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1.
We report δ44/40Ca(SRM 915a) values for eight fused MPI‐DING glasses and the respective original powders, six USGS igneous rock reference materials, the U‐Th disequilibria reference material TML, IAEA‐CO1 (Carrara marble) and several igneous rocks (komatiites and carbonatites). Sample selection was guided by three considerations: (1) to address the need for information values on reference materials that are widely available in support of interlaboratory comparison studies; (2) support the development of in situ laser ablation and ion microprobe techniques, which require isotopically homogenous reference samples for ablation; and (3) provide Ca isotope values on a wider range of igneous and metamorphic rock types than is currently available in the scientific literature. Calcium isotope ratios were measured by thermal ionisation mass spectrometry in two laboratories (IFM‐GEOMAR and Saskatchewan Isotope Laboratory) using 43Ca/48Ca‐ and 42Ca/43Ca‐double spike techniques and reported relative to the calcium carbonate reference material NIST SRM 915a. The measurement uncertainty in both laboratories was better than 0.2‰ at the 95% confidence level. The impact of different preparation methods on the δ44/40Ca(SRM 915a) values was found to be negligible. Except for ML3‐B, the original powders and the respective MPI‐DING glasses showed identical δ44/40Ca(SRM 915a) values; therefore, possible variations in the Ca isotope compositions resulting from the fusion process are excluded. Individual analyses of different glass fragments indicated that the glasses are well homogenised on the mm scale with respect to Ca. The range of δ44/40Ca(SRM 915a) values in the igneous rocks studied was larger than previously observed, mostly owing to the inclusion of ultramafic rocks from ophiolite sections. In particular, the dunite DTS‐1 (1.49 ± 0.06‰) and the peridotite PCC‐1 (1.14 ± 0.07‰) are enriched in 44Ca relative to volcanic rocks (0.8 ± 0.1‰). The Carrara marble (1.32 ± 0.06‰) was also found to be enriched in 44Ca relative to the values of assumed precursor carbonates (< 0.8‰). These findings suggest that the isotopes of Ca are susceptible to fractionation at high temperatures by, as yet, unidentified igneous and metamorphic processes.  相似文献   

2.
Significant isotopic fractionation can occur during column chemistry and determination by mass spectrometry. Improper correction may produce uncertainties in the isotopic composition of geological samples. We investigated calcium isotopic fractionation during these two processes and set up a model to check data quality. The δ44/40Ca915a value of IAPSO seawater in different Ca cuts (e.g., 0–20, 20–40, 40–60, 60–80 and 80–100%) on column chemistry ranged from ~ 4‰ to 0‰. The more Ca was eluted, the lower the δ44/40Ca915a value of the elution was found. The isotopic fractionation of calcium on the column appeared to follow the exponential law. However, TIMS instrumental fractionation during Ca runs did not always follow the exponential law due to mixing effects from sample reservoirs on the filament. Our results show that errors could be caused if the instrumental fractionation deviates from the exponential law, especially when the fractionation degree is large. To improve the measurement uncertainty, a model is proposed to check the behaviour and degree of instrumental fractionation, which will provide a quick and reasonable verdict on the data quality of TIMS runs.  相似文献   

3.
Chromium (Cr) isotopes have been widely used in various fields of Earth and planetary sciences. However, high‐precision measurements of Cr stable isotope ratios are still challenged by difficulties in purifying Cr and organic matter interference from resin using double‐spike thermal ionisation mass spectrometry. In this study, an improved and easily operated two‐column chemical separation procedure using AG50W‐X12 (200–400 mesh) resin is introduced. This resin has a higher cross‐linking density than AG50W‐X8, and this higher density generates better separation efficiency and higher saturation. Organic matter from the resin is a common cause of inhibition of the emission of Cr during analysis by TIMS. Here, perchloric and nitric acids were utilised to eliminate organic matter interference. The Cr isotope ratios of samples with lower Cr contents could be measured precisely by TIMS. The long‐term intermediate measurement precision of δ53/52CrNIST SRM 979 for BHVO‐2 is better than ± 0.031‰ (2s) over one year. Replicated digestions and measurements of geological reference materials (OKUM, MUH‐1, JP‐1, BHVO‐1, BHVO‐2, AGV‐2 and GSP‐2) yield δ53/52CrNIST SRM 979 results ranging from ?0.129‰ to ?0.032‰. The Cr isotope ratios of geological reference materials are consistent with the δ53/52CrNIST SRM 979 values reported by previous studies, and the measurement uncertainty (± 0.031‰, 2s) is significantly improved.  相似文献   

