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1.
Olivine, clinopyroxene and orthopyroxene in variably metasomatised peridotite xenoliths from three lithospheric mantle sections beneath the East African Rift in Tanzania (Lashaine, Olmani, Labait) show systematic differences in their average Li concentrations (2.4 ppm, 2.0 ppm and 1.5 ppm, respectively) and intermineral isotopic fractionations, with olivine being heaviest (δ7Li = + 2.3 to + 13.9‰, average + 5.0‰), followed by orthopyroxene (? 4.1 to + 6.5‰, average + 0.8‰) and clinopyroxene (? 6.7 to + 4.1‰, average ? 1.6‰). These features are ascribed to the effects of kinetic Li isotope fractionation combined with different Li diffusivities in mantle minerals.Two main mechanisms likely generate diffusion-driven kinetic Li isotope fractionation in mantle xenoliths (1) Li diffusion from grain boundary melt into minerals during recent metasomatism or entrainment in the host magma and (2) subsolidus intermineral Li-redistribution. The latter can produce both isotopically light (Li-addition) and heavy (Li-loss) minerals and may occur in response to changes in pressure and/or temperature.Modelling shows that non-mantle-like δ7Li in clinopyroxene (< + 2‰), combined with apparent equilibrium olivine-clinopyroxene elemental partitioning in most peridotite xenoliths from all three Tanzanian localities probably reflects incipient Li addition during interaction with the host magma. Low δ7Li (< ? 3‰), combined with high Li concentrations (> 3 ppm) in some clinopyroxene may require very recent (minutes) Li ingress from a Li-rich melt (100s of ppm) having mantle-like δ7Li. This might happen during late fragmentation of some mantle xenoliths caused by a volatile- (and Li-) rich component exsolved from the host basalt. In contrast, high Li concentrations (> 2 ppm) and δ7Li (> 4‰) in olivine from many Labait and Olmani samples are attributed to an older, pre-entrainment enrichment event during which isotopic equilibrium was attained and whose signature was not corrupted during xenolith entrainment. Low Li concentrations and mantle-like isotopic composition of olivine from most Lashaine xenoliths indicate limited metasomatic Li addition.Thus, Li concentrations and isotope compositions of mantle peridotites worldwide may reflect two processes, with olivine mainly preserving a signature of depletion in refractory samples (low Li contents and δ7Li) or of older (precursory) melt addition in metasomatised samples (high Li contents and δ7Li), while non mantle-like, low δ7Li in almost all clinopyroxene can be due to Li ingress during transport in the host magma and/or slow cooling, if the samples were erupted in lavas. In Tanzania, the peridotites experienced rift-related heating prior to entrainment and were quenched upon eruption, so Li ingress is the most likely process responsible for the isotopically light clinopyroxene here.  相似文献   

