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Sulfur isotope investigations carried out on elemental sulfur and sulfates of the Nea Kameni solfataras, Santorini, Aegean Sea, Greece, show a clear enrichment in the heavy sulfur isotope 34S against the assumed primordial ratio of 22,220. Within the same crater, different vents, only a few meters apart from each other, produced δ differences up to 10‰, which remained constant for several years. This enrichment is most probably due to contamination by heavy sulfur from a nonvolcanic source. An enrichment in the same order of magnitude was observed in sulfur of recent and older lavas ().Potential contaminants like sulfide sulfur in hydrothermal ore veins of Athinios has a δ 34S mean value close to 0‰, sulfide and sulfate in the sedimentary basement has a δ 34S mean value of Seawater sulfate from the area gives a value of , while sulfide from bacterial reduction of pore-water sulfate in recent iron ore sediments has δ 34S values between ?8 and Sulfate remaining in the pore solutions gave .The most probable explanation for the observed high δ 34S values in the solfataric sulfur and in some of the lavas of the Santorini area is contamination of the volcanic vents by Mediterranean Sea water. 相似文献
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and ratios have been measured for whole-rock samples and mineral separates from the mafic and ultramatic rocks of the Cambro-Ordovician Highland Border Suite. The H- and O- isotopic compositions of these rocks record individual stages in a relatively complex 500 Myr old hydrothermal/metamorphic history. Lizardite serpentinites (δD ～ ? 105‰; δ18O ～ + 6.2‰) record a premetamorphic history and indicate that parent harzburgites, dunites, and pyroxenites were serpentinized through low-temperature interaction with meteoric waters during cooling. The other rocks of the Highland Border Suite record subsequent interaction with metamorphic fluids. Amphibolite facies hornblende schists were produced through thrust-related (dynamothermal) metamorphism of spilitic pillow lavas. During dehydration, D-enriched fluids were driven off from the spilites thus leaving the hornblende schists to equilibrate with a relatively D-depleted internal fluid reservoir (δD ～ ? 45‰). The expelled D-enriched fluids may have mixed with more typical Dalradian metamorphic waters which then exchanged with the remaining mafic rocks and lizardite serpentinites during greenschist facies regional metamorphism to produce antigorite serpentinites (δD ～ ? 62‰; δ18O ～ + 8‰) and greenschist metaspilites (δD ～ ? 57‰; δ18O ～ + 7.3‰) with similar H- and O-isotopic compositions. Serpentinites which have been only partially metamorphosed show intermediate H-isotopic compositions between that of metamorphic antigorite (δD ～ ? 62‰) and non-metamorphic lizardite δD ～ ? 105‰) end members. 相似文献
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The isotopic composition of strontium of pore water and of authigenic minerals leached from the sediment of core 119K with hot aqua regia is similar to that of the brine in the Discovery deep and differs from that of normal seawater. The average ratio of strontium removed by acid leaching is 0.7077 ± 0.0007 (1σ) compared to a value of 0.70904 for the Red Sea. The detrital silicate fraction exhibits an approximate inverse correlation between ratios and strontium concentrations which provides tentative support for a model in which the detrital silicate fraction of deep-sea sediment is considered to be a mixture of terrigenous dust of sialic composition enriched in radiogenic 87Sr and of volcanogenic material of basaltic composition and low 87Sr abundance. The ratios of the shells of foraminifers and pteropods, expressed as δ 87Sr‰ relative to 0.70904 for seawater, decrease from at 90 cm to at 273 cm and remain constant at this value to a depth of 450 cm. The lowering of the δ 87Sr values is attributed both to the presence of aragonite overgrowths on pteropod shells and to possible isotope exchange with strontium in the connate fluid. 相似文献
