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1.
Abstract: The occurrences of the Early Archean carbonate minerals are compiled and their precipitation processes are investigated for the Warrawoona Group, Pilbara Craton. Sedimentary carbonate rocks such as limestone and dolostone are very rare, and only a small amount of sedimentary carbonate minerals are sometimes contained in the hydrothermal bedded chert, implying that a sink of CO 2 was minor in the Early Archean sediments. Moreover, it is very likely that the activity of cyanobacteria forming stromatolites was considerably low in the Early Archean. Microfossils and carbonaceous matter in the hydrothermal cherts are probably derived from a non-photosynthetic microorganisms related to the seafloor hydrothermal activity. Their preservation in sediments may play a very minor role in carbon sink of the Earth's surface. On the other hand, carbonatized volcanic rocks subjected to seafloor hydrothermal alteration occur ubiquitously in the Early Archean greenstone belts such as the Warrawoona Group, suggesting that the hydrothermally altered oceanic crust had large amounts of CO 2 as carbonate minerals. Global carbon cycle in the Early Archean is considered to have been controlled by the intense seafloor hydrothermal alteration. Large amounts of CO 2 were sunk into the oceanic crust by the alteration. The carbonatized oceanic crust was partly accreted to the continents and/or island–arcs, and partly subducted into the mantle without decomposition. Significant amounts of carbonate minerals in the carbonatized oceanic crust were very likely to store in the accretionary prisms and mantle, consequently giving rise to a decrease of atmospheric and oceanic CO 2. 相似文献
2.
New CO 2/ 3 He data from the East Sunda Arc (Indonesia) confirm earlier observations that arc volcanic gases have higher CO 2/ 3 He ratios than MOR environments.On average, > 80% of arc volcanic CO 2 is recycled, exogene carbon. Addition of a few percent of carbonate-bearing sediments to the mantle wedge explains much of the carbon abundance andcarbon isotopic data of arc gases, but can not explain the He isotope observations. The CO 2/ 3He in arc volcanoes is not strongly dependent on the composition of modem trough sediments (e.g. deep sea clays vs carbonate-rich sequences), and calcite veins in the hydrothermally altered subducted slab may provide a contribution to the recycled carbon flux of, arcs. The sum of globally deep-subducted sediment and slab carbon exceeds the estimated arc CO 2 flux, and approximately 3.5 teramole of carbon may return annually to the mantle in convergent zones. The modem combined processes of MOR volcanism, slab alteration, and subduction volcanism do not produce a substantial carbon flux into the exosphere, and rate-changes in ocean floor spreading are unlikely to cause major changes in atmospheric CO 2 as a result of changes in the volcanic CO 2 fluxes. Intense pulses of flood basalt volcanism, however, may alter the CO 2 contents of the atmosphere over the course of a millenium or so, and influence global climate. 相似文献
3.
Carbon dioxide records from polar ice cores and marine ocean sediments indicate that the last glacial maximum (LGM) atmosphere CO 2 content was 80–90 ppm lower than the mid-Holocene. This represents a transfer of over 160 GtC into the atmosphere since the LGM. Palaeovegetation studies suggest that up to 1350 GtC was transferred from the oceans to the terrestrial biosphere at the end of the last glacial. Evidence from carbon isotopes in deep sea sediments, however, indicates a smaller shift of between 400 and 700 GtC. To understand the functioning of the carbon cycle this apparent discrepancy needs to be resolved. Thus, older data have been reassessed, new data provided and the potential errors of both methods estimated. New estimates of the expansion of terrestrial biomass between the LGM and mid-Holocene are 700 GtC ± > 300 GtC, using the ocean carbon isotope-based method, compared with of 1100 GtC ± > 500 GtC using the palaeovegetation estimate. If these estimates of the carbon shift to the terrestrial biosphere are equilibrated with the dissolved carbon in the oceans, and the CaCO 3 compensation of the ocean is taken into account, then the glacial atmospheric CO 2 would have been between 50 (± 30) ppm and 95 (± 50) ppm higher. The glacial atmosphere therefore should have had a CO 2 partial pressure of between 330 and 375 μatm. Hence, a rise of between 130 and 175 μatm in atmospheric CO 2, rather than 80 μatm, at the end of the last glacial must be accounted for. 相似文献
4.
