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1.
Submarine metalliferous sedimentary rocks are chemical precipitates resulted from hydrothermal exhalation near mid‐ocean ridge or faults. They record the submarine hydrothermal activity between lithosphere and hydrosphere and are critical for understanding Fe cycling in marine environment. Fe was expelled from the hydrothermal vent systems and was oxidized and precipitated in the ambient seawater, where the precipitation of hydrothermal Fe is largely controlled by oxidation state of seawater and is potentially revealed by its Fe isotope compositions. This hydrothermal process in modern hydrothermal vent systems have been well observed, but that for the ancient ones are still not well known. Umbers, or ferromanganoan sediments, overlying Troodos ophiolite in Cyprus of Mid‐Cretaceous age thus provides an excellent example for understanding the Fe cycles in ancient submarine hydrothermal process. Samples were collected from Margi village in Troodos and are mostly amorphous Fe‐Mn oxy‐hydroxides with very minor quartz, goethite, smectite and silicates such as clinopyroxene derived from the volcanic rocks. There is no terrestrial, detrital component. Samples were analyzed for their whole‐rock element and Fe isotope compositions. The results show that samples are composed mainly of SiO2 (13～80 wt%), Fe2O3 (9～54 wt%) and MnO (1.5～10.4 wt%), with minor Al2O3 (0.7～4.3 wt%). PAAS‐normalized REE patterns are near flat with significantly negative Ce anomalies (Ce/Ce* is from 0.2 to 0.5) and slightly positive Eu anomalies (Eu/Eu* is around 1.1), indicating a source from the oxidized seawater and the high‐temperature hydrothermal fluids. δ56FeIRMM‐014 values of samples are ‐0.32‰ to ‐0.15‰, with an average of ‐0.20‰, which are consistent with those of the hydrothermal fluids previously reported. The narrow Fe isotope compositions of Cyprus umbers that are close to those of submarine hydrothermal fluids indicates near complete oxidation of hydrothermal Fe2+ during its expulsion from the hydrothermal vent.  相似文献   

2.
Results of study of pore waters of bottom sediments from different Baikal basins are presented. The most typical ion distribution patterns reflecting the Baikal sediment diagenesis are given. We have established that in areas with regular sedimentation, in the absence of faults and inflows, the sediment pore waters of three lake basins inherit the chemical composition of the Baikal water, which is stable in time and space. Changes in pore water composition mark general natural anomalies, such as the presence of active faults, tectonic movements, and inflows along permeable zones. In areas with the subsurface occurrence of gas hydrates, thorough long-term research has revealed an anomalous composition of pore waters. It has been established that the anomalies are caused by a discharge of deep-level mud-volcanic fluids. The ejected mud-volcanic waters differ from each other in mineralization, ion composition, and sources, which determines the difference in hydrate formation and the composition of gas hydrates.  相似文献   

3.
The geochemical behaviors of hydrogen and oxygen isotopes in the hydrothermal system and their inher-ent relationship with the water / rock exchange are discussed in this paper In addition to the temperature con-ditions, the effective W / R ratio is another factor controlling the changes in H and O isotope compositions ofthe altered rock and hydrothermal water. Besides, the application and geological significance of the water-rockexchange theory are also discussed in the light of the H and O isotope compositions and their variation charac-teristics of the mineralizing hydrothermal water and altered rocks from several mineral deposits. Finally, abrief evolutional model of H and O istotope compositions of meteoric and magmatic hydrothermal waters in ahydrothermal system is given.  相似文献   

