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1.
Secular variations in 13C/12C ratios and chemical compositions of gas samples from October 1986 to July 1992 are reported from a 92–95 °C steam well located about 3 km north of Mt. Mihara, an active volcano on Izu-Oshima Island, Japan. The δ13C value steeply increased from −2.97‰ (relative to PDB carbonate) in December 1986 to −1.15‰ in March 1988 and then gradually decreased to −1.75‰ in July 1992. Over the same period, the CO2 content changed similarly with time, even though the experimental error is relatively large. These variations are consistent with helium isotope changes. Initially rapid and then slow enhancements of 3He/4He ratio, δ13C value and CO2 content are invoked by violent eruptions of Izu-Oshima volcano from 15 November to 18 December 1986. After the eruptive activity, depletion of magmatic gas emission and subsequent mixing with crustal fluids in the hydrothermal system may produce the gradual decreases of 3He/4He ratio, δ13C value and CO2 content. Taking into account the rates of these decreases, we suggest that helium and carbon isotope ratios will return to the situation of before the magmatic eruption within 15 years.  相似文献   

2.
China’s widespread marine carbonate rock series are mostly characterized by intensive thermal evolu- tion and low abundance of organic matter, especially the Lower Paleozoic carbonate rocks have experienced multi-episodes of tectonics and prolonged history of thermal evolution, thus making it more complicatedethe development and distribution of hydrocar- bon-source rocks reflected in the sedimentary, bio- logical and geochemical facies. Consequently, it seems much less powerful to assess the …  相似文献   

3.
Organic carbon isotope(δ13Corg) data from two well-preserved sections across a shallow-to-deep water transect of the late Ediacaran-Early Cambrian Yangtze Platform in South China show significant temporal and spatial variations. In the shallow-water Jiulongwan-Jijiapo section, δ13Corg values of the late Ediacaran Dengying Formation range from -29‰ to -24‰. In the deep-water Longbizui section, δ13Corg values from time-equivalent strata of the Dengying Formation are mostly between –35‰ and -32‰. These new data, in combination with δ13Corg data reported from other sections in South China, reveal a 6‰–8‰ shallow-to-deep water δ13Corg gradient. High δ13Corg values(-30‰) occur mostly in shallow-water carbonate rocks, whereas low δ13Corg values(-32‰) dominate the deep-water black shale and chert. The large temporal and spatial δ13Corg variations imply limited buffering effect from a large dissolved organic carbon(DOC) reservoir that was inferred to have existed in Ediacaran-Early Cambrian oceans. Instead, δ13Corg variations between platform and basin sections are more likely caused by differential microbial biomass contribution to total organic matter. High δ13Corg values(-30‰) documented from shallow-water carbonates are within the range of typical Phanerozoic δ13Corg data and may record the isotope signature of organic matter from primary(photosynthetic) production. In contrast, low δ13Corg values(-32‰) from deep-water sections may have resulted from higher chemoautotrophic or methanotrophic biomass contribution to bulk organic matter in anoxic environments. The δ13Corg data provide indirect evidence for ocean stratification and episodic chemocline fluctuations in the Ediacaran-Early Cambrian Yangtze Platform.  相似文献   

4.
The carbon isotopic composition of diagenetic dolomite and calcite in some sediments of the Gulf of Mexico varies between “normal-marine” (δ13C ca. 0‰) and −14.6‰ which suggests that biogenic CO2 contributed to the carbonate formation. The δ13O values of dolomite and coexisting calcite are very similar but variable down-core.Dolomite and calcite precipitated early from pore water where SO42− was not reduced. However, during (and after?) SO42− reduction dolomite and calcite still formed and there are at least two generations of carbonate minerals present.  相似文献   

5.
Variation in13C/12C and18O/16O ratios in the shell carbonate of several species of land snails was studied along a climatic gradient in semi-arid to arid areas in the southern Levant.13C was found to be enriched in snails from communities having plants with a C4 photosynthetic pathway. Depleted δ13C values were found in areas with high mean annual rainfall, apparently due to higher input of metabolic CO2 as a result of greater snail activity. Shell carbonate δ18O values show a weak relation to the δ18O values of rainwater. The shell δ18O values are enriched by 2–8‰ relative to isotopic equilibrium with environmental waters. Enrichment is suggested to result from metabolic effects on body water (with lower activity producing greater enrichment) but evaporation could also be a factor. Consistent differences in both13C and18O were found among species and may relate to the time when shell deposition occurs. As with most paleoenvironmental indicators, the application of shell isotopes is complicated by the multiplicity of controls of isotopic composition.  相似文献   