We present an approach for tracing the fate of anthropogenic CO 2, compiling a large data set of stable organic carbon isotope ratios from surface sediments, plankton, and sinking matter in the Atlantic Ocean. The δ 13C values of sinking matter are generally lower by 0.5–4.6‰ compared to the surface sediments. This difference increases with increasing latitude, which is explained by a stronger modern increase in surface water [CO 2 (aq)] in the Southern Ocean relative to the Tropical/Subtropical Ocean. Preindustrial dissolved CO 2 concentrations in Atlantic surface waters, estimated from the δ 13C org of surface sediments, are compared to recently measured surface water [CO 2 (aq)] values taken from literature. We obtain only a slight increase in [CO 2 (aq)] at lower latitudes but a significant change of about 7 ± 2 μ m in high latitudinal surface waters which we attribute to anthropogenic perturbation. Our results suggest that CO 2 released by human activities has been stored in Southern Ocean surface waters. 相似文献
5.
The equilibrium constant, K a, of the association reaction to form ion pairs from charged solute species in supercritical solutions can be calculated from a model based on published equations. Log K a at constant pressure is a linear function of the inverse in the dielectric constant of the fluid times temperature. The dielectric properties of H 2O and CO 2 at supercritical pressures and temperatures can also be evaluated using the Kirkwood equation. Using Looyenga mixing rules, the dielectric constant of H 2O–CO 2 mixtures can be obtained and the change in log K a with addition of CO 2 in aqueous solutions evaluated. These changes in log K a with addition of CO 2 are consistent with measured changes of log K a with addition of Ar in supercritical H 2O–Ar solutions. Log K a of KCl and NaCl increase to an increasing extent as the mole fraction of CO 2 increases in H 2O–CO 2 solutions. For instance, at 2 kbar and constant temperature between 400 and 600° C, log K a of KCl increases by about two orders of magnitude whilst that of NaCl increases by over four orders of magnitude as the CO 2 mole fraction increases from 0.0 to 0.35. Such changes in log K a will have dramatic effects on the solubility of minerals in CO 2-rich environments. 相似文献
6.
Refinements have been made to achieve over 99% yield in the conversion of CO to CO 2 in order to improve the reproducibility and accuracy of δ 18 O measurements in sulfates. BaSO 4 (10-15 mg) was mixed with an identical amount of spectrographic-grade graphite and loaded into a Pt boat. The mixture was gradually heated to 1100 °C to reduce sulfate to CO and CO 2; the former gas was simultaneously converted to CO 2 by a glow discharge between Pt electrodes immersed in a magnetic field (produced by a pair of external neodymium magnets). A small memory effect was noticed during the analysis (less than 0.3‰ per 10‰ difference in δ 18 O between two subsequently analysed samples). The memory effect, however, was suppressed by repetitive preparation of the same specimen. CO 2 produced in this way from sulfate reference samples was analysed on a dual inlet and triple collector mass spectrometer along with CO 2 equilibrated with VSMOW, GISP and SLAP water reference samples. To avoid large departures of measured isotope ratios from 18O/ 16O of the working calibrator we used CO 2 gas prepared from ocean water sulfate for this purpose. The calibrated δ 18 O values (in ‰) obtained in this way for NBS-127, IAEA SO-5 and IAEA SO-6 reference materials were 8.73 ± 0.05, 12.20 ± 0.07 and -10.43 ± 0.12, respectively. 相似文献
7.