4.
We report mass‐independent and mass‐dependent Ca isotopic compositions for thirteen geological reference materials, including carbonates (NIST SRM 915a and 915b), Atlantic seawater as well as ten rock reference materials ranging from peridotite to sandstone, using traditional ε and δ values relative to NIST SRM 915a, respectively. Isotope ratio determinations were conducted by independent unspiked and 43Ca‐48Ca double‐spiked measurements using a customised Triton Plus TIMS. The mean of twelve measurement results gave ε40/44Ca values within ± 1.1, except for GSP‐2 that had ε40/44Ca = 4.04 ± 0.15 (2SE). Significant radiogenic 40Ca enrichment was evident in some high K/Ca samples. At an uncertainty level of ± 0.6, all reference materials had the same ε43/44Ca and ε48/44Ca values. We suggest the use of δ44/42Ca to report mass‐dependent Ca isotopic compositions. The precision under intermediate measurement conditions for δ44/42Ca over eight months in our laboratory was ± 0.03‰ (with n ≥ 8 repeat measurements). Measured igneous reference materials gave δ44/42Ca values ranging from 0.27‰ to 0.54‰. Significant Ca isotope fractionation may occur during magmatic and metasomatism processes. Studied reference materials with higher (Dyn/Ybn) tend to have lower δ44/42Ca, implying a potential role of garnet in producing magmas with low δ44/42Ca. Sandstone GBW07106 had a δ44/42Ca value of 0.22‰, lower than all igneous rocks studied so far.  相似文献   

5.
The accurate and precise determination of Li isotopic composition by MC‐ICP‐MS suffers from the poor performance of traditional column chromatography. Previously established chromatographic processes cannot completely remove Na in complex geological samples, which is currently interpreted to be a result of Na breakthrough. In this study, Na breakthrough during single‐column purification was found to differ between simply artificial Na‐containing sample solutions, where a little Na residue was found, and silicate rocks, where a large amount of breakthrough occurred. A revised two‐step column purification for Li using 0.5 and 0.3 mol l?1 HCl as eluents was designed to remove the Na. This modified method achieves high‐efficiency Li purification from Na and consequently avoiding high Na/Li ratio interference for subsequent MC‐ICP‐MS analyses. The proposed method was validated by the analysis of a series of reference materials, including Li2CO3 (IRMM‐016, ‐0.10‰), basalt (BCR‐2: 2.68‰; BHVO‐2: 4.39‰), andesite (AGV‐2: 6.46‰; RGM‐2: 2.59‰), granodiorite (GSP‐2: ?0.87‰) and seawater (CASS‐5, 30.88‰). This work reports early Na appearance prior to the elution curves in chromatography and emphasises its influence for subsequent Li isotope measurement. Based on the findings, the established two‐step method would be more secure than single‐column chemistry for Li purification.  相似文献   

6.
High‐precision calcium isotopic compositions of a set of geological reference materials from the IAG (OU‐6), ANRT (UB‐N), MPI‐DING, USGS and GSJ, relative to NIST SRM 915a, are reported here. Measurements were performed by thermal ionisation mass spectrometry (Triton instrument) using a 42Ca–43Ca double spike. δ44/40Ca values of selected reference materials, mainly felsic rocks, are reported for the first time. Felsic rock values of δ44/40Ca ranged from 0.13‰ to 1.17‰, probably implying Ca isotopic fractionation could occur during magma evolution. δ44/40Ca values of ultramafic rocks, ranging from 0.74‰ to 1.51‰, were positively correlated with MgO and negatively with CaO contents, possibly owing to Ca isotopic fractionation during partial melting. δ44/40Ca of intermediate‐mafic rocks were around 0.78‰ and displayed limited variation, suggesting Ca isotopic fractionation is insignificant during magma evolution processes. As expected, δ44/40Ca of sedimentary and metamorphic rocks varied widely due to complex geological processes.  相似文献   