2.
In this study, we investigated Fe and Li isotope fractionation between mineral separates of olivine pheno- and xenocrysts (including one clinopyroxyene phenocryst) and their basaltic hosts. Samples were collected from the Canary Islands (Teneriffa, La Palma) and some German volcanic regions (Vogelsberg, Westerwald and Hegau). All investigated bulk samples fall in a tight range of Li and Fe isotope compositions (δ56Fewr = 0.06–0.17‰ and δ7Lima = 2.5–5.2‰, assuming δ7Li of the olivine-free matrix is virtually identical to that of the bulk sample for mass balance reasons). In contrast, olivine phenocrysts display highly variable, but generally light Fe and mostly light Li isotope compositions compared to their respective olivine-free basaltic matrix, which was considered to represent the melt (with δ56Feol = ? 0.24 to 0.14‰ and δ7Liol = ? 10.5 to + 6.5‰, respectively). Single olivine crystals from one sample display even a larger range of δ56Feol between ? 0.7 and + 0.1‰. One single clinopyroxene phenocryst displays the lightest Li isotope composition (δ7Licpx = ? 17.7‰), but no Fe isotope fractionation relative to melt. The olivine phenocrysts show variable Mg# and Ni (correlated in most cases) that range between 0.89 and 0.74 and between 300 and 3000 μg/g, respectively. These olivines likely grew by fractional crystallization in an evolving magma. One sample from the Vogelsberg volcano contained olivine xenocrysts (Mg# > 0.89 and Ni > 3000 μg/g), in addition to olivine phenocrysts. This sample displays the highest Li- and the second highest Fe-isotope fractionation between olivine and melt (Δ7Liol-melt = ? 13; Δ56Feol-melt = ? 0.29).Our data, i.e. the variable olivine- at constant whole rock and matrix isotope compositions, strongly indicate disequilibrium, i.e. kinetic Fe and Li isotope fractionation between olivine and melt (for Li also between cpx and melt) during fractional crystallization. Δ7Liol-melt is correlated with the Li partitioning between olivine and melt (i.e. with Liol/Limelt), indicating Li isotope fractionation due to preferential (faster) diffusion of 6Li into olivine during fractional crystallization. Olivine with low Δ7Liol-melt, also have low Δ56Feol-melt, indicating that Fe isotope fractionation is also driven by diffusion of isotopically light Fe into olivine, potentially, as Fe–Mg inter-diffusion. The lowest Δ56Feol-melt (? 0.40) was observed in a sample from Westerwald (Germany) with abundant magnetite, indicating relatively oxidizing conditions during magma differentiation. This may have enhanced equilibrium Fe isotope fractionation between olivine and melt or fine dispersed magnetite in the basalt matrix may have shifted its Fe isotope composition towards higher δ56Fe. The decoupling of Li- and Fe isotope fractionation in cpx is likely due to faster diffusion of Li relative to Fe in cpx, implying that the large investigated cpx phenocryst resided in the magma for only a short period of time which was sufficient for Li- but not for Fe diffusion. The absence of any equilibrium Fe isotope fractionation between the investigated cpx phenocryst and its basaltic host may be related to the similar Fe3 +/Fe2 + of cpx and melt. In contrast to cpx, the generally light Fe isotope composition of all investigated olivine separates implies the existence of equilibrium- (in addition to diffusion-driven) isotope fractionation between olivine and melt, on the order of 0.1‰.  相似文献   

3.
《Ore Geology Reviews》2011,41(1):27-40
Diyadin mineralization is the first reported gold deposit located in a collisional tectonic environment in Eastern Anatolia. The mineralization is related to N–S and N10–20°W-trending fault systems and hosted within the Paleozoic metamorphic basement rocks of the Anatolide–Toride microcontinent. Calc-schist, dolomitic marble and Miocene and Quaternary volcanic rocks comprise the exposed units in the mineralized area. Geochemical signatures, alteration types and host rock characteristics of the Diyadin gold deposit resemble those of Carlin-type deposits. Mineralization is constrained by alteration of overlying volcanic rocks to younger than ~ 14 Ma (K–Ar).Carbon and oxygen stable isotope measurements of carbonate rocks were made on six drill holes (n = 81) with an additional four samples of fresh carbonate rocks from surface outcrops. Background carbonate rocks have δ13CV-PDB ~ 1.8‰ and δ18OV-SMOW ~ 27‰. Isotopically-altered host rock samples have decreased δ18O (down to ~+11.4‰) and variable δ13C (from − 3.6 to + 4.8‰). Postore carbonate veins and cave-fill material have distinctly different isotopic signatures, particularly carbon (from δ13C = + 8.4 to + 9.8‰). Whether this post-ore carbonate is simply very late in mineralization associated with the gold system, or is a completely different, younger system utilizing the same pathways, is unclear at present. Within the host rock sample set, there is no correlation between gold and δ13C, and a weak correlation between gold and δ18O, indicative of water–rock interaction and isotopic alteration. Both the isotopic data and structural mapping suggest that the main upflow zone for the deposit is near the northern portion of the drill fence. Additional data at multiple scales are required to clarify the relationship(s) between fluid flow and mineralization.  相似文献   