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The geochemical history of Lake Lisan, the Pleistocene precursor of the Dead Sea, has been studied by geological, chemical and isotopic methods.Aragonite laminae from the Lisan Formation yielded (equivalent) Sr/Ca ratios in the range 0.5 × 10?2?1 × 10?2, Na/Ca ratios from 3.6 × 10?3 to 9.2 × 10?3, values between 1.5 and 7%. and from ?7.7 to 3.4%..The distribution coefficient of Na+ between aragonite and aqueous solutions, , is experimentally shown to be very sensitive to salinity and nearly temperature independent. Thus, Na/Ca in aragonite serves as a paleosalinity indicator.Sr/Ca ratios and values in aragonite provide good long-term monitors of a lake's evolution. They show Lake Lisan to be well mixed, highly evaporated and saline. Except for a diluted surface layer, the salinity of the lake was half that of the present Dead Sea (15 vs 31%).Lake Lisan evolved from a small, yet deep, hypersaline Dead Sea-like, water body. This initial lake was rapidly filled-up to its highest stand by fresh waters and existed for about 40,000 yr before shrinking back to the present Dead Sea. The chemistry of Lake Lisan at its stable stand represented a material balance between a Jordan-like input, an original large mass of salts and a chemical removal of aragonite. The weighted average depth of Lake Lisan is calculated, on a geochemical basis, to have been at least 400, preferably 600 m.The oxygen isotopic composition of Lake Lisan water, which was higher by at least 3%. than that of the Dead Sea, was probably dictated by a higher rate of evaporation.Na/Ca ratios in aragonite, which correlate well with values, but change frequently in time, reflect the existence of a short lived upper water layer of varying salinity in Lake Lisan. 相似文献
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, and ratios have been measured on the same samples for carbonatite complexes. The results show that besides the ‘carbonatite box’ of Tayloret al. (1967) there exist higher δ18O and δ13C values than can be explained by late magmatic or deuteric processes. These processes correspond to high concentrations of CO2 and lead to big enrichments in 18O and 13C as well as in some ‘volatile’ elements. Strontium results are consistent with a model of selective contamination of deep-seated material by highly radiogenic strontium. The whole study leads to the opinion that parent magmas of carbonatites differentiated in a crustal environment with or without significant contamination. 相似文献
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Bartholomew Nagy Steven M. Kunen John E. Zumberge Austin Long Carleton B. Moore Charles F. Lewis Carl R. Anhaeusser Desmond A. Pretorius 《Precambrian Research》1974,1(1):43-48
Carbon content (0.02–0.68% organic), carbonate content (0–69.7%) and carbonate 13C abundances () were obtained on samples from the Swaziland sediments of South Africa, which are among the oldest known sedimentary rocks on earth (> 3·109 years old). The carbon chemistry of these sediments may serve as evidence for early life and/or for products of chemical evolution. The variation of organic and carbonate carbon concentrations in different sedimentary horizons seems to be controlled by differences in depositional and diagenetic histories. The carbonate δ 13C values did not vary significantly from the ordinary range of Phanerozoic limestone values. 相似文献
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The isotopic ratios and show an annual periodicity in the coral Montastrea annularis from Bermuda, Jamaica and Barbados. The abundances of 18O and 13C are positively correlated in the Jamaica and Barbados samples, but inversely related in the Bermuda sample. Annual high density growth bands are formed during the season of warmest water temperature at all 3 sites and are enriched in 16O. M. Annularis has a constant displacement from oxygen isotopic equilibrium and accurately records seasonal temperature variations via the temperature-dependent aragonite-water fractionation factor. Light intensity, through the activity of the coral's endosymbiotic algae, regulates the depth-dependent and seasonal variations in the skeletal carbon isotopic composition. 相似文献
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The Lackner Lake Complex of northern Ontario consists of nepheline syenite and ijolite with associated bodies of apatite—magnetite rock and carbonatite. A whole-rock Rb-Sr isochron indicates that these rocks crystallized 1078 ± 7 Ma ago () and had an initial 87Sr/86Sr ratio of 0.070282 ± 0.00011. This date is slightly lower than a K-Ar date of 1090 Ma for biotite reported previously, but is in agreement with K-Ar dates of several nearby syenite-carbonatite complexes. 相似文献
