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1.
A calcic skarn deposit occurs along the contact zone between Oligo-Miocene Çatalda? Granitoid and Mesozoic limestones in Susurluk, northwestern Turkey. The skarn zone with little or no retrograde stage is represented by fluid inclusions with high homogenization temperatures (up to >600 °C) and a wide range of salinity (12 to >70 wt.% NaCl). Pluton-derived fluids facilitated occurrence of continuous prograde reactions in the country rocks (particularly in the proximal zone) and oxygen isotopic depletion in calc-silicate and calcite minerals. δ18O of anhydrous minerals within proximal and distal zones indicate that skarn-forming fluids had a magmatic origin. The δ18O values are 5.93–9.08‰ (mean 6.8‰) for garnet, 4.08–9.94‰ (mean 6.4‰) for pyroxene, 4.89–7.92‰ (mean 6.4‰) for wollastonite and 6.65–8.28‰ (mean 7.5‰) for vesuvianite. Temperatures estimated by isotopic compositions of mineral pairs are significantly lower than those measured from the fluid inclusions, indicating that isotopic equilibrium is not preserved between the skarn minerals. δ18O and δ13C values are systematically depleted from marbles to skarn carbonates. Calc-silicate forming reactions and permeability increase triggered by volatilization and consequent strong infiltration of H2O-rich siliceous fluids into the system promoted fluid–rock interaction causing isotopic resetting and isotopic depletion of silicates (e.g. pyroxene and wollastonite) and skarn calcites.  相似文献   

2.
Oxygen isotope signatures of ruby and sapphire megacrysts, combined with trace-element analysis, from the Mbuji-Mayi kimberlite, Democratic Republic of Congo, and the Changle alkali basalt, China, provide clues to specify their origin in the deep Earth. At Mbuji-Mayi, pink sapphires have δ18O values in the range 4.3 to 5.4‰ (N = 10) with a mean of 4.9 ± 0.4‰, and rubies from 5.5 to 5.6‰ (N = 3). The Ga/Mg ratio of pink sapphires is between 1.9 and 3.9, and in rubies, between 0.6 and 2.6. The blue or yellow sapphires from Changle have δ18O values from 4.6 to 5.2 ‰, with a mean of 4.9 ± 0.2‰ (N = 9). The Ga/Mg ratio is between 5.7 and 11.3. The homogenous isotopic composition of ruby suggests a derivation from upper mantle xenoliths (garnet lherzolite, pyroxenite) or metagabbros and/or lower crustal garnet clinopyroxenite eclogite-type xenoliths included in kimberlites. Data from the pink sapphires from Mbuji-Mayi suggest a mantle origin, but different probable protoliths: either subducted oceanic protolith transformed into eclogite with δ18O values buffered to the mantle value, or clinopyroxenite protoliths in peridotite. The Changle sapphires have a mantle O-isotope signature. They probably formed in syenitic magmas produced by low degree partial melting of a spinel lherzolite source. The kimberlite and the alkali basalt acted as gem conveyors from the upper mantle up to the surface.  相似文献   

3.
The Cipoeiro gold deposit, located in the Gurupi Belt, northern Brazil, is hosted by tonalites of 2148 Ma. The deposit is controlled by splays related to the major strike-slip Tentugal shear zone, and at the deposit scale, the mineralization is confined to ductile–brittle shear zones. Mineralization style comprises thick quartz veins and narrow and discontinuous quartz-carbonate veinlets associated with disseminations in altered host rocks. The postmetamorphic hydrothermal paragenesis is composed of quartz, calcite, chlorite, white mica (phengite), pyrite, and minor albite. Electron microprobe analysis of chlorites reveals a relatively uniform chemical composition at depths of more than 100 m. The chlorites are characterized by (Fe + Mg) ratios between 0.37 and 0.47 and AlIV ranging between 2.22 and 2.59 a.p.f.u. and are classified as Fe-chlinochlore. Temperatures calculated by applying the AlIV contents of chlorites yield a relatively narrow interval of 305 ± 15°C. Stable isotope (O, H, C, S) compositions have been determined in silicate, carbonate, and sulfide minerals. The δ18O and δD values of the mineralizing fluid range from +2.4 to +5.7 and from −43‰ to −20‰, respectively, and are interpreted as having a metamorphic origin. The δ13C values of fluid CO2 are in the range −10.7‰ to −3.9‰, whereas the fluid δ34S is around 0‰. Carbon and sulfur compositions are not diagnostic of their sources, compatible as they are with mantle, magmatic, or average crustal reservoirs. The hydrothermal paragenesis, chlorite–pyrite coexistence, temperature of ore formation, and sulfur isotope evidence indicate relatively reduced fO2 conditions for the mineralizing fluid. Geologic, chemical, and isotopic characteristics of the Cipoeiro deposit are compatible with the class of orogenic gold deposits.  相似文献   