4.
Although the effects of biodegradation on the composition and physical properties of crude oil have been well studied, effects of in-reservoir petroleum biodegradation on molecular and isotopic compositions of crude oils are not yet clearly understood. The Alberta Basin, in western Canada, is one of the world’s largest petroleum accumulations and constitutes an ideal example of a natural suite of sequentially biodegraded oils. The basin hosts moderately to severely biodegraded petroleum, regionally distributed and in single, more or less continuous, oil columns. In this study, a series of oil samples from the Alberta heavy oil and oil sands provinces, with varying degrees of biodegradation, were analyzed to assess the impact of progressive biodegradation on the molecular and C, H, N, and S isotopic compositions of oils. The results of the molecular characterization of the hydrocarbon fraction of the studied oils show that the oils have suffered biodegradation levels from 2 to 10+ (toward the Alberta–Saskatchewan border) on the Peters and Moldowan scale of biodegradation (abbreviated PM 2 to PM 10) and from tens to hundreds on the Manco scale. Within single reservoirs, increasing biodegradation was observed from top to bottom of the oil columns at all sites studied. The whole oil stable isotopic compositions of the samples varied in the ranges δ13C = −31.2‰ to −29.0‰, δ2H = −147‰ to −133‰, δ15N = 0.3–4.7‰ and δ34S = 0.4–6.4‰. The maximum differences between δ values of samples (Δ) within single oil columns were Δ13C = 1.4‰, Δ2H = 7‰, Δ15N = 1.7‰ and Δ34S = 1.0‰. Regional variations in the isotopic compositions of oil samples from different wells (averaged values from top to bottom) were 1.2‰ for δ13C, 12‰ for δ2H, 4.1‰ for δ15N and 5.5‰ for δ34S and hence generally significantly larger variations were seen than variations observed within single oil columns, especially for N and S. It appears that even severe levels of biodegradation do not cause observable systematic variations in carbon, nitrogen or sulfur isotope composition of whole oils. This indicates that sulfur and nitrogen isotopic compositions may be used in very degraded oils as indicators for oil charge from different source rock facies.  相似文献   

5.
This contribution reports our preliminary work to determine Cu isotope ratios for various granite rocks and examine the Cu isotope systematics within granite suites. A chemical procedure, modified from Maréchal [Maréchal, C.N., Télouk, P. and Albarède, F., 1999. Precise analysis of copper and zinc isotopic compositions by plasma-source mass spectrometry. Chemical Geology, 156(1–4): 251–273.], was used to separate Cu from rock matrix. Quantitative recovery (100.6 ± 1.6%), with a low total procedural blank (2.65 ± 0.66 ng) for Cu, has been achieved, allowing Cu isotopic measurements on samples with as little as 10 ppm Cu. The Cu isotope ratios (δ65Cu relative to NIST SRM 976) of 32 rock samples, ranging from mafic to felsic compositions, from 3 batholiths (2 I-type, 1 S-type) from the Lachlan Fold Belt in southeastern Australia, vary from ? 0.46‰ to 1.51‰. Most of them cluster around zero, with mean values for the I-type and S-type granites of 0.03 ± 0.15‰ and ? 0.03 ± 0.42‰ (2 sigma) respectively. These data, together with Cu isotope ratios of two loess samples, provide preliminary evidence that the baseline Cu isotopic composition of the crystalline part of upper continental crust is close to zero. The tight clustering of Cu isotope ratios of rocks from the I-type suites suggests that high-temperature magmatic processes do not produce significant Cu isotope fractionation. However, two granites with abnormally heavy Cu isotope signatures (up to 1.51‰) appears to be the result of localized hydrothermal alteration. Measurable variation in Cu isotopic composition of the S-type granite may reflect isotopic heterogeneity in the sedimentary source region as a result of redox processes or may be due to hydrothermal overprinting. Thus, Cu isotope geochemistry may be a useful tracer for studying hydrothermal alteration and source heterogeneity of granitic rocks.  相似文献   

6.
Large variations are reported in the B concentrations and isotopic ratios of river and thermal spring waters in Guadeloupe, Lesser Antilles. Rivers have δ11B values around 40‰ and B concentrations lower than 30 μg/L, while thermal springs have δ11B of 8–15‰ and B concentrations of 250–1000 μg/L. River samples strongly impacted by hydrothermal inputs have intermediate δ11B and B contents. None of these surface water samples have δ11B comparable to the local unweathered volcanic rocks (around 0‰), implying that a huge isotopic fractionation of 40‰ takes place during rock weathering, which could be explained by preferential incorporation of 10B during secondary mineral formation and adsorption on clays, during rock weathering or in the soils. The soil-vegetation B cycle could also be a cause for such a fractionation. Atmospheric B with δ11B of 45‰ represents 25–95% of the river B content. The variety of the thermal spring chemical composition renders the understanding of B behavior in Guadeloupe hydrothermal system quite difficult. Complementary geochemical tracers would be helpful.  相似文献   