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and ratios were measured for 114 petroleum samples and for several samples of related organic matter. δD of crude oil ranges from ?85 to ?181‰, except for one distillate (?250‰) from the Kenai gas field; δ13C of crude oil ranges from ?23.3 to ?32.5‰, Variation in δD and δ13C values of compound-grouped fractions of a crude oil is small, 3 and 1.1%., respectively, and the difference in δD and δ13C between oil and coeval wax is slight. Gas fractions are 53–70 and 22.6–23.2‰ depleted in D and 13C, respectively, relative to the coexisting oil fractions.The δD and δ13C values of the crude oils appear to be largely determined by the isotopic compositions of their organic precursors. The contribution of terrestrial organic debris to the organic precursors of most marine crude oils may be significant. 相似文献
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Precambrian sedimentary carbonates: carbon and oxygen isotope geochemistry and implications for the terrestrial oxygen budget 总被引：2，自引：0，他引：2
Carbon isotope values of 260 Precambrian limestones and dolomites (most of them being substantially unaltered) have yielded an overall mean of vs. PDB; the corresponding oxygen values average at vs. SMOW. Like the overall mean, the δ 13C values furnished by individual carbonate occurrences are, as a rule, fairly “modern” and almost constant as from the very beginning of the sedimentary record. A remarkable exception are the “heavy” dolomites of the Middle Precambrian Lomagundi Group, Rhodesia, with vs. PDB. As a result of our measurements, the sporadic occurrence in the geological past of anomalously heavy carbonates seems to be established.The approximate constancy around zero per mill of the δ 13C values of marine carbonates through geologic time would imply a corresponding constancy of the relative proportion of organic carbon in the total sedimentary carbon reservoir since about 3.3 · 109 y ago (with Corg/Ctotal ? 0.2). Utilizing this ratio and current models for the accumulation of the sedimentary mass as a function of time, we get a reasonable approximation for the absolute quantity of organic carbon buried in sediments and, accordingly, of photosynthetic oxygen released. Within the constraints of our model (based on a terrestrial degassing constant λ = 1.16 · 10?9 y?1) close to 80% of the amount of oxygen contained in the present oxygen budget should have been released prior to 3 · 109 y ago. Since geological evidence indicates an O2-deficient environment during the Early and most parts of the Middle Precambrian, there is reason to believe that the distribution of this oxygen between the “bound” and the “molecular” reservoir was different from that of today (with effective O2-consuming reactions bringing about an instantaneous transfer to the crust of any molecular oxygen released). Accordingly, the amount of Corg in the ancient sedimentary reservoir as derived from our isotope data is just a measure of the gross amount of photosynthetic oxygen produced, withholding any information as to how this oxygen was partitioned between the principal geochemical reservoirs. As a whole, the carbon isotope data accrued provide evidence of an extremely early origin of life on Earth since the impact of organic carbon on the geochemical carbon cycle can be traced back to almost 3.5 · 109y. 相似文献
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The Mushandike Sanctuary, near Masvingo (Fort Victoria), Zimbabwe, contains well-preserved, but metamorphosed stromatolitic limestones of Archaean age. Despite the metamorphism, textural preservation is excellent to a scale as fine as c. 100–200μ. Cores recovered from the stromatolites have (PDB), approximately, and δ13C = 0.0 to 0.5‰ (PDB), in less weathered samples. The results are consistent with a metamorphic history involving volatile loss at 200°C or less, from an original source rock which may have been isotopically similar to the Cheshire stromatolites, Belingwe. 相似文献
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David B. Wenner 《Geochimica et cosmochimica acta》1979,43(4):603-614
, and analyses were made of 14 whole rock and 28 mineral samples of rodingites associated dominantly with lizardite-chrysotile serpentinites from the West Coast of the U.S.A., New Zealand, and the Northern Appalachian Mtns. The δD values of the rodingite minerals are in three groupings: 5 monomineralic veins of pectolite, ?281 to ?429; 8 monomineralic veins of xonotlite, ?112 to ?135; all other minerals, including hydrogarnet, idocrase, prehnite, actinolite, nephrite, and chlorite, ?34 to ?80. Most calcites in rodingites have δ18O (+9.3 to +14.4) and (δ13C (?6.7 to +0.9) values similar to calcites in other Franciscan rocks, but distinct from the very low temperature calcite veins in serpentinites. The data, combined with δ18O values of xonotlite (+5.7 to +10.9) and pectolite (+8.9 to +12.4) suggest formation