4.
We present results of a detailed mineralogical and geochemical study of the progressive hydrothermal alteration of clastic sediments recovered at ODP Site 858 in an area of active hydrothermal venting at the sedimented, axial rift valley of Middle Valley (northern Juan de Fuca Ridge). These results allow a characterization of newly formed phyllosilicates and provide constraints on the mechanisms of clay formation and controls of mineral reactions on the chemical and isotopic composition of hydrothermal fluids. Hydrothermal alteration at Site 858 is characterized by a progressive change in phyllosilicate assemblages with depth. In the immediate vent area, at Hole 858B, detrital layers are intercalated with pure hydrothermal precipitates at the top of the section, with a predominance of hydrothermal phases at depth. Sequentially downhole in Hole 858B, the clay fraction of the pure hydrothermal layers changes from smectite to corrensite to swelling chlorite and finally to chlorite. In three pure hydrothermal layers in the deepest part of Hole 858B, the clay minerals coexist with neoformed quartz. Neoformed and detrital components are clearly distinguished on the basis of morphology, as seen by SEM and TEM, and by their chemical and stable isotope compositions. Corrensite is characterized by a 24?Å stacking sequence and high Si- and Mg-contents, with Fe/(Fe+Mg) ratio of ≈0.08. We propose that corrensite is a unique, possibly metastable, mineralogical phase and was precipitated directly from seawater-dominated hydrothermal fluids. Hydrothermal chlorite in Hole 858B has a stacking sequence of 14?Å with Fe/(Fe+Mg) ratios of ≈0.35. The chemistry and structure of swelling chlorite suggest that it is a corrensite/chlorite mixed-layer phase. The mineralogical zonation in Hole 858B is accompanied by a systematic decrease in δ18O, reflecting both the high thermal gradients that prevail at Site 858 and extensive sediment-fluid interaction. Precipitation of the Mg-phyllosilicates in the vent region directly controls the chemical and isotopic compositions of the pore fluids. This is particularly evident by decreases in Mg and enrichments in deuterium and salinity in the pore fluids at depths at which corrensite and chlorite are formed. Structural formulae calculated from TEM-EDX analyses were used to construct clay-H2O oxygen isotope fractionation curves based on oxygen bond models. Our results suggest isotopic disequilibrium conditions for corrensite-quartz and swelling chlorite-quartz precipitation, but yield an equilibrium temperature of 300°?C±30° for chlorite-quartz at 32?m below the surface. This estimate is consistent with independent estimates and indicates steep thermal gradients of 10–11°/m in the vent region.  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.
Mixing is a dominant hydrogeological process in the hydrothermal spring system in the Cappadocia region of Turkey. All springs emerge along faults, which have the potential to transmit waters rapidly from great depths. However, mixing with shallow meteoric waters within the flow system results in uncertainty in the interpretation of geochemical results. The chemical compositions of cold and warm springs and geothermal waters are varied, but overall there is a trend from Ca–HCO3 dominated to Na–Cl dominated. There is little difference in the seasonal ionic compositions of the hot springs, suggesting the waters are sourced from a well-mixed reservoir. Based on δ18O and δ2H concentrations, all waters are of meteoric origin with evidence of temperature equilibration with carbonate rocks and evaporation. Seasonal isotopic variability indicates that only a small proportion of late spring and summer precipitation forms recharge and that fresh meteoric waters move rapidly into the flow system and mix with thermal waters at depth. 3H and percent modern carbon (pmC) values reflect progressively longer groundwater pathways from cold to geothermal waters; however, mixing processes and the very high dissolved inorganic carbon (DIC) of the water samples preclude the use of either isotope to gain any insight on actual groundwater ages.  相似文献   

7.
Rare earth element (REE) concentrations are reported for a large suite of seafloor vent fluids from four hydrothermal systems in the Manus back-arc basin (Vienna Woods, PACMANUS, DESMOS and SuSu Knolls vent areas). Sampled vent fluids show a wide range of absolute REE concentrations and chondrite-normalized (REEN) distribution patterns (LaN/SmN ∼ 0.6-11; LaN/YbN ∼ 0.6 - 71; ). REEN distribution patterns in different vent fluids range from light-REE enriched, to mid- and heavy-REE enriched, to flat, and have a range of positive Eu-anomalies. This heterogeneity contrasts markedly with relatively uniform REEN distribution patterns of mid-ocean ridge hydrothermal fluids. In Manus Basin fluids, aqueous REE compositions do not inherit directly or show a clear relationship with the REE compositions of primary crustal rocks with which hydrothermal fluids interact. These results suggest that the REEs are less sensitive indicators of primary crustal rock composition despite crustal rocks being the dominant source of REEs in submarine hydrothermal fluids. In contrast, differences in aqueous REE compositions are consistently correlated with differences in fluid pH and ligand (chloride, fluoride and sulfate) concentrations. Our results suggest that the REEs can be used as an indicator of the type of magmatic acid volatile (i.e., presence of HF, SO2) degassing in submarine hydrothermal systems. Additional fluid data suggest that near-seafloor mixing between high-temperature hydrothermal fluid and locally entrained seawater at many vent areas in the Manus Basin causes anhydrite precipitation. Anhydrite effectively incorporates REE and likely affects measured fluid REE concentrations, but does not affect their relative distributions.  相似文献   