7.
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   

8.
Lithium (Li) is a fluid-mobile element and δ7Li in secondary deposits represents an excellent proxy for silicate weathering and authigenic mineral formation. The soil samples from 1205 to 1295 cm in the Weinan profile, one of the best developed loess-paleosol sequences covering the last glacial–interglacial climatic cycle, were collected and chemically separated into detritus and carbonate fractions for subsequent analyses of Li, δ7Li, major and trace elements. Other desert specimens (i.e., Qaidam Desert, Tengger Desert, Badain Juran Desert and Taklimakan Desert) near the Chinese Loess Plateau (CLP) and various standard clays were analyzed for assisting provenance determination. The Li and δ7Li distributions in the detritus are rather homogeneous, 1.4–2.0 μg/g and +2.5‰ to +4.7‰, respectively, compared with the carbonate fraction. The detrital δ7Li varies systematically with magnetic susceptibility and grain size changes, reflecting significant Li isotopic variation associated with sources and mineralogy of detrital material. On the other hand, Li and δ7Li in carbonates show large changes, 781–963 ng/g and −4.1‰ to +10.2‰, respectively. These carbonate δ7Li correlated well with the estimated index of chemical weathering, as a result of Li mobilization and soil formation during chemical weathering.  相似文献   

9.
In this study potential iron isotope fractionation by magmatic processes in the Earth's crust was systematically investigated. High precision iron isotope analyses by MC-ICP-MS were performed on a suite of rock samples representative for the volcanic evolution of the Hekla volcano, Iceland. The whole series of Hekla's rocks results from several processes. (i) Basaltic magmas rise and induce partial melting of meta-basalts in the lower part of the Icelandic crust. The resulting dacitic magma evolves to rhyolitic composition through crystal fractionation. During this differentiation the δ56/54FeIRMM-014 values increase successively from 0.051 ± 0.021‰ for the primitive dacites to 0.168 ± 0.021‰ for the rhyolites. This increase can be described by a Rayleigh fractionation model using a constant bulk fractionation factor between all mineral phases (M) and the silicate liquid (L) of Δ56/54FeM–L = ? 0.1‰. (ii) The basaltic magma itself differentiates by crystal fractionation to basaltic andesite composition. No Fe isotope fractionation was found in this series. All basalts and basaltic andesites have an average δ56/54FeIRMM-014 value of 0.062 ± 0.042‰ (2SD, n = 9), identical to mean terrestrial basaltic values reported in previous studies. This observation is consistent with the limited removal of iron from the remaining silicate melt through crystal fractionation and small mineral-melt Fe isotope fractionation factors expected at temperatures in excess of 1050 °C. (iii) Andesites are produced by mixing of basaltic andesite with dacitic melts. The iron isotope composition of the andesites is matching that of the basaltic andesites and the less evolved dacites, in agreement with a mixing process. In the Hekla volcanic suite Li concentrations are positively correlated with indicators of magma differentiation. All Hekla rocks have δ7Li values typical for the upper mantle and demonstrate the absence of resolvable Li isotope fractionation during crystal fractionation. As a fluid-mobile trace element, Li concentrations and isotopes are a potential tracer of magma/fluid interaction. At Hekla, Li concentrations and isotope compositions do not indicate any extensive fluid exsolution. Hence, the heavy Fe isotope composition of the dacites and rhyolites can be predominately attributed to fractional crystallisation. Iron isotope analyses on single samples from other Icelandic volcanoes (Torfajökull, Vestmannaeyjar) confirm heavy Fe isotope enrichment in evolving magmas. Our results suggest that the iron isotope composition of highly evolved crust can be slightly modified by magmatic processes.  相似文献   

10.
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   

11.
The Jinding Zn–Pb deposit located in the Mesozoic-Cenozoic Lanping Basin of southwest China has ore reserves of ∼ 220 Mt with an average grade of 6.1% Zn and 1.3% Pb. The mineralization is hosted by sandstone in the Early Cretaceous Jingxing Formation and limestone breccia in the Paleocene Yunlong Formation. Mineralization in both types of host rocks is characterized by a paragenetic sequence beginning with marcasite–sphalerite (Stage 1) followed by pyrite–marcasite–sphalerite–galena (Stage 2), and then galena–sphalerite–pyrite–sulfate–carbonate (Stage 3). Pyrite from these stages have different δ33S compositions with pyrite from Stage 1 averaging − 9.6‰, Stage 2 averaging − 8.9‰, and Stage 3 averaging + 0.3‰. Sphalerite hosted by the sandstone has similar δ66Zn values ranging from 0.10 to 0.30‰ in all stages of the mineralization, but sphalerite samples from the limestone breccia-hosted ore show variable δ66Zn values between − 0.03 and 0.20‰. Our data on sphalerite precipitated during the earlier stages of mineralization has a constant δ66Zn value and cogenetic pyrite displays a very light sulfur isotope signature, which we believe to reflect a sulfur source that formed during bacterial sulfate reduction (BSR). The Stage 3 sphalerite and pyrite precipitated from a late influx of metal-rich basinal brine, which had a relatively constant variable δ66Zn isotopic composition due to open system isotope fractionation, and a near zero δ33S composition due to the influence of abiotic thermochemical sulfate reduction from observed sulfates in the host rock.  相似文献   

