The hydrogeochemistry of methane: Evidence from English groundwaters   总被引：2，自引：0，他引：2  

W.G. Darling  D.C. Gooddy 《Chemical Geology》2006,229(4):293-312

The presence of methane (CH₄) in groundwater is usually only noticed when it rises to high concentrations; to date rather little is known about its production or natural ‘baseline’ conditions. Evidence from a range of non-polluted groundwater environments in England, including water supply aquifers, aquicludes and thermal waters, reveals that CH₄ is almost always detectable, even in aerobic conditions. Measurements of potable waters from Cretaceous, Jurassic and Triassic carbonate and sandstone aquifers reveal CH₄ concentrations of up to 500 μg/l, but a mean value of < 10 μg/l. However, aquiclude and thermal waters from the Carboniferous and Triassic typically contain in excess of 1500 μg/l. Such high concentrations have so far only been found at redox (Eh) potentials below 0 mV, but in general CH₄ concentration and Eh value are poorly correlated. This suggests a lack of thermodynamic equilibrium, which is confirmed by comparing pe values calculated from the redox couple C(4)/C(− 4) with those derived from Eh. Genesis of CH₄ appears to occur on two timescales: a rapid if low rate of production from labile carbon in anaerobic microsites in the soil, and a much longer, millennium scale of production from more refractory carbon. Methane is rarely measured in groundwater; there is no single ionic determinand which acts universally as a proxy, but a combination of high HCO₃ and low SO₄ concentrations, or the reverse, is an indication that high amounts of CH₄ may be present.  相似文献   

Na depletion in modern adakites via melt / rock reaction within the sub-arc mantle     

X.L. Xiong  B. Xia  J.F. Xu  H.C. Niu  W.S. Xiao 《Chemical Geology》2006,229(4):273-292

Interaction between slab-derived melt and mantle peridotite and the role of slab melt as a metasomatizing agent in the sub-arc mantle is being increasingly recognized. Adakite, the slab melt erupted on the surface, usually exhibits anomalously high MgO, CaO, Cr and Ni contents that indicate interaction with mantle peridotitite. Here we note that Cenozoic adakites have Na₂O contents below 5.8 wt.% with ∼95% samples lower than 5.0 wt.%, and are generally depleted in this component relative to experimental basalt partial melts (mostly beyond 5.0 wt.% and up to 9.0 wt.% Na₂O) produced under 1.5-3.0 GPa conditions that are most relevant to adakite production. We interpret the adakite Na depletion to be also a consequence of the melt / rock reaction that takes place within the hot mantle wedge. During ascent and reaction with mantle peridotite, primary adakite melts gain mantle components MgO, CaO, Cr and Ni but lose Na₂O, SiO₂ and perhaps K₂O to the mantle, leading to Na-rich mantle metasomatism. Selective assimilation of predominately mantle clinopyroxene, some spinel and minor olivine at high T/P has been considered to be an important process in producing high-Mg adakites from primary low-Mg slab melts [Killian, R., Stern, C. R., 2002. Constraints on the interaction between slab melts and the mantle wedge from adakitic glass in peridotite xenoliths. Eur. J. Mineral. 14, 25-36]. In such a process, Na depletion in the assimilated melt is the result of dilution due to the increase in melt mass. Phase relationships in the reaction system siliceous melt + peridotite and quantitative calculation suggest that assimilation of mantle clinopyroxene, olivine and spinel and fractional crystallization of sodic amphibole and orthopyroxene, under conditions of moderate T/P and increasing melt mass, is also an important process that modifies the composition of adakites and causes the Na depletion.  相似文献   

Comparative planetary mineralogy: Implications of martian and terrestrial jarosite. A crystal chemical perspective     