Abstract Nearly pure CO 2 fluid inclusions are abundant in migmatites although H 2O-rich fluids are predicted from the phase equilibria. Processes which may play a role in this observation include (1) the effects of decompression on melt, (2) generation of a CO 2-bearing volatile phase by the reaction graphite + quartz + biotite + plagioclase = melt + orthopyroxene + CO 2-rich vapour, (3) selective leakage of H 2O from CO 2+ H 2O inclusions when the pressure in the inclusion exceeds the confining pressure during decompression, and (4) enrichment of grain-boundary vapour in CO 2 by subsolidus retrograde hydration reactions. 相似文献
8.
Abstract. This study examined the effect of CO 2 on NaCl solubility in hydrothermal fluid, with the synthetic fluid inclusion technique. Fluid inclusions of 30–40 wt% NaCl and 5 mol % CO 2 were synthesized, and their halite dissolution temperatures, T m(halite), were measured. The solubilities of NaCl in CO 2-bearing aqueous fluid were obtained at 160–320C under vapor-saturated pressures. The T m(halite) value in aqueous fluid with 5 mol % CO 2 obtained in this study agrees with that of Schmidt et al. (1995), showing that 5 mol % CO 2 reduces the solubility of NaCl by about 1 wt%. Calculation of magnetite solubility suggests that 5–10 mol % CO2 decreases magnetite solubility by 4.5–8.9 % relative to the magnetite solubility in CO2-free solution. Therefore, an increase of CO2 content in ore-forming solutions may cause deposition of iron minerals and produce ore deposits. 相似文献
9.
In order to evaluate the possible influence of oceanic crust production on climatic changes during the past 100Myr variations in total oceanic crust for this period including production at mid-ocean ridges, oceanic plateaus, and back-arc basins were calculated using the most recent and accurate time-scales. The rates presented here differ from those of Larson (1991a, b) on Cenozoic fluctuations and show that (1) maximum production values occurred during the Cenomanian, Palaeocene, and late Oligocene-early Miocene and (2) minimum values occurred in Campanian-Maastrichtian, late Eocene, and middle Miocene. Significantly, variations of oceanic crust production correspond with variations in the δ 18O of deep-water benthic foraminifera: maximum values of oceanic crust production correspond with minimum values of δ 18O, and minimum production values with maximum values of δ 18O. This latter synchronism suggests that changes in land-sea relationships and atmospheric CO 2 related to major fluctuations in oceanic crust production were the main cause of mid-Cretaceous warming and Late Cretaceous cooling, and of climatic quasi-cycles having a periodicity of 33–38 million years over the last 100 Myr. This is the first report showing variations of ocean crust production synchronized with the Cenozoic climate changes. 相似文献
10.
High-density CO 2-rich fluid inclusions from a sapphirine-bearing granulite (Hakurutale, Sri Lanka) have been studied by microthermometry, Raman spectrometry and SEM analysis. Based on textural evidence, two groups of inclusions can be identified: primary, negative crystal shaped inclusions (group I) and pseudo-secondary inclusions, which experienced a local, limited post-trapping modification (group II). Both groups contain magnesite as a daughter mineral, occurring in a relatively constant fluid/solid inclusion volume ratio (vol solid =0.15 total volume). CO 2 densities for group I and II differ only slightly. Both groups contain a fluid, which was initially trapped at peak metamorphic conditions as a homogeneous (CO 2+MgCO 3) mixture. Thermodynamic calculations suggest that such a fluid (CO 2+15 vol% MgCO 3) is stable under granulite facies conditions. After trapping, magnesite separated upon cooling, while the remaining CO 2 density suffered minor re-adjustments. A model isochore based on the integration of the magnesite molar volume in the CO 2 fluid passes about 1.5–2 kbar below peak metamorphic conditions. This remaining discrepancy can be explained by the possible role of a small quantity of additional water. 相似文献
11.