from meteoric-type waters at low temperatures; the depletion of pectolite, however, is anomalous. Rodingite whole rock values range from δ18O = +4.1 to +11.5 and δD = ?50 to ?86; one sample containing minor amounts of lizardite-chrysotile serpentinite has δD = ?92, outside this range. However, most rodingites of basaltic or gabbroic parentage are more restricted in δ18O (+4.1 to +8.6). Such a wide range in δ18O is consistent with the idea that most rodingites form over a relatively broad range of hydrothermal temperatures. Hydrogen isotopic data for most rodingite minerals (except xonotlite and pectolite) and for whole rocks are suggestive of non-meteoric waters. These data overlap those observed for veins of hydrous minerals found in Franciscan igneous rocks studied by Margaritz and Taylor (1976, Geochim. Cosmochim. Acta40, 215–234), possibly suggesting evolved D-enriched, connate type metamorphic waters generated during high P, low T Franciscan-type metamorphism at temperatures (250–500°C) comparable to estimates based on mineral stabilities. Such an interpretation is supported by the and data for calcite in rodingites.The isotope data appear to contradict some of the conclusions derived from geologic and petrologic studies that indicate concomitant metasomatism and serpentinization of their presently observed host rock. These data appear most consistent with the interpretation that most rodingite minerals, with the exception of late-stage veins of xonotlite and possibly pectolite, may involve metasomatism in association with antigorite serpentinization of ultramafic rock. Subsequent upward tectonic transport in many instances may result in incorporation of the rodingites into their presently observed lizarditechrysotile host rock during or subsequent to pervasive shallow level serpentinization by meteoric waters. 相似文献
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The Archean Yellowknife Supergroup (Slave Structural Province. Canada) is composed of a thick sequence of supracrustal rocks, which differs from most Archean greenstone belts in that it contains a large proportion ( ~ 80%) of sedimentary rocks. Felsic volcanics of the Banting Formation are characterized by HREE depletion without Eu-anomalies, indicating an origin by small degrees of partial melting of a mafic source, with minor garnet in the residua. Granitic rocks include synkinematic granites [HREE-depleted; low ()I], post-kinematic granites [negative Eu-anomalies, high ()I] and granitic gneisses with REE patterns similar to the post-kinematic granites. Sedimentary rocks (turbidites) of the Burwash and Walsh Formations have similar chemical compositions and were derived from 20% mafic-intermediate volcanics, 55% felsic volcanics and 25% granitic rocks. Jackson Lake Formation lithic wackes can be divided into two groups with Group A derived from 50% mafic-intermediate volcanics and 50% felsic volcanics and Group B, characterized by HREE depletion, derived almost exclusively from felsic volcanics.REE patterns of Yellowknife sedimentary rocks are similar to other Archean sedimentary REE patterns, although they have higher . These patterns differ significantly from typical post-Archean sedimentary REE patterns, supporting the idea that Archean exposed crust had a different composition than the present day exposed crust. 相似文献
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For sulfates of Miocene evaporites in the Carpathian Foredeep, the waters of crystallization of gypsum (w.c.g.) have and (SMOW). The and values of the sulfates are uniform and consistent with a marine origin. It is proposed that the original w.c.g. was equilibrated with marine water. Subsequently, it re-equilibrated towards very isotopically light water (δD ～ ?100%., δ18O ～ ?14%) during a glacial or postglacial period and is now trending towards current waters circulating through the deposits (δD ～ ?50%., δ18 ～ ?7%). The extent of reequilibration increased with decreasing crystal size. 相似文献
