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1.
Study on Modern Plant C－13 in Western China and Its Significance   总被引：2，自引：0，他引：2  
Organic carbon isotopic composition(δ^13C) is one of the important proxies in paleoenvironment studies.In this paper modern plant δ^13C in the arid areas of China and Tibetan Plateau is studied.It is found that most terrestrial plant species in western China are C3 plants with δ^13C values ranging from -32.6‰ to -23.2‰ and only few species are C4 plants with δ^13C values from -16.8‰ to -13.3‰.The δ^13C is closely related to precipitation (or humidity),i.e., light δ^13C is related to high precipitation(or humid climate),while heavy δ^13C to low precipitation (or dry climate),but there is almost no relation between plant δ^13C and temperature.Submerged plants have δ^13C values ranging from -22.0‰ to -12.7‰,like C4 plants,while merged plants have δ^13C values ranging from -28.1‰ to -24.5‰，like C3 C4 plants,while marged plants have δ^13C values ranging from -28.1‰ to -24.5‰,like C3 plants.It can then be concluded that organic δ^13C variations in terrestrial sediments such as loeas and soil in western China can indicate precipitation changes,but those in lake sediments can reflect organic sources and the productivity of different types of aquatic plants.  相似文献   

2.
REE－fluorocarbonates as major REE minerals in the Bayan Obo deposit,the largest REE deposit in the world,were analyzed for their stable isotopic compositions,The δ^13 C and δ^18 O values of huanghoite,cebaite and bastnaesite from late-stage veins vary in the ranges of 7.8--4.0‰ and 6.7-9.4‰,respectively,These data are relatively similar to those of bastnaesites from banded ores:δ^13C-5.6--5.2‰ andδ^18O3.6-5.5‰.The REE fluorocarbonates from both late-staege veins and banded ores are characterized by lower δ^13 C and δ^18O values,especially the δ^18O values of bastnaesites from banded ores.Compared with them,the disseminated bastnaesits the dolomite-type ores possess rather highδ^13 C and δ^18O values,i.e.,-2.1-0.4‰ and 8.6-12.9‰ respectively.The high values are typical of the sedimentary host dolomite rocks as well as of the dolomite-type-ores.The carbon and oxygen isotopic characteristics of REE fluorocarbonate minerals provide new evidence for the hypothesis on the origin of Bayan Obo deposit-epigenetic hydrothermal metasomatism.  相似文献   

3.
Lake sediments can provide important historical information on records of paleoenvironments and paleoclimates and their changes.This study deals with the sedimentary history of the westem Taihu Lake based on seven geochemical indices measured in an 89-cm long sediment core. The core, corresponding to a time period from 6870 a B.P. to the present, was analyzed for δ^13Corg, δ^15N, TOC, TN, TP, C/N ratio and radiocarbon dates. Comparison of these multiple geochemical tracers helps to improve interpretations of the paleoenvironmental changes. All of the geochemical proxies used in the study change regularly, and show four major time scales that suggest different environments. During 6870-6532 a B.P., the values of all parameters slightly varied. δ^13Corg values increased in a wave pattern from -25.9‰ to -20.7‰. Similarly, δ^15N values increased from 1.6‰ to 4.5‰. TOC, TN and TP concentrations remained around 0.8%, 0.1%-3.2% and 0.5%, respectively. C/N ratios varied from 20.6 to 6.6. At 6370 a B.P., the sediment record profile showed dramatic variations in all parameters. δ^13Corg and δ^15N values dropped to -26.9‰ and 1.3‰, respectively.  相似文献   