7.
Although initial studies have demonstrated the applicability of Ni isotopes for cosmochemistry and as a potential biosignature, the Ni isotope composition of terrestrial igneous and sedimentary rocks, and ore deposits remains poorly known. Our contribution is fourfold: (a) to detail an analytical procedure for Ni isotope determination, (b) to determine the Ni isotope composition of various geological reference materials, (c) to assess the isotope composition of the Bulk Silicate Earth relative to the Ni isotope reference material NIST SRM 986 and (d) to report the range of mass‐dependent Ni isotope fractionations in magmatic rocks and ore deposits. After purification through a two‐stage chromatography procedure, Ni isotope ratios were measured by MC‐ICP‐MS and were corrected for instrumental mass bias using a double‐spike correction method. Measurement precision (two standard error of the mean) was between 0.02 and 0.04‰, and intermediate measurement precision for NIST SRM 986 was 0.05‰ (2s). Igneous‐ and mantle‐derived rocks displayed a restricted range of δ60/58Ni values between ?0.13 and +0.16‰, suggesting an average BSE composition of +0.05‰. Manganese nodules (Nod A1; P1), shale (SDO‐1), coal (CLB‐1) and a metal‐contaminated soil (NIST SRM 2711) showed positive values ranging between +0.14 and +1.06‰, whereas komatiite‐hosted Ni‐rich sulfides varied from ?0.10 to ?1.03‰.  相似文献   

8.
We report an approach for the accurate and reproducible measurement of boron isotope ratios in natural waters using an MC‐ICP‐MS (Neptune) after wet chemistry sample purification. The sample matrix can induce a drastic shift in the isotopic ratio by changing the mass bias. It is shown that, if no purification is carried out, the direct measurement of a seawater diluted one hundred times will induce an offset of ?7‰ in the isotopic ratio, and that, for the same concentration, the greater the atomic mass of the matrix element, the greater the bias induced. Whatever the sample, it is thus necessary to remove the matrix. We propose a method adapted to water samples allowing purification of 100 ng of boron with a direct recovery of boron in 2 ml of 3% v/v HNO3, which was our working solution. Boron from the International Atomic Energy Agency IAEA‐B1 seawater reference material and from the two groundwater reference materials IAEA‐B2 and IAEA‐B3, was chemically purified, as well as boron from the certified reference material NIST SRM 951 as a test. The reproducibility of the whole procedure (wet chemistry and MC‐ICP‐MS measurement) was ± 0.4‰ (2s). Accuracy was verified by comparison with positive‐TIMS values and with recommended values. Seawater, being homogeneous for boron isotope ratios, is presently the only natural water material that is commonly analysed for testing accuracy worldwide. We propose that the three IAEA natural waters could be used as reference samples for boron isotopes, allowing a better knowledge of their isotopic ratios, thus contributing to the certification of methods and improving the quality of the boron isotopic ratio measurements for all laboratories.  相似文献   

9.
The commonly used, but no longer available, reference materials NIST SRM 976 (Cu) and ‘JMC Lyon’ (Zn) were calibrated against the new reference materials ERM®‐AE633, ERM®‐AE647 (Cu) and IRMM‐3702 (Zn), certified for isotope amount ratios. This cross‐calibration of new with old reference materials provides a continuous and reliable comparability of already published with future Cu and Zn isotope data. The Cu isotope amount ratio of NIST SRM 976 yielded δ65/63Cu values of ?0.01 ± 0.05‰ and ?0.21 ± 0.05‰ relative to ERM®‐AE633 and ERM®‐AE647, respectively, and a δ66/64ZnIRMM‐3702 value of ?0.29 ± 0.05‰ was determined for ‘JMC Lyon’. Furthermore, we separated Cu and Zn from five geological reference materials (BCR‐2, BHVO‐2, BIR‐1, AGV‐1 and G‐2) using a two‐step ion‐exchange chromatographic procedure. Possible isotope fractionation of Cu during chromatographic purification and introduction of resin‐ and/or matrix‐induced interferences were assessed by enriched 65Cu isotope addition. Instrumental mass bias correction for the isotope ratio determinations by MC‐ICP‐MS was performed using calibrator‐sample bracketing with internal Ni doping for Cu and a double spike approach for Zn. Our results for the five geological reference materials were in very good agreement with literature data, confirming the accuracy and applicability of our analytical protocol.  相似文献   

