首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 321 毫秒
1.
The ion microprobe has the capability to generate high resolution, high precision isotopic measurements, but analysis of the isotopic composition of strontium, as measured by the 87Sr/86Sr ratio, has been hindered by isobaric interferences. Here we report the first high precision measurements of 87Sr/86Sr by ion microprobe in calcium carbonate samples with moderate Sr concentrations. We use the high mass resolving power (7000 to 9000 M.R.P.) of the SHRIMP-RG ion microprobe in combination with its high transmission to reduce the number of interfering species while maintaining sufficiently high count rates for precise isotopic measurements. The isobaric interferences are characterized by peak modeling and repeated analyses of standards. We demonstrate that by sample-standard bracketing, 87Sr/86Sr ratios can be measured in inorganic and biogenic carbonates with Sr concentrations between 400 and 1500 ppm with ∼2‰ external precision (2σ) for a single analysis, and subpermil external precision with repeated analyses. Explicit correction for isobaric interferences (peak-stripping) is found to be less accurate and precise than sample-standard bracketing. Spatial resolution is ∼25 μm laterally and 2 μm deep for a single analysis, consuming on the order of 2 ng of material. The method is tested on otoliths from salmon to demonstrate its accuracy and utility. In these growth-banded aragonitic structures, one-week temporal resolution can be achieved. The analytical method should be applicable to other calcium carbonate samples with similar Sr concentrations.  相似文献   

2.
In principle, garnet growth rates may be calculated from 87Rb/86Sr and 87Sr/86Sr measurements in garnet subsamples and the surrounding rock matrix. Because of low Rb/Sr, garnet should passively record the matrix decay of 87Rb to 87Sr as a progressive increase in 87Sr/86Sr from core to rim. This concept was tested by collecting Rb‐Sr data for five garnet grains from four major orogenic belts: eastern Vermont (c. 380 Ma), western New Hampshire (c. 320 Ma), southern Chile (c. 75 Ma) and northwestern Italy (c. 35 Ma). Both normal Sr isotope zoning (increasing 87Sr/86Sr from core to rim) and inverse Sr zoning (decreasing 87Sr/86Sr from core to rim) were observed. Garnet and matrix isotope data commonly yielded grossly inaccurate model ages. Incomplete Rb and Sr equilibration among matrix minerals is invoked to explain the deviations between theoretical v. measured zoning patterns and the age disparities. Initially, the reactive matrix is dominated by rapidly equilibrating Rb‐rich mica, which imparts high 87Sr/86Sr values in garnet cores. Progressive participation of slower equilibrating Sr‐rich plagioclase buffers or even reduces 87Sr/86Sr, possibly leading to flat or decreasing 87Sr/86Sr from garnet cores to rims. Unusually high 87Sr/86Sr in garnet in combination with bulk matrix compositions causes erroneously young apparent ages, so metamorphic ages, growth rates, and associated heating and loading rates are likely suspect. Although Rb‐Sr may be the most susceptible because of the profound disparities between mica and feldspar, zircon reactivity might influence the Lu‐Hf system by up to a few per cent. The Sm‐Nd system seems generally immune to these effects. Pseudosection analysis and conventional garnet geochronology, which presume complete matrix equilibration during metamorphism, may require modification to account for differences between whole‐rock v. reactive matrix compositions.  相似文献   

3.
Lead isotopes are a powerful and versatile tool to elucidate fundamental geological problems related to the formation and evolution of continental crust. K-feldspar is a popular target for Pb isotope measurement as it is prevalent in many rock types and tends to capture the initial Pb isotope composition of its parental magma. We present data for a new Pb isotope reference material: Albany K-feldspar; as well as updated data for Shap K-feldspar. Results of Pb double-spike TIMS for Albany K-feldspar are 206Pb/204Pb = 16.7872 ± 0.0062, 207Pb/204Pb = 15.5640 ± 0.0056, and 208Pb/204Pb = 36.6600 ± 0.0168 (2s). TIMS measurement results for Shap K-feldspar indicate two isotopically distinct Pb populations. LA-MC-ICP-MS, with a spatial resolution as high as 15 μm, indicates a homogeneous Pb isotopic composition in Albany K-feldspar. In accord with previous studies, our results show that scatter in the measured Pb isotope ratios, related to the low natural isotopic abundance of 204Pb, along with the effect of isobaric 204Hg-204Pb interference, increases at lower count rates. However, the mean Pb isotope ratios measured via LA-MC-ICP-MS using a range of spot sizes are in excellent agreement with TIMS results thus highlighting the feasibility of Pb isotope determination via LA-MC-ICP-MS to access geological information preserved in small crystals, including mineral inclusions.  相似文献   