The seawater 87Sr/ 86Sr curve implies a 50–100 Myr episodicity in weathering rate which requires a corresponding variation in CO 2 degassing from the solid earth to the atmosphere. It is proposed that this is caused by orogenesis, which both produces CO 2 as a result of metamorphic decarbonation reactions, and consumes extra CO 2 as a consequence of erosion-enhanced weathering. Global climate on the geological time-scale is therefore contTolled by the difference between the relatively large and variable orogenic-moderated degassing and weathering CO 2 fluxes. 相似文献
12.
Abstract Deformed quartz veins in garnet-zone schist adjacent to the active Alpine Fault, New Zealand, have fluid inclusions trapped along quartz grain boundaries. Textures suggest that the inclusions formed in their present shapes during annealing of the deformed veins. Many of the inclusions are empty, but some contain carbon dioxide with densities that range from 0.16 to 0.80 g cm −3. No water, nitrogen or methane was detected. The inclusions are considerably more CO 2-rich than either the primary metamorphic fluid (<5% CO 2) or fluids trapped in fracture-related situations in the same, or related, rocks (<50% CO 2). Enrichment of CO 2 is inferred to have resulted from selective migration (wicking) of saline water from the inclusions along water-wet grain boundaries after cooling-induced immiscibility of a water-CO 2 mixture. Inclusion volumes changed after loss of water. Non-wetting CO 2 remained trapped in the inclusions until further percolation progressively removed CO 2 in solution. This mechanism of fluid migration dominated in ductile quartz-rich rocks near, but below, the brittle-ductile transition. At deeper levels, hydraulic fracturing is also an important mechanism for fluid migration, whereas at shallower levels advection through open fractures dominates the fluid flow regime. 相似文献
13.
Combined 147Sm--- 143Nd and the now extinct [τ(1/2) 146=103×10 6 yr] 146Sm--- 142Nd isotopic systematics are reported for early Archean gneisses from Greenland (Amîtsoq and Akilia associations), and Canada (Acasta gneiss). Using both field relationships and high resolution U---Pb SHRIMP ion-probe ages, it has been possible to identify the most ancient rocks in these terrains for isotopic analyses. Preliminary 142Nd analyses of a still limited number of samples have failed to identify terrestrial 142Nd anomalies. Effects, if present, are limited to < 10 ppm and we have thus been unable to confirm the +33±4 ppm ε 142 value claimed by Harper and Jacobsen (1992a, b) for a single sample. From the lack of 146Sm--- 142Nd effects we infer that large-scale fractionation events that may have occurred in the first 200 Ma of Earth history did not leave a significant nor widespread imprint on the early Archean mantle or crust. If a terrestrial magma ocean, with associated LREE fractionation, formed as a result of planetary accretion, then it had a lifetime of at most 250 m.y. before being remixed into the Earth's mantle. The samples analysed in this study have a range of ε143 values including highly positive values of up to +4.2. This requires that the earliest known Archean crust was differentiated from a reservoir that was strongly depleted in the LREE as compared with chondritic compositions. In the early Archean it is proposed that the depletions in LREE are a consequence of extraction of a limited fraction of the Earth's continental crust ( < 10%) from the upper 200 km of the mantle. A three reservoir model, consisting of the continental crust, depleted mantle and a more primitive mantle reservoir can be extended to account for both the present-day, as well as the evolving Nd isotopic composition of the Earth's crust and mantle. In contrast to previous models, the rate of growth of the continental crust is used as an input parameter to constrain the concomitant growth and evolution of the depleted mantle reservoir. Recycling of large volumes of bulk continental crust into the mantle is not considered to be an important process, nor is the existence of an additional major enriched component in the early Archean mantle. 相似文献
14.
The results are given for all the determinations made by the co-operating laboratories of major elements in the six NIMROC rock samples (granite, syenite, lujavrite, norite, pyroxenite and dunite) prepared by the National Institute for Metallurgy in 1966. Relevant statistical data are given for the sets of results for each major constituent, and recommended values for all constituents except Al 20 3, Na 20, K 20and CO 2 in dunite, Fe 2O 3, MgO and CO 2 in the granite, Fe 23 and CO 2 in the norite and CO 2 in the pyroxenite. This report and that on the trace and minor elements issued in 1978 complete the revision of the recommended values. It is suggested that analysts should concentrate rather on those constituents for which the results have shown such a wide scatter that they can be of no usc for reference purposes, than on those for which the ualues are fairly well established. 相似文献
15.