4.
The role of sulfur in two hydrothermal vent systems, the Logatchev hydrothermal field at 14°45′N/44°58′W and several different vent sites along the southern Mid-Atlantic Ridge (SMAR) between 4°48′S and 9°33′S and between 12°22′W and 13°12′W, is examined by utilizing multiple sulfur isotope and sulfur concentration data. Isotope compositions for sulfide minerals and vent H2S from different SMAR sites range from + 1.5 to + 8.9‰ in δ34S and from + 0.001 to + 0.051‰ in Δ33S. These data indicate mixing of mantle sulfur with sulfur from seawater sulfate. Combined δ34S and Δ33S systematics reveal that vent sulfide from SMAR is characterized by a sulfur contribution from seawater sulfate between 25 and 33%. This higher contribution, compared with EPR sulfide, indicates increased seawater sulfate reduction at MAR, because of a deeper seated magma chamber and longer fluid upflow path length, and points to fundamental differences with respect to subsurface structures and fluid evolution at slow and fast spreading mid-ocean ridges.Additionally, isotope data uncover non-equilibrium isotopic exchange between dissolved sulfide and sulfate in an anhydrite bearing zone below the vent systems at fluid temperatures between 335 and 400 °C. δ34S values between + 0.2 to + 8.8‰ for dissolved and precipitated sulfide from Logatchev point to the same mixing process between mantle sulfur and sulfur from seawater sulfate as at SMAR. δ34S values between ? 24.5 and + 6.5‰ and Δ33S values between + 0.001 and + 0.125‰ for sulfide-bearing sediments and mafic/ultramafic host rocks from drill cores taken in the region of Logatchev indicate a clear contribution of biogenic sulfides formed via bacterial sulfate reduction. Basalts and basaltic glass from SMAR sites with Δ33S = ? 0.008‰ reveal lower Δ33S lower values than suggested on the basis of previously published isotopic measurements of terrestrial materials.We conclude that the combined use of both δ34S and Δ33S provides a more detailed picture of the sulfur cycling in hydrothermal systems at the Mid-Atlantic Ridge and uncovers systematic differences to hydrothermal sites at different mid-ocean ridge sites. Multiple sulfur isotope measurements allow identification of incomplete isotope exchange in addition to isotope mixing as a second important factor influencing the isotopic composition of dissolved sulfide during fluid upflow. Furthermore, based on Δ33S we are able to clearly distinguish biogenic from hydrothermal sulfides in sediments even when δ34S were identical.  相似文献   

5.
Concentrations and isotopic compositions of Hg and Pb were measured in a sediment core collected from Lake Ballinger, near Seattle, Washington, USA. Lake Ballinger has been affected by input of metal contaminants emitted from the Tacoma smelter, which operated from 1887 to 1986 and was located about 53 km south of the lake. Concentrations and loadings of Hg and Pb in Lake Ballinger increased by as much as three orders of magnitude during the period of smelting as compared to the pre-smelting period. Concentrations and loadings of Hg and Pb then decreased by about 55% and 75%, respectively, after smelting ended. Isotopic compositions of Hg changed considerably during the period of smelting (δ202Hg = −2.29‰ to −0.38‰, mean −1.23‰, n = 9) compared to the pre-smelting period (δ202Hg = −2.91‰ to −2.50‰, mean −2.75‰, n = 4). Variations were also observed in 206Pb/207Pb and 208Pb/207Pb isotopic compositions during these periods. Data for Δ199Hg and Δ201Hg indicate mass independent fractionation (MIF) of Hg isotopes in Lake Ballinger sediment during the smelting and post-smelting period and suggest MIF in the ore smelted, during the smelting process, or chemical modification at some point in the past. Negative values for Δ199Hg and Δ201Hg for the pre-smelting period are similar to those previously reported for soil, peat, and lichen, likely suggesting some component of atmospheric Hg. Variations in the concentrations and isotopic compositions of Hg and Pb were useful in tracing contaminant sources and the understanding of the depositional history of sedimentation in Lake Ballinger.  相似文献   