10.
In recent years, the 187Re–187Os isotope system has been increasingly used to study samples containing very small quantities of Os. For such samples, optimisation of measurement procedures is essential to minimise the loss of Os before mass spectrometric measurements. Micro‐distillation is a necessary purification step that is applied after the main Os chemical separation procedure, prior to Os isotope ratio measurements by negative‐thermal ionisation mass spectrometry (N‐TIMS). However, unlike the other separation steps, this procedure has not yet been optimised for small samples. In this study, we present a refined micro‐distillation method that achieved higher yields and allowed high‐precision R(187Os/188Os) expressed as 187Os/188Os measurements for small‐sized geological samples that contain only a few pg Os. The Os recovery in the micro‐distillation step was tested by changing the operating conditions including heating time and temperature, and amounts of oxidant and reductant. Recoveries were measured by the isotope dilution ICP‐MS method after the addition of 190Os‐enriched spike solution. We found that the most critical factor controlling the chemical yield of Os during micro‐distillation is the extent of dilution of the reductant (HBr) by H2O evaporated from the oxidant. A refined micro‐distillation method, in which the amount of oxidant solution is reduced from the conventional method, achieved an improved chemical yield of Os (~ 90%). This refined method was applied to the measurement of 187Os/188Os by N‐TIMS of varying test portions of the geological reference material BIR‐1a. The resulting 187Os/188Os ratios of BIR‐1a matched the literature data, with propagated uncertainties of 0.2, 1.1 and 11% digested sample quantities containing 150, 10 and 1 pg of Os, respectively.  相似文献   

11.
The high‐precision δ60/58Ni values of twenty‐six geological reference materials, including igneous rocks, sedimentary rocks, stream sediments, soils and plants are reported. The δ60/58Ni values of all samples were determined by double‐spike MC‐ICP‐MS (Nu Plasma III). Isotope standard solution (NIST SRM 986) and geological reference materials (BHVO‐2, BCR‐2, JP‐1, PCC‐1, etc.) were used to evaluate the measurement bias and intermediate precision over a period of six months. Our results show that the intermediate precision of Ni isotope determination was 0.05‰ (2s, n = 69) for spiked NIST SRM 986 and typically 0.06‰ for actual samples, and the δ60/58Ni NIST SRM 986 values were in excellent agreement with previous studies. Eighteen high‐precision Ni isotope ratios of geological reference materials are first reported here, and their δ60/58Ni values varied from ?0.27‰ to 0.52‰, with a mean of 0.13 ± 0.34‰ (2s, n = 18). Additionally, SGR‐1b (0.56 ± 0.04‰, 2s), GSS‐1 (?0.27 ± 0.06‰, 2s), GSS‐7 (?0.11 ± 0.01‰, 2s), GSD‐10 (0.46 ± 0.06‰, 2s) and GSB‐12 (0.52 ± 0.06‰, 2s) could potentially serve as candidate reference materials for Ni isotope fractionation and comparison of Ni isotopic compositions among different laboratories.  相似文献   

12.
Calcium isotopic compositions of sixteen Ca‐bearing USGS geological reference materials including igneous and sedimentary rocks are reported. Calcium isotopic compositions were determined in two laboratories (GPMR, State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan; and CIG, Centre for Isotope Geochemistry, University of California, Berkeley) using the 42Ca‐48Ca double‐spike technique by thermal ionisation mass spectrometry. As opposed to common cation exchange resin, a micro‐column filled with Ca‐selective resin (DGA resin) was used in order to achieve high recovery (> 96%) and efficient separation of Ca from the sample matrix. The intermediate measurement precision was evaluated at 0.14‰ (2s) for δ44/40CaSRM915a at GPMR, based on replicate measurements of pure Ca reference material NIST SRM 915a, NIST SRM 915b and seawater. Overall, the measurement uncertainties in both laboratories were better than 0.15‰ at the 2s level. Result validation was carried out for all available data sets. The Ca isotopic compositions of USGS reference materials are not only in agreement between GPMR and CIG, but also in agreement with previously published data within quoted uncertainties. The comprehensive data set reported in this study serves as a reference for both quality assurance and interlaboratory comparison of high precision Ca isotopic study.  相似文献   