4.
Sixty-five natural gas samples were collected from 19 oil-gasfields in the Qaidam basin, China. The chemical composition and carbon isotope values of the samples were measured, and the geochemical characteristics and origin of the natural gases were studied. The gases can be divided into biogenic gases, sapropelic oil-type gases, mixed type oil-type gases, coal-type gases and mixed gas. The δ13Ci values of the biogenic gases are very small and the C2+ contents of them are very low, ranging from -68.2‰ to -61.8‰ and 0.06% to 0.20% respectively. They have heavy δD and δ13Cco2> showing a CC>2 reduction pathway. They are distributed in the East depression region and derived from the Quaternary source rocks. The sapropelic oil-type gases have small δ13C2 values and high C2+ ranging from -36.6‰ to -28.6‰ and from 33.01% to 47.15% respectively. The mixed type oil-type gases have <5I3C2 values and C2+ contents varying from -28.6‰ to -24.8‰ and from 4.81% to 26.06% respectively. Both sapropelic oil-typ  相似文献   

5.
Early carbonate cements in the Yanchang Formation sandstones are composed mainly of calcite with relatively heavier carbon isotope （their δ^18O values range from -0.3‰- -0.1‰） and lighter oxygen isotope （their δ^18O values range from -22.1‰- -19.5‰）. Generally, they are closely related to the direct precipitation of oversaturated calcium carbonate from alkaline lake water. This kind of cementation plays an important role in enhancing the anti-compaction ability of sandstones, preserving intragranular volume and providing the mass basis for later disso- lution caused by acidic fluid flow to produce secondary porosity. Ferriferous calcites are characterized by relatively light carbon isotope with δ^13C values ranging from -8.02‰ to -3.23‰, and lighter oxygen isotope with δ^18O values ranging from -22.9‰ to -19.7‰, which is obviously related to the decarboxylation of organic matter during the late period of early diagenesis to the early period of late diagenesis. As the mid-late diagenetic products, ferriferous cal- cites in the study area are considered as the characteristic authigenic minerals for indicating large-scaled hydrocarbon influx and migration within the clastic reservoir. The late ankerite is relatively heavy in carbon isotope with δ^13C values ranging from -1.92‰ to -0.84‰, and shows a wide range of variations in oxygen isotopic composition, with δ^18O values ranging from -20.5‰ to -12.6‰. They are believed to have nothing to do with decarboxylation, but the previously formed marine carbonate rock fragments may serve as the chief carbon source for their precipitation, and the alkaline diagenetic environment at the mid-late stage would promote this process.  相似文献   

6.
This paper studies the Late Cretaceous dinosaur eggshell fossils from the Xixia Basin,Henan Province,by icroscopy and carbon isotope method.Careful observation under microscope revealed that all dinosaur eggshell fossils consist of primary calcite and secondary calcite.The content of the former is about 60.5% and the latter 39.5% according to image analysis.The δ13C values of secondary carbonate mineral filling within the dinosaur eggs in the strata range from-5.63‰ to -5.68‰,with an average value of -5.64‰,the δ^13 C values of sixteen dinosaur eggshell fossils are in the range from-5.88‰ to -7.79‰,Then we calculated the δ^13 C values of primary carbonate minerals,ranging from-6.03‰ to-9.19‰.Bsed on the δ^13C values of the primary carbonate ,the ancient food type and food proportion of dinosaurs were deduced.The dinosaru‘s food proportions of C3 plant and C4 plant are 61% and 39%,respectively.Finally,it is inferred that the palaeoclimate in the Xixia Basin should be a warm-sub-humid-sub-arid climate in the subtropical zone of the Xixia Basin.  相似文献   