J.J. Papike  C.K. Shearer 《Geochimica et cosmochimica acta》2006,70(5):1309-1321

The importance of the discovery of jarosite at the Meridiani Planum region of Mars is discussed. Terrestrial studies demonstrate that jarosite requires a unique environment for its formation, crystallizing from highly acidic (pH < 4) S-rich brines under highly oxidizing conditions. A likely scenario for jarosite formation on Mars is that degassing of shallow magmas likely released SO₂ that reacted with aqueous solutions in shallow aquifers or on the martian surface. This interaction forms both H₂SO₄ and H₂S. A martian oxidant must be identified to both oxidize H₂S to produce the required acidity of the fluid, and to oxidize Fe²⁺ to Fe³⁺. We suggest that reactions involving both sulfur and the reduction of CO₂ to CO may provide part of the answer. The jarosite crystal structure is truly remarkable in terms of its tolerance for the substitution of a large number of different cations with different ionic radii and charges. The structure accommodates hydrogen, oxygen, and sulfur, the stable isotope systematics of which are strong recorders of low-temperature fluid-rock-atmosphere interactions. Jarosite has been proven to be a robust chronometer for Ar-Ar and K-Ar dating techniques, and there is every reason to believe that U-Pb, Rb-Sr, and Nd-Sm techniques for older jarosite from Mars will also be robust. Although the discovery of jarosite on Mars alone, with no other analytical measurements on the phase, has given us insights to martian surficial processes, the true power of jarosite can not be exploited until jarosite is sampled and returned from Mars. Mars sample return is a long way off but, until then, we should be vigilant about examining martian meteorites for alteration assemblages that contain jarosite. A suite of jarosite samples representing a significant time span on Mars may hold the key to reading the record of martian atmospheric evolution.  相似文献   

Carbon isotope fractionation in wood during carbonization   总被引：1，自引：0，他引：1  

C.S.M. Turney  D. Wheeler  Allan R. Chivas 《Geochimica et cosmochimica acta》2006,70(4):960-964

A significant uncertainty exists as to whether δ¹³C values in charcoal meaningfully represent the stable isotopic content of the original material, with studies suggesting variable responses to both natural and laboratory heating. An extensive study was undertaken using fully homogenised samples of wood taken from Eucalyptus spp., Quercus robur and Pinus radiata. The results demonstrate that the duration of heating had no tangible effect on the final composition of the charred material, with the δ¹³C and carbon content of wood fixed after 30 min of heating. Furthermore, all three wood types become progressively depleted in ¹³C with increasing temperature. The results demonstrate that even at temperatures commonly reached in natural fires (<450 °C) isotopic fractionation of up to 1.3‰ can take place indicating that the absolute values obtained from charcoal extracted for paleoenvironmental reconstruction must be interpreted with caution.  相似文献   

Incorporating solid solutions in reactive transport equations using a kinetic discrete-composition approach     

Peter C. Lichtner  J. William Carey 《Geochimica et cosmochimica acta》2006,70(6):1356-1378

A new approach is proposed for incorporating solid solution reactions into mass conservation equations describing reaction paths in both closed and open systems. The method is applicable to problems involving advective, dispersive, and diffusive transport in a porous medium. By representing the continuously variable solid solution composition with a discrete set of stoichiometric solids that span composition space, combined with a kinetic formulation of their rates of reaction, a self-determining spatial and temporal evolution of the solid solution concentration and composition is obtained. It is demonstrated that equilibrium of an aqueous solution with a stoichiometric solid derived from a solid solution corresponds to equilibrium of the solid solution itself if and only if equilibrium of the stoichiometric solid is stable. One advantage of this approach is that it is unnecessary to introduce any additional compositional variables to represent the solid solution. Discretization may be over the entire range of composition space, or over some subset depending on the system. A major consequence of the kinetic discrete-composition solid solution representation is that modeling solid solutions is similar to modeling pure mineral phases with the exception of a weighting factor applied to reaction rates of stoichiometric solids corresponding to a common solid solution. With this approach, precipitation leads to a discrete zonation of the solid solution that approximates the continuous variation in composition expected for the actual solid solution. The approach is demonstrated for a hypothetical ideal and non-ideal binary solid solution A_xB_1−xC for a reaction path formulation and reactive transport involving advection and diffusion.  相似文献   

