, and magnetite 42%. Mineral compositions are nearly constant across grains and the section, except for a small variability in amphibole compositions reflecting the edenite exchange couple (ANa + IVAl ↔ A□ + Si). These mineral compositions, the absence of Fe-Ni metal, and the oxygen isotope data support its classification as an R (Rumuruti) chondrite. LAP 04840 is classified as petrologic grade 5, based on the chemical homogeneity of its minerals, and the presence of distinctly marked chondrules and chondrule fragments in a fine-grained crystalline matrix. The mineral assemblage of LAP 04840 allows calculation of physical and chemical conditions at the peak of its metamorphism: T = 670 ± 60 °C from a amphibole-plagioclase thermometer; PH2O between 250 and 500 bars as constrained by the assemblage phlogopite + orthopyroxene + olivine + feldspar and the absence of diopside; PCO2 unconstrained; fO2 at QFM + 0.5 log units; . The hydrogen in LAP 04840 is very heavy, an average δD value of +3660 ± 75‰ in the magnesiohornblende. Only a few known sources of hydrogen have such high δD and are suitable sources for LAP 04840: ordinary chondrite phyllosilicates (as in the Semarkona chondrite), and insoluble organic matter (IOM) in ordinary chondrites and CR chondrites. Hydrogen from the IOM could have been released by oxidation, and then reacted with an anhydrous R chondrite (at high temperature), but it is not clear whether this scenario is correct.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号