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1.
Despite its location on sediment-free basalt, vent fluids from the Main Endeavour Field (MEF) contain chemical species that indicate fluids have interacted with sediments during circulation. We report on the distribution and isotopic abundances of organic compounds (C1-C3 alkanes and alkenes, benzene and toluene) in fluids collected from the Main Endeavour Field (MEF) in July, 2000, to understand the processes that regulate their abundances and characterize fluid sources. Aqueous organic compounds are derived from the thermal alteration of sedimentary organic matter and subsequently undergo further oxidation reactions during fluid flow. Fluid:sediment mass ratios calculated using ΣNH4 concentrations indicate that the sediments are distal to the MEF, resulting in a common reservoir of fluids for all of the vents. Following the generation from sediment alteration, aqueous organic compounds undergo secondary alteration reactions via a stepwise oxidation reaction mechanism. Alkane distributions and isotopic compositions indicate that organic compounds in MEF fluids have undergone a greater extent of alteration as compared to Middle Valley fluids, either due to differences in subsurface redox conditions or the residence time of fluids at subsurface conditions. The distributions of the aromatic compounds benzene and toluene are qualitatively consistent with the subsurface conditions indicated by equilibration of aqueous alkanes and alkanes. However, benzene and toluene do not achieve chemical equilibrium in the subsurface. Methane and CO2 also do not equilibrate chemically or isotopically at reaction zone temperatures, a likely result of an insufficient reaction time after addition of CO2 from magmatic sources during upflow. The organic geochemistry supports the assumption that the sediments with which MEF fluids interact has the same composition as sediments present in Middle Valley itself, and highlight differences in subsurface reaction zone conditions and fluid flow pathways at these two sites.  相似文献   

2.
《Applied Geochemistry》2002,17(11):1381-1399
Geochemical and isotopic studies of pore fluids and solid phases recovered from the Dead Dog and Bent Hill hydrothermal sites in Middle Valley (Ocean Drilling Program Leg 169) have been compared with similar data obtained previously from these sites during Ocean Drilling Program Leg 139. Although generally the hydrothermal systems reflect non-steady state conditions, the data allow an assessment of the history of the hydrothermal processes. Sediment K/Al ratios as well as the distribution of anhydrite in the sediments suggest that the Dead Dog hydrothermal field has been, and still is, active. In contrast, similar data in the Bent Hill hydrothermal field indicate a waning of hydrothermal activity. Pore fluid and hydrothermal vent data in the Dead Dog hydrothermal field are similar in nature to the data collected during ODP Leg 139. In the area of the Bent Hill sulfide deposit, however, the pore water data indicate that recent wholesale flushing of the sediment column with relatively unaltered seawater has obliterated a previous record of hydrothermal activity in the pore fluids. Data from the deepest part of Hole 1035A in the Bent Hill locality show the presence of hydrothermal fluids at greater depths in this area. This suggests the origin of the hydrothermal fluids found to be emanating from Hole 1035F, which constitutes one of the first man made hydrothermal vents in the Middle Valley hydrothermal system. Similarly, CORKed Hole 858G, because of seal failures, has acted as a hydrothermal vent, with sulfide deposits forming inside the CORK.  相似文献   

3.
Four seep sites located within an ∼20 km2 area offshore Georgia (Batumi seep area, Pechori Mound, Iberia Mound, and Colkheti Seep) show characteristic differences with respect to element concentrations, and oxygen, hydrogen, strontium, and chlorine isotope signatures in pore waters, as well as impregnation of sediments with petroleum and hydrocarbon potential. All seep sites have active gas seepage, near surface authigenic carbonates and gas hydrates. Cokheti Seep, Iberia Mound, and Pechori Mound are characterized by oil-stained sediments and gas seepage decoupled from deep fluid advection and bottom water intrusion induced by gas bubble release. Pechori Mound is further characterized by deep fluid advection of lower salinity pore fluids. The Pechori Mound pore fluids are altered by mineral/water reactions at elevated temperatures (between 60 and 110 °C) indicated by heavier oxygen and lighter chlorine isotope values, distinct Li and B enrichment, and K depletion. Strontium isotope ratios indicate that fluids originate from late Oligocene strata. This finding is supported by the occurrence of hydrocarbon impregnations within the sediments. Furthermore, light hydrocarbons and high molecular weight impregnates indicate a predominant thermogenic origin for the gas and oil at Pechori Mound, Iberia Mound, and Colkheti Seep. C15+ hydrocarbons at the oil seeps are allochtonous, whereas those at the Batumi seep area are autochthonous. The presence of oleanane, an angiosperm biomarker, suggests that the hydrocarbon source rocks belong to the Maikopian Formation. In summary, all investigated seep sites show a high hydrocarbon potential and hydrocarbons of Iberia Mound, Colkheti Seep, and Pechori Mound are predominantly of thermogenic origin. However, only at the latter seep site advection of deep pore fluids is indicated.  相似文献   