13.
Isotope ratios of heavy elements vary on the 1/10000 level in high temperature materials, providing a fingerprint of the processes behind their origin. Ensuring that the measured isotope ratio is precise and accurate depends on employing an efficient chemical purification technique and optimised analytical protocols. Exploiting the disparate speciation of Cu, Fe and Zn in HCl and HNO3, an anion exchange chromatography procedure using AG1‐×8 (200–400 mesh) and 0.4 × 7 cm Teflon columns was developed to separate them from each other and matrix elements in felsic rocks, basalts, peridotites and meteorites. It required only one pass through the resin to produce a quantitative and pure isolate, minimising preparation time, reagent consumption and total analytical blanks. A ThermoFinnigan Neptune Plus MC‐ICP‐MS with calibrator‐sample bracketing and an external element spike was used to correct for mass bias. Nickel was the external element in Cu and Fe measurements, while Cu corrected Zn isotopes. These corrections were made assuming that the mass bias for the spike and analyte element was identical, and it is shown that this did not introduce any artificial bias. Measurement reproducibilities were ± 0.03‰, ± 0.04‰ and ± 0.06‰ (2s) for δ57Fe, δ65Cu and δ66Zn, respectively.  相似文献   

14.
Measurements of sulfur stable isotope ratios (34S/32S) have suffered from technical difficulties in analysing low‐S materials reducing their use despite their undeniable scientific interest. The measurement of 34S/32S ratios is a powerful tool for deciphering problems such as determining the sources of environmental pollutants, to detect adulteration, tracking the evolution of the redox state of the oceans and quantifying the role of the bacterial activity in sulfide minerals genesis. We have used a high‐precision method of sulfur isotope determination using a new type of elemental analyser based on ‘purge and trap’ technology. This new technique demonstrates the high quality of 34S/32S measurements for samples with S concentrations lower than 1% m/m. International calibrated references of diverse sulfur‐bearing materials were used to calibrate two low (< 1%) S‐bearing phosphorites used as compositional reference material for future use as isotopic references: BCR 32 and NBS 120c. δ34SCDT values of, respectively, 18.2‰ (1s = 0.3; n = 23) and 18.3‰ (1s = 0.4; n = 20) are proposed for these. Calibration of both phosphorites with international reference materials led to calculation of a mean standard error close to 0.4‰. The demonstration of a capability to reliably measure S isotope ratios in low‐S phosphate minerals or rocks opens up new fields of palaeoenvironmental reconstructions.  相似文献   

15.
This study presents a high‐precision method to measure barium (Ba) isotope compositions of international carbonate reference materials and natural carbonates. Barium was purified using chromatographic columns filled with cation exchange resin (AG50W‐X12, 200–400 mesh). Barium isotopes were measured by MC‐ICP‐MS, using a 135Ba–136Ba double‐spike to correct mass‐dependent fractionation during purification and instrumental measurement. The precision and accuracy were monitored by measuring Ba isotope compositions of the reference material JCp‐1 (coral) and a synthetic solution obtained by mixing NIST SRM 3104a with other matrix elements. The mean δ137/134Ba values of JCp‐1 and the synthetic solution relative to NIST SRM 3104a were 0.21 ± 0.03‰ (2s, n = 16) and 0.02 ± 0.03‰ (2s, n = 6), respectively. Replicate measurements of NIST SRM 915b, COQ‐1, natural coral and stalagmite samples gave average δ137/134Ba values of 0.10 ± 0.04‰ (2s, n = 18), 0.08 ± 0.04‰ (2s, n = 20), 0.27 ± 0.04‰ (2s, n = 16) and 0.04 ± 0.03‰ (2s, n = 20), respectively. Barium mass fractions and Ba isotopes of subsamples drilled from one stalagmite profile were also measured. Although Ba mass fractions varied significantly along the profile, Ba isotope signatures were homogeneous, indicating that Ba isotope compositions of stalagmites could be a potential tool (in addition to Ba mass fractions) to constrain the source of Ba in carbonate rocks and minerals.  相似文献   