8.
The Eastern Lau Spreading Center (ELSC) is the southernmost part of the back-arc spreading axis in the Lau Basin, west of the Tonga trench and the active Tofua volcanic arc. Over its 397-km length it exhibits large and systematic changes in spreading rate, magmatic/tectonic processes, and proximity to the volcanic arc. In 2005, we collected 81 samples of vent water from six hydrothermal fields along the ELSC. The chemistry of these waters varies both within and between vent fields, in response to changes in substrate composition, temperature and pressure, pH, water/rock ratio, and input from magmatic gases and subducted sediment. Hot-spring temperatures range from 229° to 363 °C at the five northernmost fields, with a general decrease to the south that is reversed at the Mariner field. The southernmost field, Vai Lili, emitted water at up to 334 °C in 1989 but had a maximum venting temperature of only 121 °C in 2005, due to waning activity and admixture of bottom seawater into the subseafloor plumbing system. Chloride varies both within fields and from one field to another, from a low of 528 mmol/kg to a high of 656 mmol/kg, and may be enriched by phase separation and/or leaching of Cl from the rock. Concentrations of the soluble elements K, Rb, Cs, and B likewise increase southward as the volcanic substrate becomes more silica-rich, especially on the Valu Fa Ridge. Iodine and δ7Li increase southward, and δ11B decreases as B increases, apparently in response to increased input from subducted sediment as the arc is approached. Species that decrease southward as temperature falls are Si, H2S, Li, Na/Cl, Fe, Mn, and 87Sr/86Sr, whereas pH, alkalinity, Ca, and Sr increase. Oxygen isotopes indicate a higher water/rock ratio in the three systems on Valu Fa Ridge, consistent with higher porosity in more felsic volcanic rocks. Vent waters at the Mariner vent field on the Valu Fa Ridge are significantly hotter, more acid and metal-rich, less saline, and richer in dissolved gases and other volatiles, including H2S, CO2, and F, than the other vent fields, consistent with input of magmatic gases. The large variations in geologic and geophysical parameters produced by back-arc spreading along the ELSC, which exceed those along mid-ocean ridge spreading axes, produce similar large variations in the composition of vent waters, and thus provide new insights into the processes that control the chemistry of submarine hot springs.  相似文献   

9.
《Applied Geochemistry》2002,17(11):1457-1466
Ocean Drilling Program (ODP) Leg 169, which was conducted in 1996 provided an opportunity to study the gas geochemistry in the deeper part of the sediment-rich hydrothermal system in Escanaba Trough. Gas void samples obtained from the core liner were analyzed and their results were compared with analytical data of vent fluid samples collected by a submersible dive program in 1988. The gas geochemistry of the pore fluids consisted mostly of a hydrothermal component and was basically the same as that of the vent fluids. The He isotope ratios (R/RA=5.6–6.6) indicated a significant mantle He contribution and the C isotopic compositions of the hydrocarbons [δ13C(CH4)=−43‰, δ13C(C2H6)=−20‰] were characterized as a thermogenic origin caused by hydrothermal activity. On the other hand, the pore fluids in sedimentary layers away from the hydrothermal fields showed profiles which reflected lateral migration of the hydrothermal hydrocarbons and abundant biogenic CH4. Helium and C isotope systematics were shown to represent a hydrothermal component and useful as indicators for their distribution beneath the seafloor. Similarities in He and hydrocarbon signatures to that of the Escanaba Trough hydrothermal system were found in some terrestrial natural gases, which suggested that seafloor hydrothermal activity in sediment-rich environments would be one of the possible petroleum hydrocarbon generation scenarios in unconventional geological settings.  相似文献   

10.
王焰新  陈德隆 《地球科学》1993,18(5):661-670
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11.
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12.
The study of bottom sediments of Lake Baikal recovered by submarine drilling at the Selenga–Buguldeika saddle (core VER93-2 st. 24GC) allowed us to reconstruct the climatic events in the Baikal region in the last 20–25 k.y. On the basis of the data on distribution of chemical elements in the core section, the mineral composition of sediments was calculated by the physicochemical modeling method. A study of how ratios of clay minerals changed in the section allowed us to identify the Pleistocene–Holocene boundary, Bølling–Allerød postglacial warming, and Late Dryas cooling. The calculated data on mineral composition of bottom sediments from the core VER93-2 demonstrate a good fit to the X-ray diffraction analysis results. The proposed approach can be used in calculation of mineral compositions of other sedimentary sequences with known chemical composition.  相似文献   