4.
Supercritical CO2 (scCO2) is a good solvent for organic compounds such as benzene, toluene, ethyl-benzene, and xylene (BTEX), phenols, and polycyclic aromatic hydrocarbons (PAHs). Monitoring results from geological carbon sequestration (GCS) field tests have shown that organic compounds are mobilized following CO2 injection. Such results have raised concerns regarding the potential for groundwater contamination by toxic organic compounds mobilized during GCS. Knowledge of the mobilization mechanism of organic compounds and their transport and fate in the subsurface is essential for assessing risks associated with GCS. Extraction tests using scCO2 and methylene chloride (CH2Cl2) were conducted to study the mobilization of volatile organic compounds (VOCs, including BTEX), the PAH naphthalene, and n-alkanes by scCO2 from representative reservoir rock and caprock obtained from depleted oil reservoirs and coal from an enhanced coal-bed methane recovery site. Results showed that the extent of mobilization for the organic compounds was a function of the source rock. In fate and transport sand column experiments, moisture content was found to have an important influence on the transport of the organic compounds. In dry sand columns the majority of the compounds were retained in the column except benzene and toluene. In wet sand columns the mobility of the BTEX was much higher than that of naphthalene. Based upon the results determined for the reservoir rock, caprock, and coal samples studied here, the risk to aquifers from contamination by organic compounds appears to be relatively low; however, further work is necessary to fully evaluate the risks.  相似文献   

5.
The composition and temperature of vent fluids sampled from the active hydrothermal system in Escanaba Trough, Gorda Ridge in 2000 and 2002 remain unchanged from the only time this field was previously sampled, in 1988. ODP Leg 169 drilled nine bore holes at this site in 1996, some within meters of the vents, yet this disturbance has not impacted the measured compositions or temperatures of the fluids exiting at the seafloor. The fluids have maximum measured temperatures of 218°C and contain ∼20% more chloride than local ambient seawater. Our interpretation is that the fluid compositions are generated by supercritical phase separation of seawater, with much of the water-rock reaction occurring within the ∼400m thick sedimentary section that overlies the basalt at this site. The ODP drilling results provide information on the mineralogy and composition of materials below the seafloor, as well as direct constraints not typically available on the physical conditions occurring below the seafloor hydrothermal system. Calculations utilizing geochemical modeling software suggest the fluids are close to saturation with a suite of minerals found subsurface, suggesting equilibrium between the fluids and substrate. These results provide an explanation for why the fluids have remained chemically stable for 14 yrs. The pore water data from drilling suggest that the hydrology and chemistry of the hydrothermal system are much more complex within the sediment cover than would be expected from the surface manifestations of the hydrothermal system. While the pore waters have chloride contents both greater and less than the local seawater, only fluids with higher chloride contents vent at the seafloor. Our calculations suggest that at the current conditions the “brines” (fluids with chlorinity greater than seawater) are actually less dense than the “vapors” (fluids with chlorinity less than seawater). These density relationships may provide an explanation for why the “brines” are now venting preferentially to the “vapors,” a situation opposite to what is usually observed or inferred.  相似文献   