Abstract. Geochemical studies for gas hydrate, gas and organic matter collected from gas hydrate research wells drilled at the landward side of the eastern Nankai Trough, offshore Tokai, Japan, are reported. Organic matter in the 2355 m marine sediments drilled to Eocene is mainly composed of Type III kerogen with both marine and terrigenous organic input. The gas hydrate-bearing shallow sediments are immature for hydrocarbon generation, whereas the sediments below 2100 mbsf are thermally mature. The origins of gases change from microbial to thermogenic at around 1500 mbsf. Carbon isotope compositions of CH 4 and CO 2, and hydrocarbon compositions consistently suggest that the CH 4 in the gas hydrate-bearing sediments is generated by microbial reduction of CO 2. The δ 13C depth-profiles of CH 4 and CO 2 suggest that the microbial methanogenesis is less active in the Nankai Trough sediments compared with other gas hydrate-bearing sediments where solid gas hydrate samples of microbial origin were recovered. Since in situ generative-potential of microbial methane in the Nankai Trough sediments is interpreted to be low due to the low total organic carbon content (0.5 % on the average) in the gas hydrate-bearing shallow sediments, upward migration of microbial methane and selective accumulation into permeable sands should be necessary for the high concentration of gas hydrate in discrete sand layers. 相似文献
16.
桑托斯盆地盐下油气田中发现了大量CO 2,给油气勘探开发和生产都带来诸多困难和挑战.利用地层测试、样品分析及文献资料等,明确了CO 2成因及来源,统计分析了其区域分布特征,并基于区域重磁和深源地震等资料,剖析了控制CO 2分布的地质因素.盆内CO 2主要为幔源—岩浆成因,且幔源CO 2贡献了至少92%的CO 2总量.区域上,CO 2自陆向海呈增加趋势,并相对集中在盆地东部隆起带上.地壳减薄和地幔局部隆升是控制CO 2宏观分布最重要的背景因素.极端的地壳伸展造成了圣保罗地台下部陆壳强烈拉伸减薄,形成了一个面积约5.1×104 km2的地壳减薄区,造成了富含CO 2的地幔物质上拱进入陆壳,宏观上决定了盆内CO 2区域分布.此区域之外,出现高含量CO 2的可能性大幅降低.岩浆侵入和活动断层都是沟通隆升地幔和浅部储层的重要路径,但以断裂沟通最常见.NW-SE向区域走滑断裂和NE-SW向I-II级正断层对CO 2在浅部地层中的分配起控制作用,两组断裂交汇部位或周缘是幔源岩浆或CO 2最集中发育区. 相似文献
17.
A salt crust from an ephemeral saline lake in northern Namibia includes a basal thenardite layer (Na 2SO 4), followed by intervals consisting of burkeite (Na 6(CO 3)(SO 4) 2) with a palisade-type fabric that are separated by layers of fine-grained burkeite. All palisade burkeite formed as coatings along the sides of the neighbouring intervals and around thin lenses of fine-grained burkeite. During the development of these coatings, the original constituents of the intervals that they now occupy were dissolved. Textural features indicate that the dissolved deposits were composed at least in part of halite crystals. The study of this crust illustrates the complexity of evaporite sedimentation in ephemeral saline lakes where more than one dominant mineral is formed. In the studied basin, selective dissolution of the most soluble salts occurred during flooding stages, partly affecting subsurface occurrences that consisted of continuous beds. The final product is a nearly monomineralic deposit, derived from a layered non-monomineralic formation. The deposit developed to a large extent by subsurface crystal growth in a subaqueous setting, rather than by mineral formation along the brine–sediment interface or under subaerial conditions. 相似文献
18.