6.
Exploration of unconventional natural gas reservoirs such as impermeable shale basins through the use of horizontal drilling and hydraulic fracturing has changed the energy landscape in the USA providing a vast new energy source. The accelerated production of natural gas has triggered a debate concerning the safety and possible environmental impacts of these operations. This study investigates one of the critical aspects of the environmental effects; the possible degradation of water quality in shallow aquifers overlying producing shale formations. The geochemistry of domestic groundwater wells was investigated in aquifers overlying the Fayetteville Shale in north-central Arkansas, where approximately 4000 wells have been drilled since 2004 to extract unconventional natural gas. Monitoring was performed on 127 drinking water wells and the geochemistry of major ions, trace metals, CH4 gas content and its C isotopes (δ13CCH4), and select isotope tracers (δ11B, 87Sr/86Sr, δ2H, δ18O, δ13CDIC) compared to the composition of flowback-water samples directly from Fayetteville Shale gas wells. Dissolved CH4 was detected in 63% of the drinking-water wells (32 of 51 samples), but only six wells exceeded concentrations of 0.5 mg CH4/L. The δ13CCH4 of dissolved CH4 ranged from −42.3‰ to −74.7‰, with the most negative values characteristic of a biogenic source also associated with the highest observed CH4 concentrations, with a possible minor contribution of trace amounts of thermogenic CH4. The majority of these values are distinct from the reported thermogenic composition of the Fayetteville Shale gas (δ13CCH4 = −35.4‰ to −41.9‰). Based on major element chemistry, four shallow groundwater types were identified: (1) low (<100 mg/L) total dissolved solids (TDS), (2) TDS > 100 mg/L and Ca–HCO3 dominated, (3) TDS > 100 mg/L and Na–HCO3 dominated, and (4) slightly saline groundwater with TDS > 100 mg/L and Cl > 20 mg/L with elevated Br/Cl ratios (>0.001). The Sr (87Sr/86Sr = 0.7097–0.7166), C (δ13CDIC = −21.3‰ to −4.7‰), and B (δ11B = 3.9–32.9‰) isotopes clearly reflect water–rock interactions within the aquifer rocks, while the stable O and H isotopic composition mimics the local meteoric water composition. Overall, there was a geochemical gradient from low-mineralized recharge water to more evolved Ca–HCO3, and higher-mineralized Na–HCO3 composition generated by a combination of carbonate dissolution, silicate weathering, and reverse base-exchange reactions. The chemical and isotopic compositions of the bulk shallow groundwater samples were distinct from the Na–Cl type Fayetteville flowback/produced waters (TDS ∼10,000–20,000 mg/L). Yet, the high Br/Cl variations in a small subset of saline shallow groundwater suggest that they were derived from dilution of saline water similar to the brine in the Fayetteville Shale. Nonetheless, no spatial relationship was found between CH4 and salinity occurrences in shallow drinking water wells with proximity to shale-gas drilling sites. The integration of multiple geochemical and isotopic proxies shows no direct evidence of contamination in shallow drinking-water aquifers associated with natural gas extraction from the Fayetteville Shale.  相似文献   

7.
The Korba aquifer on the east coast of Cape Bon has been overexploited since the 1960s with a resultant reversal of the hydraulic gradient and a degradation of the quality due to seawater intrusion. In 2008 the authorities introduced integrated water resources planning based on a managed aquifer recharge with treated wastewater. Water quality monitoring was implemented in order to determine the different system components and trace the effectiveness of the artificial recharge. Groundwater samples taken from recharge control piezometers and surrounding farm wells were analyzed for their chemical contents, for their B isotopes, a proven tracer of groundwater salinization and domestic sewage, and their carbamazepine content, an anti-epileptic known to pass through wastewater treatment and so recognized as a pertinent tracer of wastewater contamination. The system equilibrium was permanently disturbed by the different temporal dynamics of continuous processes such as cation exchange, and by threshold processes linked to oxidation–reduction conditions. The B isotopic compositions significantly shifted back-and-forth due to mixing with end-members of various origin. Under the variable contribution of meteoric recharge, the Plio-Quaternary groundwater (δ11B of 35–40.6‰, a mean B concentration of 30 μmol/L, no carbamazepine, n = 7) was subject to seawater intrusion that induced a high δ11B level (δ11B of 41.5–48.0‰, a mean B concentration of 36 μmol/L, and n = 8). Fresh groundwater (δ11B of 19.89‰, B concentration of 2.8 μmol/L, no carbamazepine) was detected close to the recharge site and may represent the deep Miocene pole which feeds the upper Plio-Quaternary aquifer. The managed recharge water (δ11B of 10.67–13.8‰, n = 3) was brackish and of poor quality with a carbamazepine content showing a large short term variability with an average daily level of 328 ± 61 ng/L. A few piezometers in the vicinity of the recharge site gradually acquired a B isotopic composition close to the wastewater signature and showed an increasing carbamazepine content (from 20 to 910 ng/L). The combination of B isotopic signatures with B and carbamazepine contents is a useful tool to assess sources and mixing of treated wastewaters in groundwaters. Effluent quality needs to be greatly improved before injection to prevent further degradation of groundwater quality.  相似文献   