6.
Accumulation rates of marine and terrigenous organic carbon in the continental margin sediments off southwestern Taiwan were estimated from the measured concentrations and isotopic compositions of total organic carbon (TOC) and previously reported sedimentation rates. Surficial sediments were collected from the study area spanning from the narrow shelf near the Kaoping River mouth to the deep slope with depths reaching almost 3000 m. The average sediment loading of Kaoping River is 17 Mt/yr, which yields high sediment accumulation rates ranging from 0.08 to 1.44 g cm−2 yr−1 in the continental margin. About half of the discharged sediments were deposited on the margin within 120 km of the river mouth. Carbon isotopic compositions of terrestrial and marine end-members of organic matter were determined, respectively, based on suspended particulate matter (SPM) collected from three major rivers in the southwestern Taiwan and from an offshore station. All samples were analyzed for the TOC content and its isotopic composition (δ13Corg). The SPM samples were also analyzed for the total nitrogen (TN) content. TOC content in marine sediments ranges from 0.45% to 1.35% with the highest values on the upper slope near the Kaoping River mouth. The TOC/TN ratio of the SPM samples from the offshore station is 6.8±0.6, almost identical to the Redfield ratio, indicating their predominantly marine origin; their δ13Corg values are also typically marine with a mean of −21.5±0.3‰. The riverine SPM samples exhibit typical terrestrial δ13Corg values around −25‰. The δ13Corg values of surficial sediments range from −24.8‰ to −21.2‰, showing a distribution pattern influenced by inputs from the Kaoping River. The relative contributions from marine and terrestrial sources to sedimentary organic carbon were determined by the isotope mixing model with end-member compositions derived from the riverine and marine SPM. High fluvial sediment inputs lead to efficient trapping of organic carbon over a wide range of water depth in this continental margin. The marine organic accumulation rate ranges from 1.6 to 70 g C m−2 yr−1 with an area weighted mean of 4.2 g C m−2 yr−1, which is on a par with the mean terrestrial contribution and accounts for 2.3% of mean primary production. The depth-dependent accumulation rate of marine organic carbon can be simulated with a function involving primary productivity and mineral accumulation rate, which may be applicable to other continental margins with high sedimentation rates. Away from the nearshore area, the content of terrigenous organic carbon in surficial sediments decreases with distance from the river mouth, indicating its degradation in marine environments.  相似文献   

7.
Springs are complex and taxa rich ecosystems. Diatom assemblages have received very little attention in spring ecosystems in Iran; hence, the diatom assemblage in three selected springs in northeast Iran, were investigated using multivariate analysis together with hydro chemical measures. For this purpose, water and diatom samples collected during four seasons of 2019. Hydrochemistry results revealed that water-rock interaction is the most important factor in changing the water chemistry of studied springs and their waters produced from carbonate reservoir rocks (mainly calcite). As a result, Ca-HCO3 is predominate water type in all three springs. In this study, 75 diatom taxa identified, from which 55 were included in the analysis. Cluster analysis based on diatoms relative abundance, clustered samples in two major groups and third small group. Results of the indicator species analysis for groups and DCA analysis were in the absolute conformity. Results revealed that the diatom assemblage dominated by combination of early colonizer taxa, spring indicator taxa and cosmopolitan taxa; most of them also reported from different spring types. The results also showed that the spring’s diatom composition influence by size and morphology of the springs.  相似文献   