Many high-temperature–low-pressure (high- T –low- P ) metamorphic terranes show evidence for peak mineral growth during crustal thickening strain increments at pressures near the maximum attained during the heating–cooling cycle. Such terranes are not readily explained as the conductive response to crustal thickening since the resulting Moho temperatures would greatly exceed the crustal liquidus and because heating due to conductive equilibration on length scales appropriate to lithospheric-scale strains must greatly outlast the deformation. Consequently, high- T –low- P metamorphism may be generated during crustal thickening only when significant heat is advected within the crust, as for example may occur during the segregation of granitic melts. We show that without the addition of asthenospheric melts and at strain rates appropriate to continental deformation the conditions required for significant lower crustal melting during deformation are only likely to be attained if heat flow into the lower crust during crustal thickening is increased substantially, for example, by removing the mantle part of the lithosphere. A simple parameterization of lithospheric deformation involving the vertical strain on the scale of the crust, c, and the lithosphere, 1 respectively, allows the potential energy of the evolving orogen to be readily evaluated. Using this parameterization we show that an important isostatic consequence of the deformation geometries capable of generating such high- T –low- P metamorphism during crustal thickening (with c1) is an imposed upper limit to crustal thicknesses which is much lower than for homogeneous deformations (f c= f 1) for the same initial lithospheric configuration. 相似文献
19.
Results of electron microprobe and microthermometric studies of samples collected from the Bouvet Triple Junction Region (BTJR) during a joint Russian-Italian geological expedition on the R/V Academician Nikolaj Strakhov (1994) have revealed new data on the composition of basaltic magmas and oceanic hydrothermal fluids connected with magmatic processes. Detailed analysis of basaltic glasses shows that the modem Mid-Atlantic Ridge (MAR) rift valley is composed of normal mid-ocean ridge basalts with low concentrations of K 2 O and TiO z (N-MORB), while its flanks are more enriched with these components approaching E-MORB. A marked influence of the Bouvet hot spot volcanism on magma generation on the South-West Indian Ridge (SWIR) near Bouvet Island is observed. Basaltic melts in this area belong to alkalic and transitional series and have maximum contents of K 2O, TiO 2, H 2O. Microthermometric analyses of fluid inclusions in the samples from the BTJR have revealed major differences in the oceanic hydrothermal fluid systems on the MAR and near SWIR, which depends on the peculiarities of magma. In the area of the MAR (with dry melts) only H 2O solution inclusions in quartz were found; thus, seawater is probably the only primary source of hydrothermal fluids (NaCl + MgCl 2+ H 2O; T = 170–200°C). In the SWIR area (with the high content of water in melts) syngenetic liquid CO 2 and H 2O solution inclusions in quartz indicate the influence of the magmatic fluid component on the ore-forming water/carbon dioxide solutions (NaCl + CaC1 2+ H 2O + CO 2; T = 200–310 °C; P = 900–1700 bar). 相似文献
20.
The Fourier transform infrared spectra of individual conodonts with various CAI values indicate definite changes in conodont francolite during diagenesis. Steady decarbonation can be observed in the increasing intensity of the band with the wavenumber 2340 an -1, which is assigned to trapped molecular CO 2. Carbon dioxide originates from decomposing CO 2-3-ions occupying the B-site in francolite. Furthermore, the intensity of the water-deformation band at 1644 cm -1 decreases with higher CAI values. These changes of intensity can be quantified by calculating the maximum extinction (E max) for the corresponding absorption maxima and correlated with the CAI. Unaltered conodonts have a relative variable water 'content' and very little CO 2. Up to a CAI value of 4 conodont francolite continuously expells water and carbon dioxide is trapped. Conodonts with CAI = 5 have similar water 'contents' to CAI = 4 conodonts, but decarbonation continues to take place during this stage of diagenesis. 相似文献
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