16.
The utility of 40Ca/44Ca as a tracer of pre-existing crustal contributions in early Archaean cratons has been explored to identify traces of Hadean crust and to assess the style of continental growth. The relatively short half-life of 40K (∼1.3 Gy) means that its decay to 40Ca occurs dominantly during early Earth History. If Archaean crust had a significant component derived from a more ancient protolith, as anticipated by “steady state” crustal evolution models, this should be clearly reflected in radiogenic 40Ca/44Ca ratios (or positive initial εCa) in different Archaean cratons. A high precision thermal ionisation technique has been used to analyse the 40Ca/44Ca ratios of plagioclase separates and associated whole rocks in ∼3.6 Ga (early Archaean) samples from Zimbabwe and West Greenland. Three out of four tonalite, trondhjemite, granodiorite (TTG) suite samples from Zimbabwe display initial 40Ca/44Ca ratios indistinguishable from our measured modern MORB value (i.e., εCa(3.6) ∼ 0). Greenland samples, however, are very diverse ranging from εCa(3.7) = 0.1 in mafic pillow lavas and felsic sheets from the Isua supracrustal belt, up to very radiogenic signatures (εCa(3.7) = 2.9) in both mafic rocks of the Akilia association and felsic TTG from the coastal Amîtsoq gneisses.At face value, these results imply the Zimbabwe crust is juvenile whereas most Greenland samples include an earlier crustal component. Yet the west Greenland craton, as with many Archaean localities, has experienced a complex geological history and the interpretation of age-corrected initial isotope values requires great care. Both felsic and mafic samples from Greenland display εCa(3.7) so radiogenic that they are not readily explained by crustal growth scenarios. The presence of such radiogenic 40Ca/44Ca found in low K/Ca plagioclases requires Ca isotope exchange between plagioclase and whole rock during later metamorphic event(s). In addition the unexpectedly radiogenic Ca isotope ratios in some mafic samples reflect anomalous K/Ca ratios as a result of intense K-metasomatism ∼3.6 Ga. Thus Ca isotope measurements are not a robust tracer of crustal growth in the presence of intense tectono-metamorphic processes. Coupled with other isotope data, however, the degree of overprint can be estimated and the 40Ca/44Ca ratio of a little disturbed sample hints at a small contribution of Hadean protocrust in the coastal part of the Godthåbsfjord area (Southwest Greenland). In the majority of Zimbabwe TTG samples, unradiogenic initial Ca isotope ratios point to very little prior crustal history and minor subsequent disturbance. We thus infer that the modest initial εNd ∼0.8 of the Zimbabwean samples is representative of the depleted mantle at ∼3.6 Ga. Furthermore, Ca isotope systematics provide little support for a “steady state” model of crustal growth.  相似文献   

17.
Measurement of Ba isotope ratios of widely available reference materials is required for interlaboratory comparison of data. Here, we present new Ba isotope data for thirty‐four geological reference materials, including silicates, carbonates, river/marine sediments and soils. These reference materials (RMs) cover a wide range of compositions, with Ba mass fractions ranging from 6.4 to 1900 µg g?1, SiO2 from 0.62% to 90.36% m/m and MgO from 0.08% to 41.03% m/m. Accuracy and precision of our data were assessed by the analyses of duplicate samples and USGS rock RMs. Barium isotopic compositions for all RMs were in agreement with each other within uncertainty. The variation of δ138/134Ba in these RMs was up to 0.7‰. The shale reference sample, affected by a high degree of chemical weathering, had the highest δ138/134Ba (0.37 ± 0.03‰), while the stream sediment obtained from a tributary draining carbonate rocks was characterised by the lowest δ138/134Ba (?0.30 ± 0.05‰). Geochemical RMs play a fundamental role in the high‐precision and accurate determination of Ba isotopic compositions for natural samples with similar matrices. Analyses of these RMs could provide universal comparability for Ba isotope data and enable assessment of accuracy for interlaboratory data.  相似文献   

18.
Located in the western Yangtze Block, the Qingshan Pb–Zn deposit, part of the Sichuan–Yunnan–Guizhou Pb–Zn metallogenic province, contains 0.3 million tonnes of 9.86 wt.% Pb and 22.27 wt.% Zn. Ore bodies are hosted in Carboniferous and Permian carbonate rocks, structurally controlled by the Weining–Shuicheng anticline and its intraformational faults. Ores composed of sphalerite, galena, pyrite, dolomite, and calcite occur as massive, brecciated, veinlets, and disseminations in dolomitic limestones.