13.
Exchange of water between groundwater and surface water could alter water quality of the surface waters and thereby impact its ecosystem. Discharges of anoxic groundwater, with high concentrations of sulfate and chloride and low concentrations of nitrate and oxygen, from three sinkhole vents (El Cajon, Middle Island and Isolated) in Lake Huron have been recently documented. In this investigation, we collected and analyzed a suite of water samples from these three sinkhole vents and lake water samples from Lake Huron for Ra, radon-222, stable isotopes of oxygen and hydrogen, and other ancillary parameters. These measurements are among the first of their kind in this unique environment. The activities of Ra are found to be one to two orders of magnitude higher than that of the lake water. Isotopic signatures of some of the bottom lake water samples indicate evidences for micro-seeps at distances farther from these three vents. A plot of δD versus δ18O indicates that there are deviations from the Global Meteoric Line that can be attributed to mixing of different water masses and/or due to some subsurface chemical reactions. Using the Ra isotopic ratios, we estimated the transit times of the vent waters from the bottom to the top of the vent (i.e., sediment–water interface) to be 4–37 days. More systematic studies on the distribution of the radioactive and stable isotope studies are needed to evaluate the prevalence of micro-seeps in Lake Huron and other Great Lakes system.  相似文献   

14.
Uranium (U) concentrations and activity ratios (δ234U) of authigenic carbonates are sensitive recorders of different fluid compositions at submarine seeps of hydrocarbon-rich fluids (“cold seeps”) at Hydrate Ridge, off the coast of Oregon, USA. The low U concentrations (mean: 1.3 ± 0.4 μg/g) and high δ234U values (165-317‰) of gas hydrate carbonates reflect the influence of sedimentary pore water indicating that these carbonates were formed under reducing conditions below or at the seafloor. Their 230Th/234U ages span a time interval from 0.8 to 6.4 ka and cluster around 1.2 and 4.7 ka. In contrast, chemoherm carbonates precipitate from marine bottom water marked by relatively high U concentrations (mean: 5.2 ± 0.8 μg/g) and a mean δ234U ratio of 166 ± 3‰. Their U isotopes reflect the δ234U ratios of the bottom water being enriched in 234U relative to normal seawater. Simple mass balance calculations based on U concentrations and their corresponding δ234U ratios reveal a contribution of about 11% of sedimentary pore water to the bottom water. From the U pore water flux and the reconstructed U pore water concentration a mean flow rate of about 147 ± 68 cm/a can be estimated. 230Th/234U ages of chemoherm carbonates range from 7.3 to 267.6 ka. 230Th/234U ages of two chemoherms (Alvin and SE-Knoll chemoherm) correspond to time intervals of low sealevel stands in marine isotope stages (MIS) 2, 4, 5, 6, 7 and 8. This observation indicates that fluid flow at cold seep sites sensitively reflects pressure changes of the hydraulic head in the sediments. The δ18OPDB ratios of the chemoherm carbonates support the hypothesis of precipitation during glacial times. Deviations of the chemoherm δ18O values from the marine δ18O record can be interpreted as to reflect temporally and spatially varying bottom water and/or vent fluid temperatures during carbonate precipitation between 2.6 and 8.6°C.  相似文献   

15.
The Wakamiko submarine crater is a small depression located in Kagoshima Bay, southwest Japan. Marine shallow‐water hydrothermal activity associated with fumarolic gas emissions at the crater sea floor (water depth 200 m) is considered to be related with magmatic activity of the Aira Caldera. During the NT05‐13 dive expedition conducted in August 2005 using remotely operated vehicle Hyper‐Dolphine (Japan Agency for Marine‐Earth Science and Technology), an active shimmering site was discovered (tentatively named the North site) at approximately 1 km from the previously known site (tentatively named the South site). Surface sediment (up to 30 cm) was cored from six localities including these active sites, and the alteration minerals and pore fluid chemistry were studied. The pore fluids of these sites showed a drastic change in chemical profile from that of seawater, even at 30 cm below the surface, which is attributed to mixing of the ascending hydrothermal component and seawater. The hydrothermal component of the North site is estimated to be derived from a hydrothermal aquifer at 230°C based on the hydrothermal end‐member composition. Occurrence of illite/smectite interstratified minerals in the North site sediment is attributed to in situ fluid–sediment interaction at a temperature around 150°C, which is in accordance with the pore fluid chemistry. In contrast, montmorillonite was identified as the dominant alteration mineral in the South site sediment. Together with the significant low potassium concentration of the hydrothermal end‐member, the abundant occurrence of low‐temperature alteration mineral suggests that the hydrothermal aquifer in the South site is not as high as 200°C. Moreover, the montmorillonite is likely to be unstable with the present pore fluid chemistry at the measured temperature (117°C). This disagreement implies unstable hydrothermal activity at the South site, in contrast to the equilibrium between the pore fluid and alteration minerals in the North site sediment. This difference may reflect the thermal and/or hydrological structure of the Wakamiko Crater hydrothermal system.  相似文献   