6.
The surface sediments of two mud mounds (“Mound 11” and “Mound 12”) offshore southwest Costa Rica contain abundant authigenic carbonate concretions dominated by high-Mg calcite (14–20 mol-% MgCO3). Pore fluid geochemical profiles (sulfate, sulfide, methane, alkalinity, Ca and Mg) indicate recent carbonate precipitation within the zone of anaerobic oxidation of methane (AOM) at variable depths. The current location of the authigenic carbonate concretions is, however, not related to the present location of the AOM zone, suggesting mineral precipitation under past geochemical conditions as well as changes in the flow rates of upward migrating fluids. Stable oxygen and carbon isotope analysis of authigenic carbonate concretions yielded δ18Ocarbonate values ranging between 34.0 and 37.7 ‰ Vienna standard mean ocean water (VSMOW) and δ13Ccarbonate values from ?52.2 to ?14.2 ‰ Vienna Pee Dee belemnite (VPDB). Assuming that no temperature changes occurred during mineral formation, the authigenic carbonate concretions have been formed at in situ temperature of 4–5 °C. The δ18Ocarbonate values suggest mineral formation from seawater-derived pore fluid (δ18Oporefluid = 0 ‰ VSMOW) for Mound 12 carbonate concretions but also the presence of an emanating diagenetic fluid (δ18Oporefluid ≈5 ‰) in Mound 11. A positive correlation between δ13Ccarbonate and δ18Ocarbonate is observed, indicating the admixing of two different sources of dissolved carbon and oxygen in the sediments of the two mounds. The carbon of these sources are (1) marine bicarbonate (δ13Cporefluid ≈0 ‰) and (2) bicarbonate which formed during the AOM (δ13Cporefluid ≈?70 ‰). Furthermore, the δ18Oporefluid composition, with values up to +4.7 ‰ Vienna standard mean ocean water (VSMOW), is interpreted to be affected by the presence of emanating, freshened and boron-enriched fluids. Earlier, it has been shown that the origin of 18O-enriched fluids are deep diagenetic processes as it was indicated by the presence of methane with thermogenic signature (δ13CCH4 = ?38 ‰). A combination of present geochemical data with geophysical observations indicates that Mounds 11 and 12 represent a single fluid system interconnected by deep-seated fault(s).  相似文献   

7.
This paper deals with natural temperature records in the heavy (asphaltenes) and the light fractions (C7—light hydrocarbons) of petroleum. Two sets of marine oils formed from different source rocks and petroleum systems were studied using asphaltene kinetics and light hydrocarbon analysis. Both fractions have been reported to contain information about the temperature the respective oils have been exposed to in the subsurface. These indicated temperatures generally correspond to the conditions in the source rock when expulsion occurred. Bulk kinetic analysis of reservoir oil asphaltenes as well as light hydrocarbon (LH) analysis (of dimethylpentanes) were used here in order to evaluate the expulsion temperatures. Surprisingly, when considering information coming from both fractions, an inverse trend between LHs expulsion temperatures (Ctemp) and asphaltenes (Tasph.) can be observed—high Tasph (asphaltene temperatures) occur with low LH Ctemp (light hydrocarbon expulsion temperatures) and low Tasph can be observed when Ctemp is high. These differences are of fundamental importance for the use of such geochemical data in calibrating numerical basin models. The reason for this inverse behaviour is possibly due to the different expulsion behaviour of light hydrocarbons and the heavy fraction of oils, especially when the source rocks contain only moderate amounts of organic matter. In addition it has to be considered that the temperature predictions obtained using asphaltene kinetic analysis are related to the onset temperature of petroleum expulsion, while light hydrocarbons provide, at best, average expulsion temperatures.  相似文献   

8.
Variation in the degree of aromatic condensation of chars   总被引：1，自引：0，他引：1  
Char is ubiquitous in the environment and contributes significantly to the inert soil organic carbon pool. Its roles as a carbon sink and a strong sorbent of organic contaminants have come under increasing attention. However, quantifying these roles is difficult because properties such as degradability and sorption affinity vary greatly amongst chars, especially those produced at different temperatures. Here we describe a simple method for gauging the degree of aromatic condensation of chars, a molecular-scale property that affects both their degradability and sorption affinity. The method involves adding 13C6 benzene directly to char. The 13C nuclear magnetic resonance (NMR) chemical shift of the sorbed benzene is affected by diamagnetic ring currents in the fused aromatic structures in the char. These ring currents increase in magnitude with increasing extent of aromatic condensation. Seventeen heat-treated materials were analysed in this way. Our results confirm that aromatic condensation increases with increasing heat treatment temperature (HTT) and that activated chars contain the most highly condensed aromatic structures, but also show the importance of starting material and heat treatment time on aromatic condensation. We also show for four of the materials that the ring current effect on chemical shift was similar for other 13C-labelled molecules, including MeOH, CH3CN and toluene.  相似文献   