8.
Previous research has shown that Cu and Fe isotopes are fractionated by dissolution and precipitation reactions driven by changing redox conditions. In this study, Cu isotope composition (65Cu/63Cu ratios) was studied in profiles through sulphide-bearing tailings at the former Cu mine at Laver and in a pilot-scale test cell at the Kristineberg mine, both in northern Sweden. The profile at Kristineberg was also analysed for Fe isotope composition (56Fe/54Fe ratios). At both sites sulphide oxidation resulted in an enrichment of the lighter Cu isotope in the oxidised zone of the tailings compared to the original isotope ratio, probably due to preferential losses of the heavier Cu isotope into the liquid phase during oxidation of sulphides. In a zone with secondary enrichment of Cu, located just below the oxidation front at Laver, δ65Cu (compared to ERM-AE633) was as low as −4.35 ± 0.02‰, which can be compared to the original value of 1.31 ± 0.03‰ in the unoxidised tailings. Precipitation of covellite in the secondary Cu enrichment zone explains this fractionation. The Fe isotopic composition in the Kristineberg profile is similar in the oxidised zone and in the unoxidised zone, with average δ56Fe values (relative to the IRMM-014) of −0.58 ± 0.06‰ and −0.49 ± 0.05‰, respectively. At the well-defined oxidation front, δ56Fe was less negative, −0.24 ± 0.01‰. Processes such as Fe(II)–Fe(III) equilibrium and precipitation of Fe-(oxy)hydroxides at the oxidation front are assumed to cause this Fe isotope fractionation. This field study provides additional support for the importance of redox processes for the isotopic composition of Cu and Fe in natural systems.  相似文献   

9.
《Sedimentary Geology》2006,183(1-2):15-30
Carbonate concretions in the Miocene sedimentary rocks of the Yeonil Group in the Pohang Basin (Korea) were investigated in terms of stable oxygen and carbon isotope composition to delineate the origin and associated diagenetic environment for their formation. Carbonate concretions are widely distributed in all the sedimentary rocks in the Pohang Basin, showing that the calcitic concretions are preserved within the mass-flow deposits and the dolomitic ones mostly in the hemipelagic siliceous rocks (diatomites). Concretions can be classified into four different types, on the basis of the stable isotopic signatures, each of which represents its own geochemical range.Type I concretions are calcitic and are composed of micrite to microspar. They occur in the conglomerates and sandstones which were deposited by mass flows (debris flow to turbidity current). It shows relatively lower δ18O (− 14.0 to − 9.3‰) and δ13C (− 19.6 to − 8.4‰) values. These concretions grew in a sulfate reducing zone under the influence of residual ambient seawater which had been significantly modified by volcanogenic sediments. Type II concretions are also calcitic, composed mostly of micrite with minor microspar and found in the sandstones. These concretions are characterized by relatively high δ18O (+ 1.8 to + 2.4‰) and variable δ13C (− 17.3 to − 0.4‰) values. These isotopic signatures reflect that Type II concretions formed from just beneath the sediment/water interface down to the sulfate reducing zone through the early stage of methanogenesis. Type III concretions are also calcitic, and composed largely of micrite with a minor contribution of microspar. They are observed in hemipelagic mudrocks which were deposited under the influence of mass flows. They are characterized by intermediate to high δ18O (− 4.6 to + 1.6‰) and high δ13C (− 1.3 to + 8.8‰) values. These concretions grew in a methanogenic zone by residual ambient seawater and/or seawater slightly modified by reaction with volcanogenic sediments. Type IV concretions are dolomite with calcite inclusion, and occur in hemipelagic siliceous rocks. These concretions are mostly composed of micrite and characterized by variable δ18O (− 9.1 to + 0.7‰) and high δ13C (+ 3.1 to + 17.9‰) values, suggesting formation in the methanogenic zone, although the residual ambient seawater is slightly modified by volcanogenic sediments.The same type of the concretions is widely distributed throughout the basin and always shows its own distinctive stable isotopic signature. This means that the formation of the given type depends upon the lithology and composition of host sediments that are closely related to the depositional process of the fan-delta systems regardless of their localities. Further, the different types of concretions are also found at the different, but closely spaced stratigraphic levels in the same locality, displaying the distinctive diagenetic conditions for each type. Such preservation of the unique diagenetic signatures in individual type of concretion suggests that the concretions formed in a completely closed diagenetic system. Therefore, caution should be made to simplify and generalize the diagenetic condition for the formation of any concretions in a large sedimentary basin.  相似文献   

10.
《Applied Geochemistry》2006,21(6):1016-1029
Nitrate concentrations approaching and greater than the maximum contaminant level are impairing the viability of many groundwater basins as drinking water sources. Nitrate isotope data are effective in determining contaminant sources, especially when combined with other isotopic tracers such as stable isotopes of water and 3H–He ages to give insight into the routes and timing of NO3 inputs to the flow system. This combination of techniques is demonstrated in Livermore, CA, where it is determined that low NO3 reclaimed wastewater predominates in the NW, while two flowpaths with distinct NO3 sources originate in the SE. Along the eastern flowpath, δ15N values greater than 10‰ indicate that animal waste is the primary source. Diminishing concentrations over time suggest that contamination results from historical land use practices. The other flowpath begins in an area where rapid recharge, primarily of low-NO3 imported water (identified by stable isotopes of water and a 3H–He residence time of <1 year), mobilizes a significant local NO3 source, bringing groundwater concentrations up to 53 mg NO3 L−1. In this area, artificial recharge of imported water via local arroyos increases the flux of NO3 to the regional aquifer. The low δ15N value (3.1‰) in this location implicates synthetic fertilizer. In addition to these anthropogenic sources, natural NO3 background levels between 15 and 20 mg NO3 L−1 are found in deep wells with residence times greater than 50 a.  相似文献   