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Sulfur isotopic studies of pyrite from metasediments in the >2.6 Byr old Deer Lake green-stone sequence, Minnesota, have been conducted in order to evaluate the possible importance of sulfate reducing bacteria in sulfide formation. Pyrite occurs as ovules up to 2 cm in diameter within graphitic slates, and as fine disseminations in metagraywacke units. SEM studies indicate the pyrite is framboidal in morphology. values of pyrite from the Deer Lake sediments range from ?2.3 to 11.1‰, with a peak at ~ +2‰ Isotopic data are consistent with either high temperature inorganic reduction of circulating seawater sulfate, or low temperature bacterial reduction. However, the lack of sulfide bands or massive occurrences in the sediments, the restriction of pyrite mineralization to the sediments, and the absence of evidence for hot spring activity suggest that a diagenetic origin of pyrite is more feasible. Sulfide in such an environment would be produced principally by the action of sulfate reducing bacteria.Results of the study are in agreement with those of Goodwinet al. (1976) who suggest that dissimilatory sulfate reduction was operative in the Archean ocean some 2.75 Byr ago. 相似文献
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Twenty-four groundwater samples from seven operating mines at Sudbury, Yellow-knife and Thompson (Ontario, North West Territories and Manitoba, resp.), all from depths greater than 1 km and ranging in total dissolved solids (TDS) from 1900 to 250,000 mg l?1, were measured for their values. Each geographic location gives a limited range in values and each location is distinct from the others. This is interpreted as the result of extensive water-rock interaction on a local scale. For most of the time, these brines were isolated and only recently have been exposed to surface water as a result of the mining operations. The extent of the isolation is shown by the contrasting isotopic values of two “pockets” of water (0.711 vs. 0.716) located on opposite sides of the same fault system on the North Range at Sudbury. The exchange at all sites probably has continued until the present, as indicated by the close agreement between water and present-day whole-rock values. If so, it suggests that there is no single age for such brines, but it may be possible to date stages in the water's evolution by determining the age of secondary minerals that equilibrated with the water. 相似文献
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The Hemlo deposit, near Marathon, Ontario, is one of the largest gold deposits in North America. It is stratiform within Archean metamorphosed volcano-sedimentary rocks. The main ore zone is composed of pyritic, sericitic schist, and massive barite. This is the first report of stratiform barite in the Archean of North America, but other occurrences have since been found west of Hemlo. The mineralization is substantially enriched in Au, Mo, Sb, Hg, Tl and V and lacks carbonate. Because of metamorphism and deformation of the body its genesis is uncertain. of .7017 for barite from the deposit is similar to that of the sedimentary barite west of Hemlo and to initial ratios of contemporaneous volcanic rocks. At the base of the main ore zone, barite with δ34S of +8 to +12%. was deposited with ~0%. pyrite. Upward, both barite and pyrite get isotopically lighter, with minimum values for pyrite, to ?17.5%, in non-baritic schist forming the upper part of the ore zone. In drill section, Au grades correlate with the isotopic composition of pyrite. This, and the association of fractionated sulphide with sulphate, suggests that Au, pyrite and barite were deposited contemporaneously. The linked, asymmetric distributions of S minerals and isotopic distributions, which are continuous from section to section, and the isotopic similarity of the Hemlo and western barites are consistent with a syngenetic depositional model.Two sources for the S minerals are considered. In the first, exogenous sulphate from a restricted basin were partially reduced in a geothermal system to form 34S-depleted sulphide. In the second, the sulphate and sulphide are of magmatic-hydrothermal origin. Sulphate and fractionated sulphide are uncommon in Archean rocks, but one or both occur with unusual frequency in major Archean gold deposits. Hydrothermal fluids of moderately high , containing sulphate and permitting isotopic fractionation between oxidized and reduced S species, may have favoured the dissolution, transport and precipitation of Au. 相似文献
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Six authigenic feldspars and three detrital feldspars in limestones and dolostones of Eocene to Preeambrian ages were analyzed for their content. The difference in δO18 between the authigenic feldspars (δO18range = + 18.2 to + 24.7%.) and carbonate host rocks, both limestones and dolostones, was found to be ?0.5 to ?1.4%. Detrital feldspars (δO18 = + 11.2, + 22.5 and + 17.0%.) exhibit Δfeldsparcarbonate values of ?12.0, ?2.4 and ?1.6‰, respectively, and appear to have undergone increased isotopic exchange as a function of decreased grain size under solid-state conditions. 相似文献