9.
In recent surface sediments there is no indication of any of the saturated C3–C7 gasolinerange hydrocarbons which are so common in petroleum. Appreciable gasoline-range hydrocarbon generation (85–180°C) of 80 ppm by weight of dry rock, or more, occurs only with increased temperature due to deeper burial, below about 8000 ft in the Los Angeles basin and below 12,500 ft in the Ventura basin. Because of the lower temperature gradient in the Ventura basin, the zone of substantial gasoline generation is considerably deeper there than in the Los Angeles basin. However, the subsurface temperature range over which substantial gasoline generation occurs is practically the same in the two basins. This demonstrates that the subsurface temperature, not depth, is the controlling factor in gasoline generation in source rocks. For appreciable gasoline generation somewhat higher subsurface temperatures are required than for equivalent generation of heavy hydrocarbons boiling above 325°C. Appreciable generation of the C1–C4 wet gas components of 75 ppm by weight of dry rock, or more, takes place quite deep also; in the Los Angeles basin it occurs below 10,000 ft.The composition of the gasoline-range hydrocarbons generated changes gradually with increasing depth, temperature and age of the shales. In deep strata the gasolines from shale cannot be distinguished from the gasolines of waxy crude oils in the same basin. The gasoline-range hydrocarbons mature with depth, temperature and age of the sediments, very much like the heavy hydrocarbons investigated earlier.Based on the similarity of analyses of heavy as well as of gasoline-range hydrocarbons from crude oils and from certain deep shales, a secure identification has been made of mature oil source beds in the Los Angeles and San Joaquin Valley oil basins of California. The combined results of these studies provide strong evidence for the origin of petroleum from the organic matter of sediments.  相似文献   

10.
Calcite veins with fluid and solid bitumen inclusions have been discovered in the south-western shoulder of the Dead Sea rift within the Masada-Zohar block, where hydrocarbons exist in small commercial gas fields and non-commercial fields of heavy and light oils. The gas–liquid inclusions in calcite are dominated either by methane or CO2, and aqueous inclusions sometimes bear minor dissolved hydrocarbons. The enclosed flake-like solid bitumen matter is a residue of degraded oil, which may be interpreted as “dead carbon”. About 2/3 of this matter is soot-like amorphous carbon and 1/3 consists of n-C8C18 carboxylic acids and traces of n-alkanes, light dicarboxylic acids, and higher molecular weight (>C20) branched and/or cyclic carboxylic acids. Both bitumen and the host calcites show genetic relationship with mature Maastrichtian chalky source rocks (MCSRs) evident in isotopic compositions (δ13C, δ34S, and δ18O) and in REE + Y patterns. The bitumen precursor may have been heavy sulfur-rich oil which was generated during the burial compaction of the MCSR strata within the subsided blocks of the Dead Sea graben. The δ18O and δ13C values and REE + Y signatures in calcites indicate mixing of deep buried fluids equilibrated with post-mature sediments and meteoric waters. The temperatures of fluid generation according to Mg–Li-geothermometer data range from 55 °С to 90 °С corresponding to the 2.5–4.0 km depths, and largely overlap with the oil window range (60–90 °С) in the Dead Sea rift (Hunt, 1996; Gvirtzman and Stanislavsky, 2000; Buryakovsky et al., 2005). The bitumen-rich vein calcites originated in the course of Late Cenozoic rifting and related deformation, when tectonic stress triggers damaged small hydrocarbon reservoirs in the area, produced pathways, and caused hydrocarbon-bearing fluids to rise to the subsurface; the fluids filled open fractures and crystallized to calcite with entrapped bitumen. The reported results are in good agreement with the existing views of maturation, migration, and accumulation of hydrocarbons, as well as basin fluid transport processes in the Dead Sea area.  相似文献   

11.
The Murchison meteorite contains aliphatic and aromatic hydrocarbons similar to those made in static Fischer-Tropsch-type syntheses. Principal compound classes above C8 are n-alkanes, mono- and dimethylalkanes, alkenes, alkylbenzenes and -naphthalenes. Below C8, n-alkanes are virtually absent; instead, benzene, toluene, branched alkanes dominate. The CH4/C2H6 ratio is greater than 30, possibly greater than 700. Isoprenoids from C17 to C20 occur in a surface rinse but not in subsequent extracts and appear to be terrestrial contaminants. Thiophenes, porphyrin-like pigments and chlorobenzenes were also found; the latter appear to be contaminants. In the Allende meteorite, only methane, benzene, toluene and an aromatic polymer seem to be indigenous.  相似文献   