12.
The Han-Xing iron mineralization in the central North China Craton is a typical Fe skarn deposit associated with altered diorites. Here we report the Fe isotopic compositions of whole rocks and mineral separates from this deposit with a view to evaluate the Fe isotope fractionation during the formation of Fe skarn deposit, and to constrain the metal source. The Fe isotopes show a large variation both in whole rocks and mineral separates. Altered diorites show a wide range in δ56Fe values (− 0.07‰ to + 0.21‰ relative to the Fe isotope standard IRMM-014) which positively correlate with their TFe2O3/TiO2 ratios (Fe2O3 and FeO calculated as TFe2O3). The positive correlation indicates that heavy Fe isotopes were preferentially leached from diorites during the skarn-type alteration. Among the metallic minerals, pyrite and pyrrhotite are isotopically heavier (+ 0.12‰ to + 0.48‰) than the magnetite (+ 0.07‰ to + 0.21‰). Fe isotope fractionation between mineral pairs demonstrates that magnetite did not attain Fe isotopic equilibrium with pyrite and pyrrhotite, whereas pyrite and pyrrhotite might have attained isotopic equilibrium. Petrological observations and major element data also suggest that iron was leached from the diorites during the skarn-type alteration. If the leached iron provides the main Fe budget of the Han-Xing Fe skarn deposit, magnetite in ores would be isotopically heavier than the unaltered diorite. However, our results are in contrast with the magnetite being isotopically lighter than the unaltered diorite. This suggests that the major Fe source of the Han-Xing Fe skarn deposit is not from the leaching of diorites, and might be from magmatic fluid which is isotopically lighter than the silicate melt. Our data demonstrate that Fe isotopes can be used as important tracers in deciphering the metal source of Fe skarn deposits.  相似文献   

13.
Lithium-rich brine within the sub-surface of the Salar del Hombre Muerto (SHM) salt pan in the Andes of northwestern Argentina has a chemical and isotopic composition which is consistent with Li derived from several sources: the modern halite saturated lagoon, Li-rich salts and brines formed recently, and dissolution of halite which precipitated from ancient saline lakes. SHM lies in the closed basin that includes part of the massive Cerro Galán caldera which is drained by the Río los Patos, which is responsible for 90% of surface runoff into the salar. The low Li isotope composition, +3.4‰, of this river is consistent with significant contributions of geothermal spring water. As water drains through the volcaniclastic deposits which cover a large proportion of the basin, Li removal, as indicated by decreasing Li/Na, occurs but without significant isotope fractionation. This indicates a mechanism of surface sorption onto smectite or ferrihydrite rather than Li incorporation into octahedral structural sites of clays. These observations suggest that conditions in this high altitude desert have limited the dilution of hydrothermal spring water as well as the formation of clay minerals, which jointly have allowed the Li resource to accumulate rapidly. Changes in climate on a multi-millennial time scale, specifically in the hydrologic budget, have resulted in solute accumulation rates that have been variable through time, and decoupled Li and Na fluxes. Inflow to the salar under modern conditions has high Li/Na (7.9 × 10−3 by wt) with δ7Li indistinguishable from basement rocks (−0.3‰ to +6.4‰), while under pluvial climate conditions the Li/Na of the saline lake was 40 times lower than the modern lagoon (0.1–0.3 × 10−3 compared to 10.6–13.4 × 10−3) with slightly higher δ7Li, +6.9‰ to +12.3‰, reflecting the uptake of 6Li into secondary minerals which formed under a wetter climate.  相似文献   