8.
The Campi Flegrei (Naples, Campanian Plain, southern Italy) geothermal system is hosted by Quaternary volcanic rocks erupted before, during and after the formation of the caldera that represents one of the major structural features in the Neapolitan area. The volcanic products rest on a Mesozoic carbonate basement, cropping out north, east and south of the area. Chemical (major, minor and trace elements) and stable isotope (C, H, O) analyses were conducted on drill-core samples recovered from geothermal wells MF-1, MF-5, SV-1 and SV-3, at depths of ˜ 1100 to 2900 m. The study was complemented by petrographic and SEM examination of thin sections. The water which feeds the system is both marine and meteoric in origin. Mineral zonation typical of a high-temperature geothermal system exists in all the geothermal wells; measured temperatures in wells are as high as ˜ 400 °C. The chemical composition of the waters suggests the existence of two reservoirs: a shallow reservoir (depth < 2000 m) fed by seawater that boiled at 320 °C and became progressively diluted by steam-heated local meteoric water during its ascent; and a deeper reservoir (depth > 2000 m) of hypersaline water. The drill-cores are mainly hydrothermally altered volcanics of trachy-latitic affinity, but some altered pelites and limestones are also present. Published Na, Mg and K concentrations of selected geothermal waters indicate that the hydrothermal fluids are in equilibrium with their host rocks, with respect to K-feldspar, albite, sericite and chlorite. The measured δ18O(SMOW) values of rocks range from +4.3 to + 16.5%. The measured δD(SMOW) values range from − 79 to − 46%. The calculated isotopic composition of the fluids at equilibrium with the samples vary from + 1 to + 8.3%. δ18O and from − 52 to + 1%. δD. The estimated isotopic composition of the waters at equilibrium with the studied samples confirmed the existence of two distinct fluid types circulating in the geothermal system. The shallower has a marine water signature, while the deeper water has a signature consistent both with magmatic and meteoric origins. In the latter case, the recharge of this aquifer likely occurs at the outcrop of the Mesozoic Limestones surrounding the Campanian Plain; after infiltration, the water percolates through evaporitic layers, becoming hypersaline and D-depleted.  相似文献   

9.
Noncondensible gases from hot springs, fumaroles, and deep wells within the Valles caldera geothermal system (210–300°C) consist of roughly 98.5 mol% CO2, 0.5 mol% H2S, and 1 mol% other components. 3He/4He ratios indicate a deep magmatic source (R/Ra up to 6) whereas δ13C–CO2 values (−3 to −5‰) do not discriminate between a mantle/magmatic source and a source from subjacent, hydrothermally altered Paleozoic carbonate rocks. Regional gases from sites within a 50-km radius beyond Valles caldera are relatively enriched in CO2 and He, but depleted in H2S compared to Valles gases. Regional gases have R/Ra values ≤1.2 due to more interaction with the crust and/or less contribution from the mantle. Carbon sources for regional CO2 are varied. During 1982–1998, repeat analyses of gases from intracaldera sites at Sulphur Springs showed relatively constant CH4, H2, and H2S contents. The only exception was gas from Footbath Spring (1987–1993), which experienced increases in these three components during drilling and testing of scientific wells VC-2a and VC-2b. Present-day Valles gases contain substantially less N2 than fluid inclusion gases trapped in deep, early-stage, post-caldera vein minerals. This suggests that the long-lived Valles hydrothermal system (ca. 1 Myr) has depleted subsurface Paleozoic sedimentary rocks of nitrogen. When compared with gases from many other geothermal systems, Valles caldera gases are relatively enriched in He but depleted in CH4, N2 and Ar. In this respect, Valles gases resemble end-member hydrothermal and magmatic gases discharged at hot spots (Galapagos, Kilauea, and Yellowstone).  相似文献   

10.
Characteristics of organic matter content, hydrocarbon generation and expulsion of carbonate rocks are discussed by analysis of organic carbon and pyrolysis. There is a strong inhomogeneity in distribution of organic matter in carbonate rocks. The organic matter abundance is higher in stylolites, carbonate varves or marls, while it is the lowest in matrixes (purer carbonate rocks around stylolites). Because of stable thickness and broad area, marls and carbonate varves may become good source rocks. At the same depth, stylolites, carbonate varves and matrixes generate and expel hydrocarbons almost at the same time. Expulsion efficiency of carbonate varve is the highest; that of rnatrixes is the lowest and that of stylolites is between marl’s or carbonate varve’s and matrix’s. Project supported by the Science and Technology Research of the 9th Five-Year Plan from China National Petroleum & Gas Company.  相似文献   