7.
The alternative development of coal-bearing hydrocarbon source rocks and low-porosity and low-permeability tight sandstone reservoirs of the Triassic Xujiahe Formation in the Sichuan Basin is favorable for near-source hydrocarbon accumulation. The natural gas composition of the Xujiahe Formation in the Sichuan Basin is dominated by hydrocarbon gases, of which the methane content is80.16%-98.67%. Typically, the C_2~+ content is larger than 5% in main wet gas. The dry gas is mainly distributed in the western and northern regions of the basin. The non-hydrocarbon gases mainly contain nitrogen, carbon dioxide, hydrogen, and helium, with a total content of 2%. The carbon isotope ranges of methane and its homologues in natural gas are: δ~(13)C_1 of-43.8‰ to-29.6‰, δ~(13)C_2 of-35.4‰ to-21.5‰, δ~(13)C_3 of-27.6‰ to-19.8‰,and δ~(13)C_4 of-27.7‰ to-18.8‰. δ~(13)C_3δ~(13)C_4 occurs in some natural gas with a low evolution degree; such gas is mainly coal-related gas from humic-type source rocks of the Xujiahe Formation. As for the natural gas, δ~2 H_(CH4) values ranged from-195‰ to-161‰,δ~2 H_(C2H6) values ranged from-154‰ to-120‰, and δ~2 H_(C3H8) values ranged from-151‰ to-108‰. The dry coefficient,δ~(13)C and δ~2 H_(CH4) are all positively correlated with the maturity of source rocks. The higher the maturity of source rocks is, the larger the natural gas dry coefficient is and the larger the δ~(13)C and δ~2 H_(CH_4) values are, indicative of the characteristic of near-source accumulation. The δ~2 H_(C2H6) value of natural gas is influenced by paleosalinity to a relatively large extent; the higher the paleosalinity is, the larger the δ~2 H_(C2H6) value is. The Pr/Ph value of the condensate oil ranged from 1.60 to 3.43, illustrating light oxidization-light reduction and partial-oxidization characteristics of the depositional environment of coal-bearing source rocks of the Xujiahe Formation. The natural gas light hydrocarbon(C_5-C_7) from the Xujiahe Formation presented two characteristics: the first was the relatively high aromatic hydrocarbon content(19%-32.1%), which reveals the characteristic of natural gas with humic substances of high-maturity; the second was the low content of aromatic hydrocarbon(0.4%-9.3%),reflecting water-washing during the accumulation of the natural gas. The reported research outcomes indicate a potential mechanism for natural gas accumulation in the Xujiahe Formation, which will further guide natural gas exploration in this region.  相似文献   

8.
In this study, the δ^13C and δ^18O values were systematically measured on NBS-18, NBS-19 and IAEA-CO-1 with different sample sizes, with the objective to examine the stability and reproducibility of previously developed linearity correction strategy especially for small-sized samples （e.g. 〈50 μg）. Firstly, the δ^13C and δ^18O values of NBS-19 standards （6-10 samples per run） with sample sizes scattered below -100 μg were determined in three different runs. The logarithmic regressions were performed on the plots of δ-values vs. peak area （sample size） for each run and the correction was applied using peak area of the first peak. Results show that two of the three data sets have almost the same regressive equations for both δ^13C and δ^18O values. The maximum difference in δ^13C values calculated by three equations when sample size varies between -10 and -100 μg is better than 0.15‰, compared with the maximum 0.82‰ for δ^18O values. Since alteration of phosphoric acids could not influence carbon isotope, the 〈0.15‰ difference in calculated δ^13C values should reflect the stability of mass spectrometer conditions. In contrast, the large difference in regressive equations for δ^18O values may be attributed to changed oxygen isotope in phosphoric acids due to exchange with atmosphere through time. It means that standards with sample sizes properly distributed should be arranged in every run for subsequent linearity correction of δ^18O values of small-sized samples （e.g. marine ostracode）.  相似文献   

9.
Hydrogen isotopic compositions of pyroxenes in peridotite xenohths from the Nushan volcano, Anhui Province, SE China have been obtained using an ion probe (Cameca 1270). D/H ratios are constant within a single grain and among the different grains from the same sample. The lack of correlation between the D/H ratios and the hydrogen contents indicates that the hydrogen isotopic compositions are inherited from their mantle source. Combining with the δD values of coexisting amphiboles, it is inferred that the Nushan mantle experienced at least a two-stage metasomatic event.One was responsible for formation of amphiboles with extremely variable δD values, and the other was probably responsible for the high δD values (up to -20‰) of some clinopyroxenes from peridotites, clinopyroxene and mica megacrysts. High δD values point to a subduction-related fluid being involved in one metasomatic event. The primary δD values (-90‰ to -140‰) of the Nushan pyroxenes, together with data gleaned from the literature, suggest that the D/H ratios of the nominally anhydrous mineral reservoir might have differed from that of the other mantle hydrogen in being relatively depleted in D.  相似文献   