The C–O isotope compositions of hydrothermal calcite and S–Pb–Sr isotope compositions of Qingshan sulphide minerals were analysed in order to trace the sources of reduced sulphur and metals for the Pb–Zn deposit. δ13CPDB and δ18OSMOW values of calcite range from –5.0‰ to –3.4‰ and +18.9‰ to +19.6‰, respectively, and fall in the field between mantle and marine carbonate rocks. They display a negative correlation, suggesting that CO2 in the hydrothermal fluid had a mixed origin of mantle, marine carbonate rocks, and sedimentary organic matter. δ34S values of sulphide minerals range from +10.7‰ to +19.6‰, similar to Devonian-to-Permian seawater sulphate (+20‰ to +35‰) and evaporite rocks (+23‰ to +28‰) in Carboniferous-to-Permian strata, suggesting that the reduced sulphur in hydrothermal fluids was derived from host-strata evaporites. Ores and sulphide minerals have homogeneous and low radiogenic Pb isotope compositions (206Pb/204Pb = 18.561 to 18.768, 207Pb/204Pb = 15.701 to 15.920, and 208Pb/204Pb = 38.831 to 39.641) that plot in the upper crust Pb evolution curve, and are similar to those of Devonian-to-Permian carbonate rocks. Pb isotope compositions suggest derivation of Pb metal from the host rocks. 87Sr/86Sr ratios of sphalerite range from 0.7107 to 0.7136 and (87Sr/86Sr)200Ma ratios range from 0.7099 to 0.7126, higher than Sinian-to-Permian sedimentary rocks and Permian Emeishan flood basalts, but lower than Proterozoic basement rocks. This indicates that the ore strontium has a mixture source of the older basement rocks and the younger cover sequence. C–O–S–Pb–Sr isotope compositions of the Qingshan Pb–Zn deposit indicate a mixed origin of the ore-forming fluids and metals.  相似文献   

19.
The importance of oxygen isotope geochemistry in studies of terrestrial and extra-terrestrial silicate rocks was recognized nearly sixtyeight years ago soon after the discovery of O18 and O17. As early as 1934, the significance of oxygen isotope variations in rocks and minerals was stressed by Russian geochemists who also pioneered the discipline of silicate oxygen isotope geochemistry. It is now known that processes involving isotopic interaction between rock and water, magmatic differentiation, and metamorphic recrystallization fractionate oxygen isotopes in the lithosphere. δO18 (the conventional notation for reporting O18/O16 ratios in rocks and minerals) is highest in sedimentary rocks (17 to 35 ‰) and lowest in igneous rocks (4 to 12‰). Metamorphic rocks have intermediate values. δO18 in mafic minerals (1 to 8‰) is lower than in felsic minerals (8 to 16‰). In igneous and metamorphic rocks, quartz is most enriched in O18 (10 to 16‰) and magnetite the least (1 to 2‰). An important application of O18/O16 techniques is in geothermometry, where these are capable of elucidating several petrological processes.  相似文献   

20.
The Ohori ore deposit is one of the Cu–Pb–Zn deposits in the Green Tuff region, NE Japan, and consists of skarn‐type (Kaninomata) and vein‐type (Nakanomata) orebodies. The former has a unique origin because its original calcareous rocks were made by hydrothermal precipitation during Miocene submarine volcanism. Carbon and oxygen isotope ratios of skarn calcite and sulfur isotope ratios of sulfides were measured in and around the deposit. Carbon and oxygen isotope ratios of the skarn calcite are δ13C = ?15.51 to ?5.1‰, δ18O = +3.6 to +22.5‰. δ13C values are slightly lower than those of the Cretaceous skarn deposits in Japan. These isotope ratios of the Kaninomata skarn show that the original calcareous rocks resemble the present submarine hydrothermal carbonates at the CLAM Site, Okinawa Trough, than Cenozoic limestones, even though some isotopic shifts had occurred during later skarnization. δ34S ratios of the sulfide minerals from the Kaninomata and Nakanomata orebodies are mostly in a narrow range of +4.0 to +7.0‰ and they resemble each other, suggesting the same sulfur origin for the both deposits. The magnetite‐series Tertiary Kaninomatasawa granite is distributed just beneath the skarn layer and has δ34S ratios of +7.5 to 8.1‰. The heavy sulfur isotope ratio of the skarn sulfides may have been affected by the Kaninomatasawa granite.  相似文献   

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