12.
Volatile C1C8 organic compounds were analyzed in seven cores recovered from a transect across the Peru Shelf Upwelling Region. Compounds detected in ng/g dry weight quantities included methane, ethane, propane and other C4C7 alkanes as well as functionalized compounds including alkenes, furans and aldehydes. Strong correlations were observed between some groups of compounds with similar structures. Maxima in two groups of compounds were observed: (1) C1, C2, C3, furans, and butanals (plus sporadic alkanes) as nitrate levels decreased below about 2–4 μM in the subsurface, and (2) C4C6 alkenes together with predominantly branched and cycloalkanes at the sediment-water interface in cores recovered from oxygenated bottom waters. The levels of C1C7 alkanes in these sediments are comparable to levels found in DSDP cores buried to depths of less than 600 m (or not exposed to geothermal temperatures exceeding about 30°C).  相似文献   

13.
Ozokerite, a natural mixture of long-chain paraffins, is found at Ein-Humar (Jordan) on the eastern escarpment of the Dead Sea. The Ein-Humar ozokerite is mainly composed of saturated n-paraffins from C30 to presumably well into the C50′s with a maximum at C38. IR, NMR and chromatographic data indicate the absence of cycloparaffinic and aromatic molecules. The ozokerite is assumed to be related to the heavy asphalts and oils in the Dead Sea area. It probably forms by separation of high-molecular paraffins from non-paraffinaceous heavy oils or asphalts by geothermal heat and precipitation in the conduits due to lowering of temperature.  相似文献   

14.
Using chemical analyses and 25° pH measurements of quenched high-temperature waters, we calculate in situ pH and distribution of aqueous species at high temperature. This is accomplished by solving simultaneous mass action equations for complexes and redox equilibria and mass balance equations, on all components, including a H+ equation with as many as 60 terms (depending on water composition). This calculation provides accurate values for the activities of aqueous ions in a given water at high temperature, which are used to calculate an ion activity product (Q) for each of more than 100 minerals. The value of log(Q/K) for each mineral, where K is the equilibrium constant, provides a measure of proximity of the aqueous solution to equilibrium with the mineral. By plotting log Q/Kvs. T for natural waters, it is possible to determine: a) whether the water was in equilibrium with a host rock mineral assemblage, b) probable minerals in the equilibrium assemblage and c) the temperature of equilibrium. In cases where the fluid departs from equilibrium with a host rock assemblage, it is possible to determine whether this may result from boiling or dilution, and an estimate of amount of lost gas or diluting water can be determined.The calculation is illustrated by application to geothermal waters from Iceland, Broadlands, and Sulphur Bank, hot spring waters from Jemez, Yellowstone and Blackfoot Reservoir (Idaho) and fluid inclusions from the Sunnyside Mine, Colorado. It is shown that most geothermal waters approach equilibrium with a subsurface mineral assemblage at a temperature close to measured temperatures and that some hot springs also approach equilibrium with the host rock at temperatures above outlet temperatures but commonly below the Na-K-Ca temperatures. The log Q/K plots show that some discrepancies between Na-K-Ca temperatures on spring waters and actual temperatures result from a failure of alkali feldspars to equilibrate with the fluid and with each other.Calculations on Sulphur Bank fluids show that boiling probably caused cinnabar precipitation near 150°C and that the boiled fluids equilibrated with secondary minerals near 150° even though temperatures up to 185° have been measured at depth. For the fluid inclusions, the measured bubble temperatures are close to those calculated for equilibration of the fluid with the observed sulfide mineral assemblage.New estimates of stability constants for aluminum hydroxide complexes are included at the end of the paper.  相似文献   