Origin of SiO₂-rich components in ordinary chondrites     

Dominik C. Hezel  Herbert Palme  Frank E. Brenker 《Geochimica et cosmochimica acta》2006,70(6):1548-1564

Silica-rich objects are common minor components in ordinary chondrites (OC), occurring as fragments and as chondrules. Their typical paragenesis is orthopyroxene + SiO₂ (with bulk SiO₂ >65 wt%) and occasionally with additional olivine and/or spinel. Individual silica-rich components (SRC) have previously been studied in various types of OCs, although there is only one comprehensive study of these objects by Brigham et al. [Brigham, C.A., Murrell, M.T., Yabuki, H., Ouyang, Z., El Goresy, A., 1986. Silica-bearing chondrules and clasts in ordinary chondrites. Geochim. Cosmochim. Acta 50, 1655-1666]. Several different explanations of how SRCs formed have been published. The main question is how silica-enrichment was achieved, because CI-chondritic atomic Mg/Si-ratio is 1.07 and as a consequence only olivine and pyroxene, but no free silica should be stable. There are two basic possibilities for the SiO₂-enrichment: (1) a RedOx-mechanism or magmatic fractionation on the parent body and (2) fractional condensation or recycling of chondrule mesostasis in the solar nebula. To better constrain the origin of these objects, we measured major and rare earth elements in SRCs of various types of ordinary chondrites, and in addition, we studied silica polymorphism in these objects using an in situ micro-Raman technique. Bulk chondrule compositions define mixing lines between the compositions of olivine and pyroxene. The SRCs extend these lines to an SiO₂ end member. In contrast, magmatic trends grossly deviate from these mixing lines. Concentrations of CaO, Al₂O₃, and REE in the pyroxenes of the SRCs are low (0.01 to 1× CI) and the CI-normalized REE-patterns are virtually flat, typical of bulk chondrules, but untypical of magmatic trends. We therefore conclude that SiO₂-rich objects are not of magmatic origin. They are the result of fractional condensation in the solar nebula. The silica in SRCs occurs mainly as tridymite and sometimes as cristobalite or—in very rare cases—as quartz. Some SiO₂-phases yielded a yet unknown micro-Raman spectrum, which we were unable to identify. The often chondrule-like shape of SRCs as well as the presence of high-temperature SiO₂-polymorphs lead to the following model for the origin of SRCs: formation of SiO₂-rich precursors in the solar nebula by fractional condensation, reheating to temperatures between 1140 and >1968 K, thereby forming the SRCs,—probably during the chondrule-forming process—followed by rapid cooling.  相似文献   

Upper Cretaceous palaeogeography of the British Isles and adjacent areas     

John C.W. Cope 《Proceedings of the Geologists' Association. Geologists' Association》2006,117(2):129-143

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Assessment of the mycoflora of commercial poultry feeds sold in the humid tropical environment of Imo State, Nigeria     

I. C. Okoli DVM  M. Sc.  Ph.D.  C. U. Nweke B.S. Agric. Tech.  C. G. Okoli B.Sc  M. Sc.  Ph.D.  M. N. Opara DVM  M.Sc 《International Journal of Environmental Science and Technology》2006,3(1):9-14

This study was carried out to identify the common moulds growing in selected commercial poultry feed sold in Owerri, Imo State, Nigeria. Forty-eight bulk samples derived froml92 bags of feeds were collected from broiler starter (BS), broiler finishers (BF), grower mash (GM) and layer mash (LM) across 4 different brands of ommercial poultry feeds, which included Livestock (LF), Top (TF) Guinea (GF) and Vital (VF) feeds. The feed samples were collected during the rainy season months of June, July and August. The common moulds isolated from these feeds were Aspergillus sp., Peicillium sp.,Mucor sp., Yeast sp., Rhizopus sp., Epicoecum sp., Gymnoaseus sp., Cladosporium sp., Mortierella sp. as well as Bacteria. Generally, more organisms were isolated in June than the other months with Mortierella sp. being the only one not isolated in that month. Vital feed with 8 different isolates had the highest diversity of fungal species while the others had between 4 and 5 species. Prevalence rates across the feed types, generally ranged from 18.76% in layer mash to 30.03% in broiler finisher. The four Aspergillus sp. isolated came from GM and BF. This study highlights the need for constant monitoring of moulds in commercial feedstuff produced in the humid tropical environments such as Imo state, Nigeria. There is also the need to routinely include fungal growth nhibitors in commercial feeds since moulds are capable of reducing the nutritional values of feedstuff as well as elaborating pathogenic toxins.  相似文献   