10.
This study dealt with the distribution characteristics of soil organic carbon （SOC） and the variation of stable carbon isotopic composition （δ^13C values） with depth in six soil profiles, including two soil types and three vegetation forms in the karst areas of Southwest China. The δ^13C values of plant-dominant species, leaf litter and soils were measured using the sealed-tube high-temperature combustion method. Soil organic carbon contents of the limestone soil profiles are all above 11.4 g/kg, with the highest value of 71.1 g/kg in the surface soil. However, the contents vary between 2.9 g/kg and 46.0 g/kg in three yellow soil profiles. The difference between the maximum and minimum δ^13C values of soil organic matter （SOM） changes from 2.2‰ to 2.9‰ for the three yellow soil profiles. But it changes from 0.8‰ to 1.6‰ for the limestone soil profiles. The contrast research indicated that there existed significant difference in vertical pattems of organic carbon and δ^13C values of SOM between yellow soil and limestone soil. This difference may reflect site-specific factors, such as soil type, vegetation form, soil pH value, and clay content, etc., which control the contents of different organic components comprising SOM and soil carbon turnover rates in the profiles. The vertical variation patterns of stable carbon isotope in SOM have a distinct regional character in the karst areas.  相似文献   

11.
Basalts interbedded with oil source rocks are discovered frequently in rift basins of eastern China, where CO2 is found in reservoirs around or within basalts, for example in the Binnan reservoir of the Dongying Depression. In the reservoirs, CO2 with heavy carbon isotopic composition (δ13C>-10‰ PDB) is in most cases accounts for 40% of the total gas reserve, and is believed to have resulted from degassing of basaltic magma from the mantle. In their investigations of the Binnan reservoir, the authors suggested that the CO2 would result from interactions between the source rocks and basalts. As the source rocks around basalts are rich in carbonate minerals, volcanic minerals, transition metals and organic matter, during their burial history some of the transition metals were catalyzed on the thermal degradation of organic matter into hydrocarbons and on the decomposition of carbonate minerals into CO2, which was reproduced in thermal simulations of the source rocks with the transition metals (Ni and Co). This kind of CO2 accounts for 55%-85% of the total gas reserve generated in the process of thermal simulation, and its δ13C values range from -11‰- -7.2‰ PDB, which are very similar to those of CO2 found in the Binnan reservoir. The co-generation of CO2 and hydrocarbon gases makes it possible their accumulation together in one trap. In other words, if the CO2 resulted directly from degassing of basaltic magma or was derived from the mantle, it could not be accumulated with hydrocarbon gases because it came into the basin much earlier than hydrocarbon generation and much earlier than trap formation. Therefore, the source rocks around basalts generated hydrocarbons and CO2 simultaneously through catalysis of Co and Ni transition metals, which is useful for the explanation of co-accumulation of hydrocarbon gases and CO2 in rift basins in eastern China.  相似文献   