11.
《Applied Geochemistry》2005,20(4):789-805
Mineralogical, hydrochemical and S isotope data were used to constrain hydrogeochemical processes that produce acid mine drainage from sulfidic waste at the historic Mount Morgan Au–Cu mine, and the factors controlling the concentration of SO4 and environmentally hazardous metals in the nearby Dee River in Queensland, Australia. Some highly contaminated acid waters, with metal contents up to hundreds of orders of magnitude greater than the Australia–New Zealand environmental standards, by-pass the water management system at the site and drain into the adjacent Dee River.Mine drainage precipitates at Mt. Morgan were classified into 4 major groups and were identified as hydrous sulfates and hydroxides of Fe and Al with various contents of other metals. These minerals contain adsorbed or mineralogically bound metals that are released into the water system after rainfall events. Sulfate in open pit water and collection sumps generally has a narrow range of S isotope compositions (δ34S = 1.8–3.7‰) that is comparable to the orebody sulfides and makes S isotopes useful for tracing SO4 back to its source. The higher δ34S values for No. 2 Mill Diesel sump may be attributed to a difference in the source. Dissolved SO4 in the river above the mine influence and 20 km downstream show distinctive heavier isotope compositions (δ34S = 5.4–6.8‰). The Dee River downstream of the mine is enriched in 34S (δ34S = 2.8–5.4‰) compared with mine drainage possibly as a result of bacterial SO4 reduction in the weir pools, and in the water bodies within the river channel. The SO4 and metals attenuate downstream by a combination of dilution with the receiving waters, SO4 reduction, and the precipitation of Fe and Al sulfates and hydroxides. It is suggested here that in subtropical Queensland, with distinct wet and dry seasons, temporary reducing environments in the river play an important role in S isotope systematics.  相似文献   

12.
Despite the occurrence of highly variable lithium (Li) elemental distribution and isotopic fractionation in mantle mineral, the mechanism of Li heterogeneity and fractionation remains a controversial issue. We measured Li contents and isotopic compositions of olivine and clinopyroxene xenocrysts and phenocrysts from kamafugite host lavas, as well as minerals in melt pockets occurring as metasomatic products in peridotite xenoliths from the Western Qinling, central China. The olivine xenocrysts in the kamafugites show compositional zonation. The cores have high Mg# (100 × Mg/(Mg+Fe); 91.0–92.2) and Li abundances (5.63–21.7 ppm), low CaO contents (≤0.12 wt%) and low δ7Li values (−39.6 to −6.76‰), which overlap with the compositional ranges of the olivines in the melt pockets as well as those in peridotite xenoliths. The rims of the olivine xenocrysts display relatively low Mg# (85.9–88.2), high CaO contents (0.19–0.38 wt%) and high δ7Li values (18.3–26.9‰), which are comparable to the olivine phenocrysts (Mg#: 86.4–87.1; CaO: 0.20–0.28 wt%; Li: 12.4–36.8 ppm; δ7Li: 18.1–26.0‰) and the silicate-melt metasomatized olivines. The clinopyroxene phenocrysts and clinopyroxenes in the melt pockets have no distinct characteristics with respect to the Li abundances and δ7Li values, but show higher and lower CaO contents, respectively, than the clinopyroxenes from silicate and carbonatite metasomatized samples. These features indicate that Li concentration and isotopic signatures of the cores of the xenocrysts recorded carbonatite melt-peridotite reaction (carbonatite metasomatism) at mantle depth, and the variations in the rims probably resulted from xenocryst–host magma interaction during ascent. Our results reveal that the interaction with carbonatite and silicate melts gave rise to an increase in Li abundance in minerals of peridotite xenoliths at mantle depth or during transportation. In terms of δ7Li, the carbonatite and silicate melts produced remarkably contrasting δ7Li variations in olivine. Based on the systematic variations of Li abundances and Li isotopes in olivines, we suggest that the δ7Li value of olivine is a more important indicator than that of clinopyroxene in discriminating carbonatite and silicate melt interaction agents with peridotites.  相似文献   