Elucidation of different forms of organic carbon in marine sediments from the Atlantic coast of Spain using thermal analysis coupled to isotope ratio and quadrupole mass spectrometry     

Elisa Lopez Capel  Jos Maria de la Rosa Arranz  Francisco J. Gonzlez-Vila  Jos Antonio Gonzlez-Perez  David A.C. Manning 《Organic Geochemistry》2006,37(12):1983

Analysis of river, estuary and marine sediments from the Atlantic coast of Spain using thermogravimetry–differential scanning calorimetry–quadrupole mass spectrometry–isotope ratio mass spectrometry (TG–DSC–QMS–IRMS) was used to (a) distinguish bulk chemical hosts for C within a sediment and humic acid fraction, (b) track C pools with differing natural C isotope ratios and (c) observe variation with distance from the coast. This is the first application of such a novel method to the characterisation of organic matter from marine sediments and their corresponding humic acid fractions. Using thermal analysis, a labile, a recalcitrant and a refractory carbon pool can be distinguished. Extracted humic fractions are mainly of recalcitrant nature. The proportion of refractory carbon is greatest in marine sediments and humic acid fractions. Quadrupole mass spectrometry confirmed that the greatest proportion of m/z 44 (CO₂) and m/z 18 (H₂O) were detected at temperatures associated with recalcitrant carbon (510–540 °C). Isotope analysis detected progressive enrichment in δ¹³C for the sediment samples with an increase in marine influence. Isotopic heterogeneity in the refractory organic matter in marine sediments could be due to products of anthropogenic origin or natural combustion products. Isotope homogeneity of humic acids confirms the presence of terrigenous C in marine sediments, allowing the terrestrial input to be characterised.  相似文献   

An electrochemical study of pyrite oxidation in the presence of carbon coatings in cyanide medium     

H. Tan  D. Feng  J.S.J. van Deventer  G.C. Lukey 《International Journal of Mineral Processing》2006,80(2-4):153-168

The anodic and cathodic behaviour of pyrite with clay and different carbon coatings of activated carbon, graphite and carbonaceous matter in cyanide medium was investigated using the potentiodynamic method. The presence of clay coating did not change the polarisation curve appearance for either the anodic oxidation of pyrite or the cathodic reduction of oxygen or the potential of the current plateau, but only decreased the plateau current especially at a higher coating thickness. The presence of the carbon coatings marginally shifted the rest potential for pyrite to a more anodic position and slightly changed the polarisation curve appearance for pyrite oxidation. The current density for pyrite oxidation largely increased in the presence of the carbon coatings, the potential at the plateau shifted to more cathodic positions, and the plateau width became smaller. These effects became more noticeable at a higher coating thickness. The activated carbon, graphite and carbonaceous matter coatings performed similarly in affecting pyrite oxidation at a similar thickness. The carbon coatings significantly increased the limiting current densities for oxygen reduction on pyrite, and the limiting current plateau became steeper at a higher coating thickness. The carbon coatings increased the limiting current density for oxygen reduction to a similar extent at a low coating thickness, but increased to varied extents at a higher coating thickness. The carbon coatings also greatly increased the cathodic current density for gold reduction on pyrite. The enhancement of pyrite oxidation and oxygen or gold reduction on pyrite by the carbon coatings was likely attributed to the electrochemical interaction between pyrite and the carbon materials with electron-rich surfaces and high conductivity. The presence of the carbon coatings significantly increased the oxidation of pyrite in aerated cyanide solutions and the preg-robbing of pyrite especially at a higher coating thickness.  相似文献