14.
Natural gas in the Xujiahe Formation of the Sichuan Basin is dominated by hydrocarbon (HC) gas, with 78–79% methane and 2–19% C2+ HC. Its dryness coefficient (C1/C1–5) is mostly < 0.95. The gas in fluid inclusions, which has low contents of CH4 and heavy hydrocarbons (C2+) and higher contents of non-hydrocarbons (e.g. CO2), is a typical wet gas produced by thermal degradation of kerogen. Gas produced from the Upper Triassic Xujiahe Formation (here denoted field gas) has light carbon isotope values for methane (δ13C1: −45‰ to −36‰) and heavier values for ethane (δ13C2: −30‰ to −25‰). The case is similar for gas in fluid inclusions, but δ13C1 = −36‰ to −45‰ and δ13C2 = −24.8‰ to −28.1‰, suggesting that the gas experienced weak isotopic fractionation due to migration and water washing. The field gas has δ13CCO2 values of −15.6‰ to −5.6‰, while the gas in fluid inclusions has δ13CCO2 values of −16.6‰ to −9‰, indicating its organic origin. Geochemical comparison shows that CO2 captured in fluid inclusions mainly originated from source rock organic matter, with little contribution from abiogenic CO2. Fluid inclusions originate in a relatively closed system without fluid exchange with the outside following the gas capture process, so that there is no isotopic fractionation. They thus present the original state of gas generated from the source rocks. These research results can provide a theoretical basis for gas generation, evolution, migration and accumulation in the basin.  相似文献   

15.
The study presents copper (Cu) isotope data of mineral separates of chalcopyrite from four drill core samples in the Miocene Dabu porphyry Cu-Mo deposit formed in a post-collisional setting in the Gangdese porphyry copper belt, southern Tibet. Copper isotope values in hypogene chalcopyrite range from –1.48‰ to +1.12‰, displaying a large variation of up to 2.60‰, which demonstrates Cu isotope fractionation at high-temperature during hydrothermal evolution. The majority of measured chalcopyrite isotopic compositions show a gradual increasing trend from –1.48‰ to +1.12‰ with the increase of drilling depth from 130m to 483m, as the alteration assemblages change from potassic to phyllic. Similarly, the other δ65Cu values (δ65Cu = ((65Cu/63Cu)sample/(65Cu/63Cu)standard − 1) × 1000) of the chalcopyrite show a gradual increasing trend from −1.48‰ to +0.59‰ with the decrease of drilling depth from 130 m to 57 m, as the alteration assemblages change from potassic, phyllic, through argillic to relatively fresh. These observations suggest a genetic link between Cu isotope variation and silicate alteration assemblages formed at different temperatures, indicative of a Rayleigh precipitation process resulting in the large variation of δ65Cu values at Dabu. In general, samples closest to the center of hydrothermal system dominated by high-temperature potassic alteration are isotopically lighter, whereas samples dominated by low-temperature phyllic alteration peripheral to the center are isotopically heavier. The predicted flow pathways of hydrothermal fluids are from No. 0 to No. 3 exploration line, and the lightest δ65Cu values are the most proximal to the hydrothermal source. Finally, we propose that the northwest side of the No. 0 exploration line has high potential for hosting undiscovered orebodies. The pattern of Cu isotope variation in conjunction with the features of silicate alteration in porphyry system can be used to trace the hydrothermal flow direction and to guide mineral exploration.  相似文献   

16.
《Chemical Geology》2006,225(1-2):137-155
Carbon stable isotopes from carbonate minerals (mainly dolomite) from six wells from the Lower Triassic Sherwood Sandstones of the Corrib Gas Field, Slyne Basin, west of Ireland, allow stratigraphic correlation. The results also provide information on palaeoenvironmental change during the deposition of these continental redbed sedimentary rocks. The Triassic reservoir rocks have been buried to > 4000 m and heated to > 165 °C and now contain methane-rich gas. Although the oxygen isotopic signal has been at least partially reset during burial and heating, a primary carbon isotopic signal appears to have survived diagenesis. The carbon isotope ratio varies from − 3.2‰ to + 2.1‰. All six wells show similar stratigraphic changes when all the carbon isotope data are plotted relative to a major playa horizon. δ13C increases from about − 3‰ at the base of the Sherwood to about + 2‰ 170 m above the base. δ13C then decreases to about − 2‰ for the next 70 m and remains steady for the following 50 m. The top 20 m of the Sherwood contains carbonate with a δ13C values decreasing to about − 3‰. The occurrence of a stratigraphically-correlatable carbon isotope pattern implies that the primary evolution signal has been preserved. The change in δ13C correlates with indicators of aridity and biological stress such that the highest δ13C values are in sedimentary rocks deposited in a playa lake (arid times); these rocks contain the greatest quantity of dolomite cement. Conversely, the lowest δ13C values correspond to sedimentary rocks deposited from well-developed rivers (relatively humid times) from the lowest quantity of dolomite cement. The same carbon isotope evolution has been found in another well in the Slyne basin and in Belgium, suggesting that the palaeoenvironmental isotope signal in the Triassic sedimentary rocks of the Corrib Field may have a regional significance.  相似文献   