12.
The Serrinha gold deposit of the Gurupi Belt, northern Brazil, belongs to the class of orogenic gold deposits. The deposit is hosted in highly strained graphitic schist belonging to a Paleoproterozoic (∼2,160 Ma) metavolcano-sedimentary sequence. The ore-zones are up to 11 m thick, parallel to the regional NW–SE schistosity, and characterized by quartz-carbonate-sulfide veinlets and minor disseminations. Textural and structural data indicate that mineralization was syn- to late-tectonic and postmetamorphic. Fluid inclusion studies identified early CO2 (CH4-N2) and CO2 (CH4-N2)-H2O-NaCl inclusions that show highly variable phase ratios, CO2 homogenization, and total homogenization temperatures both to liquid and vapor, interpreted as the product of fluid immiscibility under fluctuating pressure conditions, more or less associated with postentrapment modifications. The ore-bearing fluid typically has 18–33mol% of CO2, up to 4mol% of N2, and less than 2mol% of CH4 and displays moderate to high densities with salinity around 4.5wt% NaCl equiv. Mineralization occurred around 310 to 335°C and 1.3 to 3.0 kbar, based on fluid inclusion homogenization temperatures and oxygen isotope thermometry with estimated oxygen fugacity indicating relatively reduced conditions. Stable isotope data on quartz, carbonate, and fluid inclusions suggest that veins formed from fluids with δ18OH2O and δDH2O (310–335°C) values of +6.2 to +8.4‰ and −19 to −80‰, respectively, which might be metamorphic and/or magmatic and/or mantle-derived. The carbon isotope composition (δ13C) varies from −14.2 to −15.7‰ in carbonates; it is −17.6‰ in fluid inclusion CO2 and −23.6‰ in graphite from the host rock. The δ34S values of pyrite are −2.6 to −7.9‰. The strongly to moderately negative carbon isotope composition of the carbonates and inclusion fluid CO2 reflects variable contribution of organic carbon to an originally heavier fluid (magmatic, metamorphic, or mantle-derived) at the site of deposition and sulfur isotopes indicate some oxidation of the originally reduced fluid. The deposition of gold is interpreted to have occurred mainly in response to phase separation and fluid-rock interactions such as CO2 removal and desulfidation reactions that provoked variations in the fluid pH and redox conditions.  相似文献   

13.
A progression from cuboid to octahedral growth has been observed in 16 natural diamonds from Yakutian kimberlites. X-ray and cathodoluminescence topography have revealed that the change in morphology of diamonds with cloudy cuboid cores may occur without mixed-habit growth but via generation of numerous octahedral apices on cuboid surfaces and subsequent gradual transformation into regular octahedral morphology. Nitrogen aggregation in both cuboid and octahedral domains of such diamonds suggests that they have had a long residence time under mantle conditions. Micro-inclusions in the cuboid domains of the diamonds testify to the nucleation and growth of cuboid cores from a hydrous-carbonatitic (oxidized) fluid. The transition from cuboid hummocky growth rich in inclusions to octahedral growth without inclusions may be linked to decreasing supersaturation in the parent fluid. Measurements of δ13C and Nppm by ion microprobe show that the chemical variations observed between inner cuboid domains and outer octahedral zones commonly have a systematic character and as such they are probably not due to purely kinetic effects. The peripheral octahedral zones are always enriched in 13C in comparison with inner cuboid ones, and the total nitrogen content decreases with the change from cuboid to octahedral growth. The octahedral outer zones show a gradual progressive increase in δ13C, with an overall change of up to 5‰ from the cuboid core (δ13C usually between −8 and −6‰) to the diamond margin (δ13C usually between −4 and −2‰). Decreases in δ13C of this magnitude with a gradual increase in 13C may be attributed to the Rayleigh fractionation operating on a single parent fluid of close to normal mantle δ13C composition with diamond precipitating by the reduction of carbonatitic fluid in a closed system. However, one sample shows a variation of δ13C of approximately −17 to −6‰ and therefore suggests a possible change of fluid source composition from one containing subducted crustal organic carbon to one with common mantle carbon. An erratum to this article can be found at  相似文献   