11.
Abstract The petrogenesis of the Ulsan carbonate rocks in the Mesozoic Kyongsang Basin of South Korea, which have previously been interpreted as limestone of Paleozoic age, is reconsidered in the present study. Within the Kyongsang Basin, a small volume of carbonate rocks, containing a magnetite deposit and spatially associated ultramafic rocks, is surrounded by sedimentary, volcanic and granitic rocks of the Mesozoic age. The simple cross‐cutting relationships and other outcrop features of the area indicate that the carbonate rocks are an intrusive phase and younger than the other surrounding Mesozoic rocks. The Ulsan carbonates have low concentrations of rare earth elements (REE) and trace elements with the carbon and oxygen isotope values in the range of δ13CPDB = 2.4 to 4.0‰ and δ18OSMOW = 17.0 to 19.5‰. Outcrop evidence and geochemical signatures indicate that the Ulsan carbonates were formed from crustal carbonate melts, which were generated by the melting/fluxing of crustal carbonate materials, caused by the emplacement‐related processes of alkaline A‐type granitic rocks. Compared to typical mantle‐derived carbonatites associated with silica‐undersaturated, strongly peralkaline systems, the relatively small size and geochemical characteristics of the Ulsan carbonates reflect carbonatite genesis in a silica‐saturated, weakly alkali intrusive system. Major deep‐seated tectonic fractures formed by the collapse of the cauldron or the rift system associated with the opening of the East Sea (Japan Sea) might have facilitated the ascent of the crustal carbonate melts.  相似文献   

12.
The Long Valley Exploratory Well, at the center of the Resurgent Dome of Long Valley caldera, penetrated pre-caldera basement rocks at a depth of 2101.72–2313.0 m, beneath the caldera-forming Bishop Tuff and post-caldera Early Rhyolite. The basement rocks contain prominent quartzites, with ubiquitous milky white quartz veins (with minor calcite and pyrite) and fractures of varied orientation and geometry. The other members of the basement sequence are very fine-grained quartz-rich graphitic pelites with calcite veins, spotted hornfels, and shallow intrusive rocks. Previous studies established the presence of a post-caldera, paleohydrothermal system (500–100 ka) to a depth of 2000 m that affected the Bishop Tuff and a recent (40 ka to present) hydrothermal system at shallow depth (<1 km). The deeper extent of these hydrothermal activities is established in this paper by a detailed oxygen isotope analysis of the drill core samples. 238 analyses of δ18O in 50 quartz veins within the 163.57 m depth interval of basement rocks reveal extreme heterogeneity in δ18O values (8–19.5‰). Majorities of the 84 bulk analyses of quartzites show variation of δ18O within a narrow range of 14–16‰. However, certain samples of these quartzites near the contacts with veins and fractures exhibit sharp drops in δ18O. The interbedded pelitic rocks and spotted hornfels have whole-rock δ18O ranging from 2.2 to 11.8‰. Clear, euhedral vuggy quartz that partially fills earlier open fractures in both the quartzites and quartz veins, has distinctive δ18O, ranging between −3.2 and +8.4‰. Low values of δ18O are also found in the hydrothermal minerals and whole rocks adjacent to the thin veins, clearly indicating infiltration of meteoric water. Three distinct observed patterns of fractionation in δ18O between veins and host quartzites are analyzed with the principles of mass balance, equilibrium oxygen isotope fractionation in closed system, and kinetically controlled oxygen isotope exchange in an open system. This analysis suggests that the early quartz veins formed due to a magmatic-hydrothermal activity with no influx of external water once the system comprising the sedimentary envelope and a magmatic-hydrothermal fluid phase became closed. Two-stage isotopic exchange processes caused fractionation in the δ values that originally formed arrays with slope 1 in a δvein quartz–δhost quartzite space. Another array in the same space, with near zero slope was also formed due to variation in temperature, initial isotopic compositions of the quartzite sequence and the fluid phase. Variation in temperature was mostly in the range of 300–400°C giving Δ (=δvein quartz–δhost quartzite)≈−2.8 to +2.8. The δ18O of the fluid could range from −5 to +10; however a narrower range of +5 to +10 can explain the data. This episode of hydrothermal activity could take place either as a single pulse or in multiple pulses but each as a closed system. A later, fracture-controlled, meteoric water (δ18O−0.46 to −12.13) flow and interaction (at 250°C) is interpreted from the analysis of δ18O values of the coexisting quartz and calcite pairs and existence of markedly 18O-depleted pelitic horizons interbedded with 18O-enriched quartzite layers. Thus, the interpreted earlier magmatic-hydrothermal activity was overprinted by a later meteoric-hydrothermal activity that resulted in steep arrays of δ18O values in the δvein quartz–δhost quartzite space. Calculations show that the likely life span of the post-caldera, hydrothermal activity in the depth range of 2.1–2.3 km beneath Long Valley was 0.08–0.12 Ma. Diffusive ±advective transport of oxygen isotopes from fracture-channelized meteoric water to nearly impermeable wall rocks caused a lowering of δ18O values in the quartz over short distances and in calcites over greater distances. Thus, the hydrothermal activity appears pervasive even though the meteoric water flow was primarily controlled by fractures.  相似文献   