15.
Subsurface reservoir temperatures of two important Mexican geothermal systems (Los Azufres and Las Tres Vírgenes) were estimated by applying all available solute geothermometers for 88 and 56 chemical data measurements of the spring waters and fluids of the deep geothermal wells, respectively. Most of the chemical data for spring water of these two geothermal fields are for HCO3 water, followed by SO4 and Cl types. For the Los Azufres geothermal field (LAGF), the reservoir temperatures estimated by Na-K geothermometers for springs of HCO3 and SO4 waters, and by Na-Li and Li-Mg geothermometers for Cl water, are close to the average bottom-hole temperature (BHT) of the geothermal wells. However, all reservoir temperatures for spring waters from the Las Tres Vírgenes geothermal field (LTVGF) estimated by all solute geothermometers indicated significantly large differences (low temperatures) compared to the BHT. Evaluation of inferred reservoir temperatures for spring waters of the LAGF and LTVGF suggests that not all springs nor all solute geothermometers provide reliable estimation of the reservoir temperatures. Even though chemical equilibrium probably was not achieved in the water–rock system, Na-K geothermometers for HCO3 water (peripheral water mainly of meteoric origin with little geothermal component) and SO4 water (geothermal steam heated) and Na-Li and Li-Mg geothermometers for Cl-rich spring water (fully mature geothermal water) of the LAGF indicated reservoir temperatures close to the BHT. However, in comparison with the geothermometry of spring water of the LAGF and LTVGF, fluid measurements from geothermal wells of these two fields indicated reservoir temperatures in close agreement with their respective BHTs. For the best use of the solute geothermometry for spring water, it is advisable to: (1) chemically classify the springs based on water types; (2) identify and eliminate the discordant outlier observations by considering each water type as a separate sampled population; (3) apply all available solute geothermometers employing a suitable computer program such as SolGeo instead of using some specific, arbitrarily chosen geothermometers; and (4) evaluate the temperatures obtained for each solute geothermometer by considering the subsurface lithology, hydrological conditions, and BHTs or static formation temperatures whenever available.  相似文献   

16.
C1-C7 volatile organic compounds were analyzed in three gravity cores taken from Walvis Bay shelf. The compounds detected included alkanes (methane, ethane, propane, i- and n-butane, and i- and n-pentane, and heptane), alkenes (2-methyl-2-butene, dimethylcyclopentenes, cyclohexene), oxygen containing compounds (2- and 3-methylfuran, 2,5-dimethylfuran, 2- and 3-methylbutanal and 3-pentanone), sulfur compounds (dimethylsulfide, thiophene, 2- and 3-methylthiophene) and aromatic compounds (benzene and toluene). In situ biological and low temperature chemical (less than 15°C) formation processes are proposed, possibly from marine terpene precursors. Subsequent to this work, these compounds were found to be widely distributed in surface gravity cores from other areas. Many of these compounds do not survive deeper burial. Furans, ketocompounds, and alkenes are generally not found in more than trace quantities in deeper (?10m subbottom) DSDP cores we have examined from other areas.  相似文献   

17.
Light hydrocarbons in subsurface sediments   总被引：1，自引：0，他引：1  
The major features and numerous compositional details of the indigenous C2–C7 hydrocarbon suites of argillaceous sediments are systematically temperature dependent. The relative concentrations of alicyclic compounds exhibit a consistent maximum at subsurface temperatures close to 170°F (77°C) without regard to the chemical nature of the bulk of the kerogen, whether rich or poor in hydrogen, though this strongly affects the specific yield. A continuous increase in relative alkane content follows at higher temperatures. Indices of paraffinicity may be devised. One such, termed the ‘heptane value’ (essentially the percentage of n-heptane in the b.p. range 80.7–100.9°C), possesses a linear association with temperature, provides an index of catagenesis, and frequently provides a means of appraising paleotemperatures. Regressions of heptane value on temperature are compared in two composite stratigraphic sections dominated by kerogens representing two extremes of composition. The regression coefficients differ by 7%. Yields of light hydrocarbons increase exponentially in these sections by more than three orders of magnitude along sub-parallel, temperature-dependent curves. These similarities infer universally similar generating reactions and compositionally similar suites of light hydrocarbons at given subsurface temperatures, regardless of kerogen type, particularly for sections which underwent burial and heating during the Tertiary period.  相似文献   