16.
The chemical, gas, and isotopic compositions of nitric thermal waters in the Baikal Rift Zone are considered. It is shown that the behavior of sulfate and carbonate ions in hydrothermal systems is different, which indicates that they are of two different origins. The studied thermal waters are of five chemical types formed in different geologic conditions. Special attention is given to the genesis of hydrotherms, the geologic and geomorphologic conditions of their recharge, and their equilibrium with rocks. It has been established that most of chemical elements of the waters migrated from rocks, but a significant portion of them is bound by secondary minerals, which results in their deep differentiation, accumulation, or precipitation. Thus, the so-called redundant elements appear, which were earlier considered to be of mantle origin.  相似文献   

17.
The complex study of the river water and pore solutions from the bottom sediments in the lower reaches of the Razdol’naya River was conducted in February 2010. The major ion composition of the waters indicates the submarine origin of the near-bottom and pore waters in the lower reaches of the Razdol’naya River in the winter. The river estuary extends upstream for more than 20 km. It was established that the studied sediments are reduced oozes containing pyrite, hydrotroilite, and iron monosulfide, which is direct evidence for sulfate-reduction in the sediments. The diagenesis of organic matter is the main reason for the considerable decrease in the amount of sulfates and the increase in the alkalinity of the sediment pore water. The sedimentary pore water sampled from the deep river pits is characterized by excess alkalinity that cannot be explained by sulfate-reduction and methane genesis. It was suggested that the chemical weathering of silicate minerals and the bacterial mineralization of salts of organic acids could result in the excess alkalinity of the sediment pore water.  相似文献   

18.
We report results of bottom temperature monitoring of 2003–2004 in the deepwater South Baikal basin (Lake Baikal) near active gas-fluid methane vents at lake depths of 1020 and 1350 m. Sediments and water temperatures were measured using an autonomous temperature recorder designed at the Institute of Geophysics (Novosibirsk). Experiments implied short-duration recording and pioneering continuous 350 day-long monitoring near the Staryi vent. Measurements within a 1 m thick layer above and below the bottom showed notable variations in water (up to 0.07 °C) and sediment temperatures and in geothermal gradient. The long temperature records include a relatively steady period (mid-June 2003-early February 2004) with smooth temperature variations (especially in sediments) and two transient unsteady periods. The steady season is the best time for heat flow studies in the South Baikal basin. The 0.04–0.05 °C drop in bottom water temperature during the unsteady periods may result from intrusion of cold surface water. A positive temperature anomaly of ∼0.04 °C recorded in April 2003 may be caused, among other reasons, by active gas venting.  相似文献   

19.
Abstract. The Onsen site is an active submarine hydrothermal system hosted by the Desmos caldera in the Eastern Manus Basin, Papua New Guinea. The hydrothermal fluid is very acidic (pH=1.5) and abundant native sulfur is deposited around the vent. The δ34S values of native sulfur range from -6.5 to -9.3 %o. δ34S values of H2S and SO4 in the hydrothermal fluid are -4.3 to -9.9 %o and +18.6 to +20.0 %o, respectively. These δ34S values are significantly lower than those of the other hydrothermal systems so far reported. These low δ34S values and the acidic nature of the vent fluids suggest that volcanic SO2 gas plays an important role on the sulfur isotope systematic of the Onsen hydrothermal system. Relationship among the δ34S values of S-bearing species can be successively explained by the model based on the disproportionation reaction starting from the volcanic SO2 gas. The predicted δ34S values of SO2 agree with the measured whole rock δ34S values. δD and δ18O values of clay minerals separated from the altered rock samples also suggest the contribution of the magmatic fluid to the hydrothermal system. Present stable isotopic study strongly suggests that the Onsen hydrothermal site in the Desmos caldera is a magmatic submarine hydrothermal system.  相似文献   

20.
This work is a review of the modern concepts of the accumulation of Fe and Mn in the Baikal bottom sediments and the regularities of formation of iron-manganese (including phosphate-bearing) nodules in the lake. Special attention is given to the probable participation of hydrothermal water in this process and the genesis of ancient nodules deeply buried in the Baikal sediments. The possible relationship between the shore ores and the nodules in the bottom sediments in the adjacent Baikal regions has been first shown.  相似文献   

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