13.
Dajing Cu-Sn-Ag-Pb-Zn ore deposit, in the Inner Mongolia Autonomous Region of China, is a fissure-filling hydrothermal ore deposit. The δD values of quartz-hosted inclusion water are centered at −100%.– −130%.. The δ34S values of sulfide ore minerals and δ13 C values of carbonate gangue minerals vary from −0.3%. to 2.6%. and from −2.9%. to −7.0%., respectively. Integrated isotopic data point to two major contributions to the mineralizing fluid that include a dominant meteoric-derived groundwater, and sulfur and carbon species from hypogene magma. Linear trends are exhibited on the gaseous H2O versus CO2 plot, and plots of CO, N2, CH4, and C2H6. It is shown by quantitative simulation that magma degassing cannot explain the linear trends. Hence, these linear trends are interpreted in terms of mixing of CO2-rich magmatic fluid with meteoric-derived groundwater. The groundwater circulated in Paleozoic sedimentary rocks and absorbed CO, N2, CH4, C2H6 and radiogenic Ar from organic matter. Cooling effects resulting from mixing have caused the precipitation of ore minerals.  相似文献   

14.
This study is undertaken to understand how calcite precipitation and dissolution contributes to depth-related changes in porosity and permeability of gas-bearing sandstone reservoirs in the Kela 2 gas field of the Tarim Basin, Northwestern China. Sandstone samples and pore water samples are col-lected from well KL201 in the Tarim Basin. Vertical profiles of porosity, permeability, pore water chem-istry, and the relative volume abundance of calcite/dolomite are constructed from 3600 to 4000 m below the ground surface within major oil and gas reservoir rocks. Porosity and permeability values are in-versely correlated with the calcite abundance, indicating that calcite dissolution and precipitation may be controlling porosity and permeability of the reservoir rocks. Pore water chemistry exhibits a sys-tematic variation from the Na2SO4 type at the shallow depth (3600-3630 m), to the NaHCO3 type at the intermediate depth (3630―3695 m),and to the CaCl2 type at the greater depth (3728―3938 m). The geochemical factors that control the calcite solubility include pH, temperature, pressure, Ca2 concen-tration, the total inorganic carbon concentration (ΣCO2), and the type of pore water. Thermodynamic phase equilibrium and mass conservation laws are applied to calculate the calcite saturation state as a function of a few key parameters. The model calculation illustrates that the calcite solubility is strongly dependent on the chemical composition of pore water, mainly the concentration difference between the total dissolved inorganic carbon and dissolved calcium concentration (i.e., [ΣCO2] -[Ca2 ]). In the Na2SO4 water at the shallow depth, this index is close to 0, pore water is near the calcite solubility. Calcite does not dissolve or precipitate in significant quantities. In the NaHCO3 water at the intermedi-ate depth, this index is greater than 0, and pore water is supersaturated with respect to calcite. Massive calcite precipitation was observed at this depth interval and this intensive cementation is responsible for decreased porosity and permeability. In the CaCl2 water at the greater depth, pore water is un-der-saturated with respect to calcite, resulting in dissolution of calcite cements, as consistent with microscopic dissolution features of the samples from this depth interval. Calcite dissolution results in formation of high secondary porosity and permeability, and is responsible for the superior quality of the reservoir rocks at this depth interval. These results illustrate the importance of pore water chemis-try in controlling carbonate precipitation/dissolution, which in turn controls porosity and permeability of oil and gas reservoir rocks in major sedimentary basins.  相似文献   