18.
Post-Variscan hydrothermal base-metal mineralization of the Taunus ore district, SE Rhenish Massif (Germany), has been studied through combination of stable (S, C, O) and radiogenic (Pb) isotope geochemistry. Based on field and textural observations, five hydrothermal mineralization types can be distinguished. These are (1) tetrahedrite–tennantite bearing quartz–ankerite veins, (2) quartz veins with Pb–Zn–Cu ores, (3) giant quartz veins, (4) metasomatic dolomite in Devonian reef complexes, and (5) calcite–(quartz) mineralization in Devonian reefs. The δ18OV-SMOW quartz values of base-metal veins are in the range of 18.0–21.5‰, whereas those of giant quartz veins have lower values of 15.9–18.6‰. This difference reflects the higher fluid fluxes and smaller extent of rock-buffering for the giant quartz veins. Hydrothermal carbonates from the tetrahedrite and Pb–Zn–Cu veins have variable but distinctly negative δ13CV-PDB values. They can be explained by contributions from fluids that had picked up low δ13CV-PDB carbon via oxidation of organic matter and from fluids that interacted with Devonian reef carbonate having positive δ13CV-PDB. Metasomatic dolomite has positive δ13CV-PDB values that closely reflect those of the precursor limestone. By contrast, carbonates of calcite–(quartz) mineralization have negative δ13CV-PDB values which are negatively correlated with the δ18O values. This pattern is explained by fluid mixing processes where contributions from descending cooler fluids with rather low salinity were dominant. The isotope data suggest that tetrahedrite veins, Pb–Zn–Cu veins, and giant quartz veins formed from fluid mixing involving two end-members with contrasting chemical features. This is supported by fluid inclusion data (Adeyemi, 1982) that show repeated alternation between two different types of fluid inclusions, which are hotter intermediate- to high-salinity NaCl–CaCl2 fluids and cooler low-salinity NaCl-dominated fluids. The metal-rich saline fluids were likely generated at the boundary between the pre-Devonian basement and the overlying Devonian–Carboniferous nappe pile. Fault activation resulted in strong fluid focusing and upward migration of large volumes of hot Na–Ca brines, which mixed with cooler and more dilute fluids at shallower crustal levels. Variable contributions from both fluid types, local fluid fluxes, temperature variations, and variations in pH and oxidation state have then controlled the vein mineralogy and metal inventory.  相似文献   

19.
Thermal cracking of kerogens and bitumens is widely accepted as the major source of natural gas (thermal gas). Decomposition is believed to occur at high temperatures, between 100 and 200°C in the subsurface and generally above 300°C in the laboratory. Although there are examples of gas deposits possibly generated at lower temperatures, and reports of gas generation over long periods of time at 100°C, robust gas generation below 100°C under ordinary laboratory conditions is unprecedented. Here we report gas generation under anoxic helium flow at temperatures 300° below thermal cracking temperatures. Gas is generated discontinuously, in distinct aperiodic episodes of near equal intensity. In one three-hour episode at 50°C, six percent of the hydrocarbons (kerogen & bitumen) in a Mississippian marine shale decomposed to gas (C1–C5). The same shale generated 72% less gas with helium flow containing 10 ppm O2 and the two gases were compositionally distinct. In sequential isothermal heating cycles (~1 hour), nearly five times more gas was generated at 50°C (57.4 μg C1–C5/g rock) than at 350°C by thermal cracking (12 μg C1–C5/g rock). The position that natural gas forms only at high temperatures over geologic time is based largely on pyrolysis experiments under oxic conditions and temperatures where low-temperature gas generation could be suppressed. Our results indicate two paths to gas, a high-temperature thermal path, and a low-temperature catalytic path proceeding 300° below the thermal path. It redefines the time-temperature dimensions of gas habitats and opens the possibility of gas generation at subsurface temperatures previously thought impossible.  相似文献   

20.
The content and distribution of mercury in Holocene–Upper Pleistocene turbidites, hemipelagic sediments intercalating therein, as well as basement basalts are studied. Samples of sediments were taken from the core of Holes 858A, 858B, 858C, 858D, and 858F. Basalt samples were taken from Holes 858F and 858G drilled during Leg 139 ODP in the Middle Valley (Juan de Fuca Ridge) in the Dead Dog hydrothermal field with a high heat flow (4–20 W/m2) and numerous vents with temperature ranging from 234 to 276°C. Samples of sediments and basalts with the background Hg content were taken from the core of Holes 855A, 855C, and 855D are located beyond the hydrothermal system in the base of the fault scarp on the eastern Middle Valley. In rocks, the content of Hg and its occurrence form were determined by the atomic absorption spectrometry with thermal atomization method; the chemical composition, by the XFA and ICP-MS methods. Sections of the sedimentary cover and basalt basement are marked by an alternation of “layer cake” type units with low and high contents of Hg. Mercury occurs in rocks in the physically adsorbed and mineral forms. The Hg concentration in some parts of the sedimentary section is anomalously high: up to 9696 ppb in Hole 858B and 7260 ppb in Hole 858C. In metalliferous sediments, the Hg content is 3130 ppb. Its maximum content (up to 23200 ppb) is recorded in basalts.  相似文献   

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