14.
Summary The intrusion of the Lower Permian Los Santos-Valdelacasa granitoids in the Los Santos area caused contact metamorphism of Later Vendian-Lower Cambrian metasediments. High grade mineral assemblages are confined to a 7 km wide contact aureole. Contact metamorphism was accompanied by intense metasomatism and development of skarns, and it generated the following mineral assemblages: diopside, forsterite, phlogopite (±clintonite) and humites and spinel-bearing assemblages or diopside, grossular, vesuvianite ± wollastonite in the marbles, depending on the bulk rock composition. Cordierite, K-feldspar, andalusite and, locally, sillimanite appear in the metapelitic rocks. Mineral assemblages of marbles and hornfelses indicate pressure conditions ranging from 0.2 to 0.25 GPa and maximum temperatures between 630 and 640 °C. 13C and 18O depletions in calcite marbles are consistent with hydrothermal fluid–rock interaction during metamorphism. Calcites are depleted in both 18O (δ18O = 12.74‰) and 13C (δ13C = −5.47‰) relative to dolomite of unmetamorphosed dolostone (δ18O = 20.79‰ and δ13C = −1.52‰). The δ13C variation can be interpreted in terms of Rayleigh distillation during continuous CO2 fluid removal from the carbonates. The δ18O values reflect hydrothermal exchange with an externally derived fluid. Microthermometric analyses of fluid inclusions from vesuvianite indicate that the fluid was water dominated with minor contents of CO2 (±CH4 ± N2) suggesting a metamorphic origin. Fluorine-bearing minerals such as chondrodite, norbergite and F-rich phlogopite indicate that contact metamorphism was accompanied by fluorine metasomatism. Metasomatism was more intense in the inner-central portion of the contact aureole, where access to fluids was extensive. The irregular geometry of the contact with small aplitic intrusives between the metasediments and the Variscan granitoids probably served as pathways for fluid circulation.  相似文献   

15.
At Sams Creek, a gold-bearing, peralkaline granite porphyry dyke, which has a 7 km strike length and is up to 60 m in thickness, intrudes camptonite lamprophyre dykes and lower greenschist facies metapelites and quartzites of the Late Ordovician Wangapeka formation. The lamprophyre dykes occur as thin (< 3 m) slivers along the contacts of the granite dyke. δ18Omagma values (+5 to +8‰, VSMOW) of the A-type granite suggest derivation from a primitive source, with an insignificant mature crustal contribution. Hydrothermal gold–sulphide mineralisation is confined to the granite and adjacent lamprophyre; metapelite country rocks have only weak hydrothermal alteration. Three stages of hydrothermal alteration have been identified in the granite: Stage I alteration (high fO2) consisting of magnetite–siderite±biotite; Stage II consisting of thin quartz–pyrite veinlets; and Stage III (low fO2) consisting of sulphides, quartz and siderite veins, and pervasive silicification. The lamprophyre is altered to an ankerite–chlorite–sericite assemblage. Stage III sulphide veins are composed of arsenopyrite + pyrite ± galena ± sphalerite ± gold ± chalcopyrite ± pyrrhotite ± rutile ± graphite. Three phases of deformation have affected the area, and the mineralised veins and the granite and lamprophyre dykes have been deformed by two phases of folding, the youngest of which is Early Cretaceous. Locally preserved early-formed fluid inclusions are either carbonic, showing two- or three-phases at room temperature (liquid CO2-CH4 + liquid H2O ± CO2 vapour) or two-phase liquid-rich aqueous inclusions, some of which contain clathrates. Salinities of the aqueous inclusions are in the range of 1.4 to 7.6 wt% NaCl equiv. Final homogenisation temperatures (Th) of the carbonic inclusions indicate minimum trapping temperatures of 320 to 355°C, which are not too different from vein formation temperatures of 340–380°C estimated from quartz–albite stable isotope thermometry. δ18O values of Stage II and III vein quartz range from +12 and +17‰ and have a bimodal distribution (+14.5 and +16‰) with Stage II vein quartz accounting for the lower values. Siderite in Stage III veins have δ18O (+12 to +16‰) and δ13C values (−5‰, relative to VPDB), unlike those from Wangapeka Formation metasediments (δ13Cbulk carbon values of −24 to −19‰) and underlying Arthur Marble marine carbonates (δ18O = +25‰ and δ13C = 0‰). Calculated δ18Owater (+8 to +11‰, at 340°C) and (−5‰) values from vein quartz and siderite are consistent with a magmatic hydrothermal source, but a metamorphic hydrothermal origin cannot be excluded. δ34S values of sulphides range from +5 to +10‰ (relative to CDT) and also have a bimodal distribution (modes at +6 and +9‰, correlated with Stage II and Stage III mineralisation, respectively). The δ34S values of pyrite from the Arthur Marble marine carbonates (range from +3 to +13‰) and Wangapeka Formation (range from −4 to +9.5‰) indicate that they are potential sources of sulphur for sulphides in the Sams Creek veins. Another possible source of the sulphur is the lithospheric mantle which has positive values up to +14‰. Ages of the granite, lamprophyre, alteration/mineralisation, and deformation in the region are not well constrained, which makes it difficult to identify sources of mineralisation with respect to timing. Our mineralogical and stable isotope data does not exclude a metamorphic source, but we consider that the source of the mineralisation can best be explained by a magmatic hydrothermal source. Assuming that the hydrothermal fluids were sourced from crystallisation of the Sams Creek granite or an underlying magma chamber, then the Sams Creek gold deposit appears to be a hybrid between those described as reduced granite Au–Bi deposits and alkaline intrusive-hosted Au–Mo–Cu deposits.  相似文献   