17.
Geothermal resources are very rich in Yunnan, China. However, source of dissolved solutes in geothermal water and chemical evolution processes remain unclear. Geochemical and isotopic studies on geothermal springs and river waters were conducted in different petrological-tectonic units of western Yunnan, China. Geothermal waters contain Ca–HCO3, Na–HCO3, and Na (Ca)–SO4 type, and demonstrate strong rock-related trace elemental distributions. Enhanced water–rock interaction increases the concentration of major and trace elements of geothermal waters. The chemical compositions of geothermal waters in the Rehai geothermal field are very complicated and different because of the magma chamber developed at the shallow depth in this area. In this geothermal field, neutral-alkaline geothermal waters with high Cl, B, Li, Rb Cs, As, Sb, and Tl contents and acid–sulfate waters with high Al, Mn, Fe, and Pb contents are both controlled by magma degassing and water–rock interaction. Geothermal waters from metamorphic, granite, and sedimentary regions (except in the Rehai area) exhibit varying B contents ranging from 3.31 mg/L to 4.49 mg/L, 0.23 mg/L to 1.24 mg/L, and <0.07 mg/L, respectively, and their corresponding δ11B values range from −4.95‰ to −9.45‰, −2.57‰ to −8.85‰, and −4.02‰ to +0.06‰. The B contents of these geothermal waters are mainly controlled by leaching host rocks in the reservoir, and their δ11B values usually decrease and achieve further equilibrium with its surrounding rocks, which can also be proven by the positive δ18O-shift. In addition to fluid–rock reactions, the geothermal waters from Rehai hot springs exhibit higher δ11B values (−3.43‰ to +1.54‰) than those yielded from other areas because mixing with the magmatic fluids from the shallow magma. The highest δ11B of steam–heated waters (pH 3.25) from the Zhenzhu spring in Rehai is caused by the fractionation induced by pH and the phase separation of coexisting steam and fluids. Given the strong water–rock interaction, some geothermal springs in western Yunnan show reservoir temperatures higher than 180 °C, which demonstrate potential for electricity generation and direct-use applications. The most potential geothermal field in western Yunnan is located in the Rehai area because of the heat transfer from the shallow magma chamber.  相似文献   

18.
Carbon (δ13CPDB) and oxygen (δ18OSMOW) isotopic compositions of auriferous quartz-carbonate veins (QCVs) of gold deposits from Sangli, Kabuliyatkatti, Nagavi, Nabapur and Mysore mining areas developed on the Central Lode system of the Gadag Gold Field (GGF) in the Neoarchaean Gadag schist belt of the Dharwar Craton, southern India have been examined for the first time to understand the origin of the mineralising fluids. In majority of the samples (46 out of 49), δ13Cpdb of carbonates of the QCVs fall in the range from − 2.2‰ to − 9.7‰ and the δ18O values range from 12.0‰ to 30.5‰ SMOW. The calculated fluid δ13C∑ C compositions for these deposits range from − 2.1‰ to − 9.6‰ and δ18OH2O from 6.8‰ to 25.9‰, respectively. Carbonate δ13C and fluid δ13C∑ C compositions of the carbonates of the QCVs of the GGF are not only distinct from the carbon isotope range of marine carbonates or meta-sedimentary carbonates of the Chitradurga schist belt, but are consistent with C-isotope values of magmatic (− 5 ± 3‰, Burrows et al., 1986) and/or mantle (− 6 ± 2‰, Ohmoto, 1986) carbonates. As dissolution/decarbonation reactions during metamorphism of pre-existing carbonate/carbonated rocks produce CO2 with δ13C values similar to or more enriched than parent rock, the carbonate or fluid δ13C ratios of the QCVs (which fall in the compositional range of mantle/magmatic derived CO2 or carbonates) obtained in this work cannot be the result of metamorphism. The present study corroborates our previous reports from Ajjanahalli and G.R. Halli gold deposits (Sarangi et al., 2012) occurring in the vicinity of the southern extension of the same crustal scale shear zone on which all the GGF deposits are located.The age of gold mineralisation in this area has been reported to be 2522 ± 6 Ma by Sarma et al., 2011. Chardon et al. (2011) have proposed large-scale remobilization of the older gneissic basement, as well as, emplacement of juvenile granites between 2559 Ma and 2507 Ma, close to the crustal scale shear zone along the eastern margin of the Chitradurga schist belt. Based on these observations and our isotope studies, it is proposed that gold mineralising fluids were derived from mantle/juvenile magmatic melts and were channelled through crustal scale shear zones to give rise to the gold deposits in the GGF.  相似文献   