4.
Twelve apatite samples have been tested as secondary ion mass spectrometry (SIMS) reference materials. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis shows that the SLAP, NUAN and GR40 apatite gems are internally homogeneous, with most trace element mass fractions having 2 standard deviations (2s) ≤ 2.0%. BR2, BR5, OL2, AFG2 and AFB1, which have U > 63 μg g-1, 206Pb/204Pb > 283, and homogeneous SIMS U-Pb data, have respective isotope dilution thermal ionisation mass spectrometry (ID-TIMS) ages of 2053.83 ± 0.21 Ma, 2040.34 ± 0.09 Ma, 868.87 ± 0.25 Ma, 478.71 ± 0.22 Ma and 473.25 ± 0.09 Ma. Minor U-Pb heterogeneity exists and accurate SIMS results require correction with the 3D Concordia-constrained common Pb composition. Among the studied samples, AFG2 and BR5 are the most homogeneous U-Pb reference materials. The SIMS sulfur isotopic compositions of eight of the apatites shows they are homogeneous, with 2s for both 103δ34S and 103δ33S < 0.55‰. One apatite, BR96, has Δ33S = -0.36 ± 0.2‰. The apatite samples have ID-TIMS 87Sr/86Sr between 0.704214 ± 0.000030 and 0.723134 ± 0.000035.  相似文献   

5.
An in situ, medium‐resolution LA‐ICP‐MS method was developed to measure the abundances of the first‐row transition metals, Ga and Ge in a suite of geological materials, namely the MPI‐DING reference glasses. The analytical protocol established here hinged on maximising the ablation rate of the ultraviolet (UV) laser system and the sensitivity of the ICP‐MS, as well minimising the production of diatomic oxides and argides, which serve as the dominant sources of isobaric interferences. Non‐spectral matrix effects were accounted for by using multiple external calibrators, including NIST SRM 610 and the USGS basaltic glasses BHVO‐2G, BIR‐1G and BCR‐2G, and utilising 43Ca as an internal standard. Analyses of the MPI‐DING reference glasses, which represent geological matrices ranging from basaltic to rhyolitic in composition, included measurements of concentrations as low as < 100 μg g?1 and as high as > 104 μg g?1. The new data reported here were found to statistically correlate with the ‘preferred’ reference values for these materials at the 95% confidence level, though with significantly better precision, typically on the order of ≤ 3% (2sm). This analytical method may be extended to any matrix‐matched geological sample, particularly oceanic basalts, silicate minerals and meteoritic materials.  相似文献   

6.
In this study we determined rubidium isotope ratios in twenty-one commonly used international geological reference materials, including igneous, sedimentary and metamorphic rocks, as well as an IAPSO seawater reference material. All δ87Rb results were obtained relative to the NIST SRM 984 reference material. For most reference materials, Rb was purified using a single column loaded with Sr-spec resin. For reference materials containing low Rb but high mass fractions of matrix elements (such as basic rock and seawater), Rb was purified using two-column chromatography, with the first column packed with AGMP-50 resin and the second column packed with Sr-spec resin. Two methods for instrumental mass bias correction, sample-standard bracketing (SSB) mode, and the combined sample-standard bracketing and Zr internal normalisation (C-SSBIN) method, were compared for Rb isotopic measurements by multi-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS). The long-term reproducibility of Rb isotopic measurements using both methods was similar, better than 0.06‰ (2s, standard deviation) for NIST SRM 984. Significant Rb isotopic fractionation was observed among the reference materials, with an overall variation in δ87Rb values of approximately 0.5‰. The δ87Rb values of igneous rocks ranged from -0.28‰ to +0.06‰, showing a trend from heavier isotopic compositions in mafic rocks to lighter δ87Rb values in the more evolved felsic rocks. The sedimentary and metamorphic rocks had Rb isotope ratios similar to those of igneous rocks. The δ87Rb values of the reference materials related to low-temperature geological processes showed a wider range than those of high-temperature processes. Notably, the IAPSO seawater reference material had a δ87Rb value of +0.14‰, which deviated from that of igneous rocks, and represents the heaviest reservoir of Rb isotopes found thus far on Earth. The comprehensive dataset presented here has the potential to serve for quality assurance purposes, and provide a framework for interlaboratory comparisons of Rb isotope ratios.  相似文献   