16.
Compared to most studied kimberlite pipes of the Yakutian province, diamonds from the Poiskovaya, Zapolyarnaya, and Leningrad pipes (about 100 determinations) are characterized by a decrease in the average δ13C (−4.59, −4.50,−4.04‰) and by relatively low average nitrogen contents (93, 254, 304 ppm, respectively). These pipes also differ in the relative abundance of alpha-1 group crystals in Galimov’s classification. Based on a stable combination of morphology and properties, we distinguished diamond populations of the common origin. All of them have significant a negative correlation between δ13C and the nitrogen content. This correlation is considered to be a result of synchronous and progressive loss of nitrogen and the light isotope 12C in the medium during the diamond formation. The conclusion is drawn that the alpha-1 crystals were formed in the open fluid system. The pyrolysis of ethane—C2H6 → CH4 + H2 + Cdiam—is assumed to be a model of diamond precipitation from fluid. The pressure release due to extension deformation of the mantle under tectonic action might be a factor shifting this reaction toward diamond formation.  相似文献   

17.
In the surroundings of the Gaoqing-Pingnan fault zone are developed quite a number of gas reservoirs. Based on gas compositions, they can be divided into two groups, i.e., CO2 and CH4. Their composition and isotope geochemistry were dealt with in this study. The CO2 contents range from 60.72%–99.99%, the δ13CCO2 values from -3.41‰– -9.8‰, and the 3He/4He ratios from 4.35×10-6–6.35×10-6 (i.e. R/Ra=4.45–4.35). Based on the data on composition and isotope geochemistry, deep geological background, deep faults and volcanic rocks, it is shown that CO2 ,distributed in the Gaoqing area, mostly originated from mantle-source inorganic matter which is associated with magmatic rocks. The favorable tectonic environment for the formation of CO2 reservoirs is the rift, which is related to great fault-magmatic activity, the formation of CO2 gas pools and their space-time correlation to the most recent magmatic activities. Hydrocarbon gas pools occur in the Huagou area. The CH4 contents are within the range of 88.83%–99.12%, and the δ13CCH4 values, -44.7‰– -54.39‰. This indicates that the hydrocarbon gas resulted from the decomposition of oil-type gas at high temperatures. Volcanic rocks in the CO2 gas pool-and CH4 gas pool-distributed areas show significant differences in Fe2O3 and FeO contents. This has proven that the hydrocarbon gas may have resulted from various chemical reactions. Magmatic activities are the primary reason for the distribution of CO2 and CH4 gas pools in the Gaoqing-Pingnan fault zone.  相似文献   