13.
《Applied Geochemistry》2006,21(4):643-655
The groundwater B concentration in the alluvial aquifer of the upper Cecina River basin in Tuscany, Italy, often exceeds the limit of 1 mg L−1 set by the European Union for drinking water. On the basis of hydrogeological and geochemical observations, the main source of the B contamination of groundwater has been attributed to past releases into streams of exhausted, B-rich geothermal waters and/or mud derived from boric acid manufacturing in Larderello. The releases were discontinued 25–30 years ago.This study confirms that the B dissolved in groundwater is anthropogenic. In fact, the δ11B values of groundwater B match the range −12.2‰ to −13.3‰ of the Turkish B mineral (colemanite) processed in boric acid manufacturing, in the course of which no significant isotopic effects have been observed. This isotopic tracing of the Cecina alluvial aquifer occurs just below the confluence of the Possera Creek, which carries the B releases from Larderello. Strontium isotope ratios support this conclusion.At about 18 km from the Possera Creek confluence, the groundwater δ11B drops to much more negative values (−22‰ to −27‰), which are believed to be produced by adsorption–desorption interactions between dissolved B and the aquifer matrix. The δ11B of B fixed in well bottom sediments shows a similar variation. At present, desorption is prevailing over adsorption because the releases of B-rich water have ceased. A theoretical model is suggested to explain the isotopic trends observed.Thus, B isotopes appear to be a powerful tool for identifying the origin of B contamination in natural waters, although isotopic effects associated with adsorption–desorption processes may complicate the picture, to some extent.  相似文献   

14.
《Organic Geochemistry》2012,42(12):1269-1276
This study sought to characterize hydrogen isotopic fractionation during biosynthesis of leaf wax n-alkanes in succulent plants capable of crassulacean acid metabolism (CAM). The metabolic and physiological features of CAM represent crucial strategies for survival in hot and dry climates and have been hypothesized to impact hydrogen isotope fractionation. We measured the stable carbon and hydrogen isotopic compositions (δ13C and δD, respectively) of individual n-alkanes in 20 species of succulent plants from a global collection of the Huntington Botanical Gardens, San Marino, California. Greenhouse conditions and irrigation with water of constant δD value enabled determination of interspecies differences in net D/H fractionation between source water and leaf wax products. Carbon isotope ratios provide constraints on the extent of CAM vs. C3 photosynthesis and indicate a wide range of CAM use, with δ13C values ranging from −33.01‰ to −18.54‰ (C27–C33 n-alkanes) and −26.66‰ to −17.64‰ (bulk tissue). Despite the controlled growth environment, we observed ca. 90‰ interspecies range in δD values from −193‰ to −107‰. A positive correlation between δ13Cbulk and δDC31 values with R2 = 0.60 (δ13CC31 and δDC31 values with R2 = 0.41) implicates a metabolic isotope effect as the dominant cause of interspecies variation in the hydrogen isotopic composition of leaf wax n-alkanes in CAM-intermediate plants.  相似文献   

15.
《Chemical Geology》2007,236(3-4):181-198
Variations in molybdenum isotopic composition, spanning the range of ∼ 2.3‰ in the terms of 97Mo/95Mo ratio, have been measured in sediment cores from three lakes in northern Sweden and north-western Russia. These variations have been produced by both isotopically variable input of Mo into the lakes due to Mo isotopic heterogeneity of bedrock in the drainage basins and fractionation in the lake systems due to temporal variations in limnological conditions. Mo isotope abundances of bedrock in the lake drainage basins have been documented by analysis of Mo isotope ratios of a suite of molybdenite occurrences collected in the studied area and of detrital fractions of the lake sediment cores. The median δ97Mo value of the investigated molybdenites is 0.26‰ with standard deviation of 0.43‰ (n = 19), whereas the median δ97Mo value of detrital sediment fractions from two lakes is − 0.40‰ with standard deviation of 0.36‰ (n = 15).The isotopic composition of Mo in the sediment cores has been found to be dependent on redox conditions of the water columns and the dominant type of scavenging phases. Hydrous Fe oxides have been shown to be an efficient scavenger of Mo from porewater under oxic conditions. Oxidative precipitation of Fe(II) in the sediments resulted in co-precipitation of Mo and significant authigenic enrichment at the redox boundary. In spite of a pronounced increase in Mo concentration associated with Fe oxides at the redox boundary the isotopic composition of Mo in this zone varies insignificantly, suggesting little or no isotope fractionation during scavenging of Mo by hydrous Fe oxides. In a lake with anoxic bottom water a chironomid-inferred reconstruction of O2 conditions in the bottom water through the Holocene indicates that increased O2 concentrations are generally associated with low δ97Mo/95Mo values of the sediments, whereas lowered O2 contents of the bottom water are accompanied by relatively high δ97Mo/95Mo values, thus confirming the potential of Mo isotope data to be a proxy for redox conditions of overlying waters. However, it is pointed out that other processes including input of isotopically heterogeneous Mo and Mn cycling in the redox-stratified water column can be a primary cause of variations in Mo isotopic compositions of lake sediments.  相似文献   