15.
Stable isotopic data are presented for 112 samples of francolite from 18 separate phosphate deposits. Values ofδ13C andδ34S in most offshore deposits suggest formation within oxic or suboxic environments either by carbonate replacement or direct precipitation of francolite from water of normal marine compositions. The exceptions are concretionary francolite from Namibia, which has an isotopic composition in keeping with its formation within organic-rich sediments, and that from offshore Morocco, which has an isotopic signature of the anoxic/suboxic interface. Onshore deposits from Jordan, Mexico, South Africa and, possibly, the Permian Phosphoria Formation in the western U.S.A., are substantially depleted in18O: they appear to be too altered for deductions to be made about their environments of formation. In other onshore deposits which are unaltered, or minimally altered, the isotopic composition suggests that some formed within sulphate-reducing sediments (Sedhura, Morocco) whilst francolite from the Georgina Basin of Australia formed at the oxic/anoxic boundary, where oxidation of biogenic H2S decreases theδ34S of pore water. In general, pelletal samples show non-oxic isotopic signatures, whilst non-pelletal samples show oxic isotopic signatures, but samples from Namibia, Peru (Ica Plateau) and the Californian and Moroccan margins are exceptions to this rule. Morphology may therefore be a misleading indicator of francolite genesis as no definitive relation exists between phosphorite type and isotopic signature.  相似文献   

16.
New analyses reveal two intervals of distinctly lower δ13C values in the terrestrial organic matter of Permian–Triassic sequences in northern Xinjiang, China. The younger negative δ13Corg spike can be correlated to the conspicuous and sharp δ13C drops both in carbonate carbon and organic carbon near the Permian–Triassic event boundary (PTEB) in the marine section at Meishan. The geochemical correlation criteria are accompanied by a magnetic susceptibility pulse and higher abundances of distinctive, chain-like organic fossil remains of Reduviasporonites.The older negative δ13Corg spike originates within a latest Permian regression. Significant changes in organic geochemical proxies are recorded in the equivalent interval of the marine section at Meishan. These include relatively higher concentrations of total organic carbon, isorenieratane, C14–C30 aryl isoprenoids and lower ratios of pristane/phytane that, together, indicate the onset of anoxic, euxinic and restricted environments within the photic zone. The massive and widespread oxidation of buried organic matter that induced these euxinic conditions in the ocean would also result in increased concentrations of 13C-depleted atmospheric CO2. The latest Permian environmental stress marked by the older negative δ13Corg episode can be correlated with the distinct changeover of ostracod assemblages and the occurrences of morphological abnormalities of pollen grains. These observations imply that biogeochemical disturbance was manifested on the land at the end of the Permian and that terrestrial organisms responded to it before the main extinction of the marine fauna.  相似文献   

17.
δ87Sr values and Ca/Sr ratios were employed to quantify solute inputs from atmospheric and lithogenic sources to a catchment in NW Germany. The aquifer consists primarily of unconsolidated Pleistocene eolian and fluviatile deposits predominated by >90% quartz sand. Accessory minerals include feldspar, glauconite, and mica, as well as disperse calcium carbonate in deeper levels. Decalcification of near-surface sediment induces groundwater pH values up to 4.4 that lead to enhanced silicate weathering. Consequently, low mineralized Ca–Na–Cl- and Ca–Cl-groundwater types are common in shallow depths, while in deeper located calcareous sediment Ca–HCO3-type groundwater prevails. δ87Sr values and Ca/Sr ratios of the dissolved pool range from 7.3 to −2.6 and 88 to 493, respectively. Positive δ87Sr values and low Ca/Sr ratios indicate enhanced feldspar dissolution in shallow depths of less than 20 m below soil surface (BSS), while equilibrium with calcite governs negative δ87Sr values and elevated Ca/Sr ratios in deep groundwater (>30 m BSS). Both positive and negative δ87Sr values are evolved in intermediate depths (20–30 m BSS). For groundwater that is undersaturated with respect to calcite, atmospheric supplies range from 4% to 20%, while feldspar-weathering accounts for 8–26% and calcium carbonate for 62–90% of dissolved Sr2+. In contrast, more than 95% of Sr2+ is derived by calcium carbonate and less than 5% by feldspar dissolution in Ca–HCO3-type groundwater. The surprisingly high content of carbonate-derived Sr2+ in groundwater of the decalcified portion of the aquifer may account for considerable contributions from Ca-containing fertilizers. Complementary tritium analyses show that equilibrium with calcite is restricted to old groundwater sources.  相似文献   