19.
The Ilímaussaq intrusion, South Greenland, provides an exceptional test case for investigating the changes of stable Fe isotope fractionation of solidus phases with changes in the Fe3+/∑Fe ratio of an evolving melt. The intrusion comprises a sequence of four melt batches that were fed from the same parental alkali basaltic magma. Differentiation produced cumulate rocks that range from augite syenite (phase I) over peralkaline granite (phase II) to agpaitic syenites (phases IIIa and IIIb). Fe3+/∑Fe ratios in amphiboles increase substantially from phase I to phase II and III rocks and mark a major change in the parental magma composition from augite syenites to peralkaline granites and agpaitic syenites. Before this transition, olivine, clinopyroxene, and amphibole in augite syenite, the most primitive rock type in the Ilímaussaq Complex, have a uniform Fe isotope composition that is identical to that of the bulk of igneous crustal rocks and approximated by the average isotopic composition of basalts (δ56/54FeIRMM-014 = 0.072 ± 0.046‰). After the transition, amphiboles in the peralkaline granites and agpaitic syenites yield significantly heavier Fe isotope compositions with δ56/54FeIRMM-014 values ranging from 0.123 to 0.237‰. Contamination of the Ilímaussaq magma by ongoing crustal assimilation as cause for this increase can be excluded on the grounds of Nd isotope data. Large-scale metasomatic overprint with an external fluid can also be dismissed based on amphibole O and Li isotope systematics. Rather, the increase towards heavy Fe isotope compositions most likely reflects the change in chemical compositions of amphiboles (calcic in augite syenite to sodic in the agpaitic syenites) and their Fe3+/ΣFe ratios that mirror changes in the chemical composition of the melt and its oxygen fugacity. A sensitive adjustment of equilibrium Fe isotope fractionation factors to amphibole ferric/ferrous ratios is also supported by beta-factors calculated from Mössbauer spetroscopy data. Comparison of the measured isotope fractionation between clinopyroxene and amphibole with that predicted from Mössbauer data reveal Fe isotope systematics close to equilibrium in augite syenites but Fe isotopic disequilibrium between these two phases in phase IIIa agpaitic syenites. These results are in agreement with O and Li isotope systematics. While amphiboles in all Ilímaussaq lithologies crystallized at temperatures between 650 and 850 °C, textural evidence reveals later clinopyroxene crystallization at temperatures as low as 300–400 °C. Therefore, isotopic equilibrium at crystallization conditions between these two phases can not be expected, but importantly, subsolidus reequilibration can also be dismissed.  相似文献   

20.
Determining Ge isotopic compositions of sulfides is important to understand the ore-forming processes. Single step anion-exchange chromatography was previously used to recover Ge from silicates and lignites. We apply this procedure to recover Ge from sulfides before determining Ge isotopic compositions by hydride generation (HG)-MC-ICP-MS. Germanium is quantitatively recovered by the proposed sample preparation method. There are no obvious isotope biases for Ge-bearing solutions containing significant amounts of Cu, Sn, and W. However, δ74Ge values show obvious shifts if the solutions contain high Zn, Pb, and Sb, which is possibly attributed to suppression of germane formation that fractionates Ge isotopes. The long-term reproducibility for Ge standard solution is about ± 0.18‰ for δ74Ge. Spex and Merck standard solutions yield mean δ74Ge values of − 0.70 ± 0.19‰ and − 0.36 ± 0.08‰, respectively. The calculated δ74Ge value (− 5.13‰) of sphalerite standard based on doping experiments is indistinguishable from those of sphalerite without doping (− 5.05‰ and − 5.01‰). Sulfides from the Jinding, Shanshulin, and Tianqiao Pb–Zn deposits in SW China have δ74Ge values of − 4.94‰ to + 2.07‰. The paragenetic sequence of sulfides from the Shanshulin and Tianqiao Pb–Zn deposits is pyrite, sphalerite and galena from early to late. Sulfides from the same ore show a trend of δ74Gepyrite < δ74Gesphalerite < δ74Gegalena, which may be controlled by the kinetic or Rayleigh fractionation.  相似文献   

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