16.
This study was conducted on recent desert samples—including (1) soils, (2) plants, (3) the shell, and (4) organic matter from modern specimens of the land snail Eremina desertorum—which were collected at several altitudes (316–360 m above sea level) from a site in the New Cairo Petrified Forest. The soils and shellE. desertorum were analyzed for carbonate composition and isotopic composition (δ18O, δ13C). The plants and organic matterE. desertorum were analyzed for organic carbon content and δ13C. The soil carbonate, consisting of calcite plus minor dolomite, has δ18O values from −3.19 to −1.78‰ and δ13C values −1.79 to −0.27‰; covariance between the two values accords with arid climatic conditions. The local plants include C3 and C4 types, with the latter being dominant. Each type has distinctive bulk organic carbon δ13C values: −26.51 to −25.36‰ for C3-type, and −13.74 to −12.43‰ for C4-type plants.The carbonate of the shellE. desertorum is composed of aragonite plus minor calcite, with relatively homogenous isotopic compositions (δ18Omean = −0.28 ± 0.22‰; δ13Cmean = −4.46 ± 0.58‰). Most of the δ18O values (based on a model for oxygen isotope fractionation in an aragonite-water system) are consistent with evaporated water signatures. The organic matterE. desertorum varies only slightly in bulk organic carbon δ13C values (−21.78 ± 1.20‰) and these values suggest that the snail consumed more of C3-type than C4-type plants. The overall offset in δ13C values (−17.32‰) observed between shellE. desertorum carbonate and organic matterE. desertorum exceeds the value expected for vegetation input, and implies that 30% of carbon in the shellE. desertorum carbonate comes from the consumption of limestone material.  相似文献   

17.
The regional geologic setting of the Adycha-Taryn metallogenic zone, one of the areas most productive for noble-metal mineralization in northeastern Russia, is discussed. The intricate metallogenic history of the zone and the prolonged geodynamic activity of its ore-hosting structures are documented. Different types of mineralization, such as hydrothermal-metamorphogenic, gold-bismuth, gold-quartz, gold-antimony, and silver-antimony, are described. New data on the isotopic compositions of oxygen in quartz, sulfur in sulfides, and oxygen and carbon in carbonates from different mineralization types are presented. The early metamorphogenic quartz beyond the ore zones has δ18O = + 20.1 ± 2.0‰. At the gold-bismuth deposits, the δ18O values of quartz are within the narrow range of + 12.5 ± 0.4‰. Quartz from the gold-quartz mineralization shows much wider variation in δ18O values, from + 14.2 to + 19.5‰. A similar range (δ18O = + 16.1 to + 19.2‰) is observed for the gold-antimony mineralization. Cryptograined quartz from the silver-antimony mineralization is enriched in light oxygen isotopes (δ18O = -3.2 to + 4.7‰). The following δ34S values (‰) have been established in sulfides of mineralization of different types: gold-bismuth -3.7 to -2.2 (Apy) and -6.7 to -6.8 (Py); gold-quartz -2.1 to + 2.4 (Apy), -6.6 to + 5.4 (Py), and -6.1 to + 4.2 (St); gold-antimony -2.0 to + 1.6 (Apy), -3.5 to + 2.1 (Py), and -5.3 to + 0.2 (St); and silver-antimony -2.0 to -1.9 (Apy), -2.2 ± 0.1 (Py), and -5.7 to -5.6 (St). The δ13C and δ18O values are contrasting in the studied types of mineralization, varying respectively from -6.9 to -5.9‰ and from + 2.1 to + 5.7‰ (gold-bismuth), from -9.1 to -6.1‰ and from + 12.4 to 18.7‰ (gold-quartz), from -12.1 to -9.5‰ and from + 15.0 to + 16.3‰ (gold-antimony), from -11.6 to -11.1‰ and from + 1.5 to + 4.7‰ (silver-antimony). Metamorphogenic calcites are rich in both heavy C (-1.1 to -1.7‰) and heavy O (+ 20.3 to + 20.5‰) isotopes. Microthermometric study and crush-leach analysis of fluid inclusions have revealed differences in the composition of ore-forming fluids and formation conditions for different types of mineralization. The isotopic compositions of O, C, and S of mineral-forming fluids suggest a significant input of magmatic fluids to the formation of gold-bismuth and gold-antimony deposits, the contribution of metamorphic fluids increases at gold-quartz deposits, and meteoric water is involved in the formation of silver-antimony deposits.  相似文献   