18.
The Eoarchean Nulliak supracrustal rocks in the Saglek Block of northeastern Labrador, Canada, contain some of the world's oldest carbonate rocks. This work attempted to reveal the origin of the carbonate rocks and estimate the surface environmental conditions of the early Earth based on their occurrence and geochemistry. They occur together with mafic and ultramafic rocks in Pangertok Inlet and St. John's Harbour South, whereas they are interlayered with pelitic rock layers with quartzofeldspathic mineral assemblages in St. John's Harbour East and Big Island. The geological occurrence suggests that the formers were formed around hydrothermal fields, whereas the latters were deposited near a continental margin. Some carbonate rocks have high SiO2, Al2O3, and Zr contents, indicating that the silicification and involvement of detrital materials influenced their composition; thus, pure carbonate rocks were selected using a combined filter of the SiO2, TiO2, Al2O3, Zr, and Ba contents. The selected carbonate rocks have positive La, Eu, Gd, Y, U, Pb, and Sr anomalies, negative Nb, Zr, and Hf anomalies, and relatively small enrichment in heavy rare earth elements (HREEs). The La and Y anomalies suggest that they originated from chemical sediments precipitated from seawater. On the other hand, the small HREE-enrichment suggests that REEs were mainly dissolved as REE-carbonate complexes in seawater or that the riverine influxes were dominated by the detritus of Eoarchean continental crusts, presumably composed of HREE-depleted TTG. The U anomaly suggests that uranium was more dissolved than Th as U-bearing carbonate complexes in seawater. The Nulliak carbonate rocks also show a positive correlation between Y and Eu anomaly values, suggesting that the precipitation of iron-oxyhydroxide causing the Y anomaly was more significant near the hydrothermal fields than the continental margin, consistent with an alkaline hydrothermal model.  相似文献   

19.
Tetsuji  Onoue  Hiroyoshi  Sano 《Island Arc》2007,16(1):173-190
Abstract   The Sambosan accretionary complex of southwest Japan was formed during the uppermost Jurassic to lowermost Cretaceous and consists of basaltic rocks, carbonates and siliceous rocks. The Sambosan oceanic rocks were grouped into four stratigraphic successions: (i) Middle Upper Triassic basaltic rock; (ii) Upper Triassic shallow-water limestone; (iii) limestone breccia; and (iv) Middle Middle Triassic to lower Upper Jurassic siliceous rock successions. The basaltic rocks have a geochemical affinity with oceanic island basalt of a normal hotspot origin. The shallow-water limestone, limestone breccia, and siliceous rock successions are interpreted to be sediments on the seamount-top, upper seamount-flank and surrounding ocean floor, respectively. Deposition of the radiolarian chert of the siliceous rock succession took place on the ocean floor in Late Anisian and continued until Middle Jurassic. Oceanic island basalt was erupted to form a seamount by an intraplate volcanism in Late Carnian. Late Triassic shallow-water carbonate sedimentation occurred at the top of this seamount. Accumulation of the radiolarian chert was temporally replaced by Late Carnian to Early Norian deep-water pelagic carbonate sedimentation. Biotic association and lithologic properties of the pelagic carbonates suggest that an enormous production and accumulation of calcareous planktonic biotas occurred in an open-ocean realm of the Panthalassa Ocean in Late Carnian through Early Norian. Upper Norian ribbon chert of the siliceous rock succession contains thin beds of limestone breccia displaced from the shallow-water buildup resting upon the seamount. The shallow-water limestone and siliceous rock successions are nearly coeval with one another and are laterally linked by displaced carbonates in the siliceous rock succession.  相似文献   

20.
Trace element and isotopic characteristics of late Carboniferous to early Permian minettes and kersantites have been determined. These lamprophyres have been sampled throughout the Western European Hercynian orogen, from Brittany to the west to Schwarzwald to the east. In spite of sharp petrological differences reflected by mineralogy and major element geochemistry, minettes and kersantites exhibit close identity with respect to trace element and isotopic features. These features comprise enrichment in incompatible elements, highCs/Rb and lowCe/Pb ratios, Ta and Ti relative depletion, high abundance in transition elements and highNi/Mg ratios. Pb isotope ratios are undistinguishable from those measured on Hercynian continental crust. Initial143Nd/144Nd ratios are between0.5120 (εi −5) and0.5122 (εi −1) for minettes and kersantites whereas initial87Sr/86Sr ratios vary between 0.7055–0.710 for minettes and 0.707–0.708 for kersantites. No simple mixing relations are visible on RbSr and SmNd isochron diagrams. The exceptional homogeneity of these geochemical characteristics along a 1000 km traverse does not allow for an hypothesis of enrichment through upper level assimilation and thus leads to propose that these rocks originated through melting of a mantle enriched by recycling of crustal material.  相似文献   

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