18.
The relation of two well-known ancient carbonate deposits to hydrocarbon seepage was confirmed by this study. Archaea are found to be associated with the formation of Oxfordian seep carbonates from Beauvoisin and with a Miocene limestone from Marmorito ("tube-worm limestone"). Carbonates formed due to a mediation by archaea exhibit extremely positive or extremely negative δ13Ccarbonate values, respectively. Highly positive values (+15‰) reflect the use of 13C-enriched CO2 produced by methanogenesis. Low δ13C values of the Marmorito carbonates (–30‰) indicate the oxidation of seepage-derived hydrocarbons. Likewise, the δ13C content of specific tail-to-tail linked isoprenoids, biomarkers for archaea, was found to be strikingly depleted in these samples (as low as –115‰). The isotopic signatures corroborate that archaea were involved in the cycling of seepage-derived organic carbon at the ancient localities. Another Miocene limestone ("Marmorito limestone") shows a strong imprint of methanotrophic bacteria as indicated by δ13C values of carbonate as low as –40‰ and biomarker evidence. Epifluorescence microscopy and field-emission scanning electron microscopy revealed that bacterial biofilms were involved in carbonate aggregation. In addition to lucinid bivalves previously reported from both localities, we infer that sponges from Beauvoisin and tube worms from Marmorito depended on chemosynthesis as well. Low δ13C values of nodules related to sponge taphonomy (–27‰) indicate that sponges might have been linked to an enhanced hydrocarbon oxidation. Tube worm fossils from Marmorito closely resemble chemosynthetic pogonophoran tube worms from Recent cold seeps and are embedded in isotopically light carbonate (δ13C –30‰). Received: 13 October 1998 / Accepted: 5 February 1999  相似文献   

19.
The Tono sandstone-type uranium mine area, middle Honsyu, Japan is composed of Miocene lacustrine sedimentary rocks in the lower part (18–22 Ma) and marine facies in the upper part (15–16 Ma). Calcite and pyrite occur as dominant diagenetic alteration products in these Neogene sedimentary rocks. The characteristics of calcite and pyrite differ significantly between lacustrine and marine facies. Abundant pyrite, calcite, organic matter, and small amounts of marcasite or pyrrhotite occur in the lacustrine facies, whereas small amounts of calcite and framboidal pyrite, organic matter and no marcasite or pyrrhotite are found within the marine units. The δ13C values of calcite in the lacustrine deposits are low (−19 to −6‰ PDB) but those in marine formation are high (−11 to +3‰). This implies that the contribution of marine carbonate is larger in upper marine sedimentary rocks, and carbon in calcite in the lower lacustrine formation was derived both from oxidation of organic matter and from dissolved marine inorganic carbon. The δ34S values of framboidal pyrite in the upper marine formation are low (−14 to −8‰ CDT), indicating a small extent of bacterial seawater sulfate reduction, whereas those of euhedral-subhedral pyrite in the lower lignite-bearing arkose sandstone are high (+10 to +43‰), implying a large extent of closed-system bacterial seawater sulfate reduction. The δ34S and δ13C data which deviate from a negative correlation line toward higher δ13C values suggest methanogenic CO2 production. During diagenesis of the lacustrine unit, large amounts of euhedral-subhedral pyrite were formed, facilitated by extensive bacterial reduction of seawater sulfate with concomitant oxidation of organic matter, and by hydrolysis reactions of organic matter, producing CH4 and CO2. Uranium minerals (coffinite and uraninite) were also formed at this stage by the reduction of U6+ to U4+. The conditions of diagenetic alteration within the lacustrine deposits and uranium mineralization is characterized by low Eh in which nearly equal concentrations of CH4 and HCO3 − existed and reduced sulfur species (H2S, HS−) are predominant among aqueous sulfur species, whereas diagenetic alteration of the marine formations was characterized by a predominance of SO4 2− among dissolved sulfur species. Modern groundwater in the lacustrine formation has a low Eh value (−335 mV). Estimated and measured low Eh values of modern and ancient interstitial waters in lacustrine environments indicate that a reducing environment in which U4+ is stable has been maintained since precipitation of uranium minerals. Received: 9 February 1996 / Accepted: 11 April 1997  相似文献   

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