18.
《Sedimentary Geology》2006,183(1-2):51-69
The Chicxulub Sedimentary Basin of the northwestern Yucatan Peninsula, Mexico, which was formed because of the largest identified Phanerozoic bolide impact on Earth, became a site of deposition of dominantly marine carbonate sediments during most of the Cenozoic Era. This is a study of the filling and diagenetic history of this basin and surrounding areas. The study makes use of lithologic, biostratigraphic, petrographic, and geochemical data obtained on core samples from boreholes drilled throughout the northwestern Yucatan Peninsula.The core sample data indicate that: 1) The Chicxulub Sedimentary Basin concentrated the deposition of pelagic and outer-platform sediments during the Paleocene and Eocene, and, in places, during the Early Oligocene, as well, and filled during the Middle Miocene, 2) deeper-water limestone also is present within the Paleocene and Lower Eocene of the proposed Santa Elena Depression, which is located immediately south of the Basin, 3) shallow-water deposits are relatively more abundant outside the Basin and Depression than inside, 4) the autigenic and allogenic silicates from the Paleogene formations are the most abundant inside the Depression, 5) sediment deposition and diagenesis within the Basin also were controlled by impact crater topography, 6) the abundance of the possible features of subaerial exposure increases upward and outward from the center of the Basin, and 7) the formation of replacive low-magnesium calcite and dolomite, dedolomitization, dissolution, and precipitation of vug-filling calcite and dolomite cement have been more common outside the Basin than inside.δ18O in whole-rock (excluding vug-filling) calcite from core samples ranges from − 7.14‰ to + 0.85‰ PDB. δ13C varies from − 6.92‰ to + 3.30‰ PDB. Both stable isotopes correlate inversely with the abundance of subaerial exposure features indicating that freshwater diagenesis has been extensive especially outside and at the edge of the Chicxulub Sedimentary Basin.δ18O and δ13C in whole-rock (excluding vug-filling) dolomite ranges from − 5.54‰ to + 0.87‰ PDB and − 4.63‰ to + 3.38‰ PDB, respectively. Most dolomite samples have negative δ18O and positive δ13C suggesting that replacive dolomitization involved the presence of a fluid dominated by freshwater and/or an anomalously high geothermal gradient.Most dolomite XRD-determined mole percent CaCO3 varies between 51 and 56. Replacive dolomite is larger, more euhedral, and less stoichiometric inside the Chicxulub Sedimentary Basin than outside.  相似文献   

19.
A high resolution analysis of benthic foraminifera as well as of aeolian terrigenous proxies extracted from a 37 m-long marine core located off the Mauritanian margin spanning the last ~ 1.2 Ma, documents the possible link between major continental environmental changes with a shift in the isotopic signature of deep waters around 1.0–0.9 Ma, within the so-called Mid-Pleistocene Transition (MPT) time period. The increase in the oxygen isotopic composition of deep waters, as seen through the benthic foraminifera δ18O values, is consistent with the growth of larger ice sheets known to have occurred during this transition. Deep-water mass δ13C changes, also estimated from benthic foraminifera, show a strong depletion for the same time interval. This drastic change in δ13C values is concomitant with a worldwide 0.3‰ decrease observed in the major deep oceanic waters for the MPT time period. The phase relationship between aeolian terrigeneous signal increase and this δ13C decrease in our record, as well as in other paleorecords, supports the hypothesis of a global aridification amongst others processes to explain the deep-water masses isotopic signature changes during the MPT. In any case, the isotopic shifts imply major changes in the end-member δ18O and δ13C values of deep waters.  相似文献   

20.
The Southern Alps are an ideal locality for studying patterns of isotopic fractionation associated with orographic precipitation. We have evaluated whether altitudinal change is reflected in the stable hydrogen isotopic composition (δ2H) of stream water, plant stem water and leaf wax lipids (n-alkanes) from living plants and soils, as well as in soil temperature. Samples were collected along an altitudinal transect from the windward side of the Southern Alps to Lake Hawea in the rain shadow. The results indicate that δ2H values of stem water overlap with stream water, demonstrating a gradual decrease with elevation that complied with modeled Rayleigh distillation, reflecting an isotopic lapse rate of −18.0 (± 1.1, 1σ)‰/km. Leaf and soil n-alkanes shared similar δ2H values and were 2H depleted relative to stem/stream waters. The values for soil n-alkanes indicated an isotopic lapse rate of −21.8 (± 2.0, 1σ)‰/km, consistent with precipitation data and long term observations. MBT/CBT derived soil temperature values based on the relative distribution of microbial tetraether lipids were similar to midsummer temperature observations, displaying an elevational decrease rate of −5.6 (± 1.5, 1σ) °C/km, consistent with regional and global observations.The results indicate that sedimentary lipid δ2H and microbial tetraether temperature estimates captured altitudinal trends in the isotopic composition of precipitation and mean temperature and further support their application in the reconstruction of past climate and surface uplift histories. However, notable differences in isotopic composition and temperature estimates between in situ soils and those with downslope transport of material emphasize the importance of facies analysis when interpreting past systems.  相似文献   

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