7.
Despite the fact that porphyry Cu deposits contain large amounts of Cu in one or more small stocks, few studies have discussed evidence for significant magma recharge in porphyry Cu deposits. A systematic elemental and Sr isotopic study of plagioclase crystals from mineralized diorite and granodiorite porphyry constrains the processes of crystallization and magma recharge at the Baogutu reduced porphyry copper deposit, western Junggar, NW-China. Large compositional changes in An (12–24 mol%) are observed along with strong positive correlations between An and FeO. Significant resorption textures are also preserved in plagioclase crystals as well as repeated oscillatory zoning in An and FeO, and complex Sr isotope variations. Three types of crystals with different core-to-rim Sr isotope variations are recognized. Type I crystals have core-to-rim increases in (87Sr/86Sr)i that could be explained by diffusion. For example, Sr isotope variations recorded in BCK2-1-2 crystal could be generated by diffusion with simulated maximum crystal residence times of 100–500 years with the proximate value of ∼300 years. Type II crystals with different variation trends in (87Sr/86Sr)i on opposite sides of the core could be produced either by multi-stage crystallization or by analytical uncertainty. Whereas, type III crystals with complex core-to-rim variations in (87Sr/86Sr)i, may record repeated magma recharge events. All these results suggest repeated recharging of the magma chamber by hotter, more mafic and less radiogenic Sr isotope melts. Therefore, more mafic melt injection recorded in plagioclase profiles may provide significant metal contributions to the magma chamber, which ultimately results in Cu mineralization at the Baogutu reduced porphyry copper deposit.  相似文献   

8.
In situ U-Pb geochronology and hafnium, oxygen and zirconium isotope measurements in zircons using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and ion microprobe techniques can provide essential isotopic data to constrain geological evolutionary histories. Developing reliable zircon reference materials is the cornerstone for in situ zircon chronology and isotopic studies. In this study, the homogeneity of U-Pb ages and Hf-O-Zr isotope ratios in three Sri Lankan zircon megacrysts (SLZA, SLZB and SLZC) were investigated using multiple analytical methods. The obtained U, Th, Pb and Hf mass fractions of the SLZA zircon were 839 ± 56 μg g-1 (1s), 151 ± 15 μg g-1 (1s), 198 ± 28 μg g-1 (1s) and 8635 ± 286 μg g-1 (1s), respectively. The mass fractions of U, Th, Pb and Hf in the SLZB zircon were 1106 ± 106 μg g-1 (1s), 331 ± 61 μg g-1 (1s), 376 ± 57 μg g-1 (1s) and 9673 ± 976 μg g-1 (1s), respectively. The U, Th, Pb and Hf mass fractions determined in the SLZC zircon were 551 ± 35 μg g-1 (1s), 111 ± 8 μg g-1 (1s), 129 ± 18 μg g-1 (1s) and 7881 ± 393 μg g-1 (1s), respectively. The chemical abrasion isotope dilution thermal ionisation mass-spectrometry (CA-ID-TIMS) method yielded a Th-corrected weighted mean 206Pb/238U age of 556.94 ± 1.29 Ma (95% conf., n = 5) for the SLZA zircon, 552.90 ± 1.29 Ma (95% conf., n = 7) for the SLZB zircon and 560.83 ± 1.29 Ma (95% conf., n = 7) for the SLZC zircon. The obtained Hf isotopic compositions of the SLZA, SLZB and SLZC zircons determined with the solution MC-ICP-MS method were 0.281651 ± 0.000014 (2s, n = 10), 0.281704 ± 0.000008 (2s, n = 10) and 0.281676 ± 0.000006 (2s, n = 10), respectively. The obtained O isotopes of the SLZA and SLZB zircons measured with the laser fluorination method were 12.14 ± 0.56‰ (2s, n = 4) and 11.91 ± 0.30‰ (2s, n = 4), respectively. The Zr isotopes of the SLZA, SLZB and SLZC zircons determined with double spike TIMS analysis yielded mean δ94/90ZrSRM3169 values of -0.03 ± 0.06‰ (2s, n = 10), -0.03 ± 0.04‰ (2s, n = 10) and 0.00 ± 0.07‰ (2s, n = 8), respectively. The SLZA zircon can be used as a primary reference or quality control material for microbeam U-Pb, Hf and Zr isotope measurements because of its slight heterogeneity. The U-Pb, Hf and Zr isotopic compositions of the SLZB and SLZC megacrysts were homogeneous. The O isotopic compositions in the SLZA and SLZB zircon were slightly dispersed, indicating that these two megacrysts can only serve as secondary reference materials for microbeam O isotope measurements.  相似文献   

9.
New zircon reference materials for in situ zircon radiogenic Hf isotope and stable Zr isotopic determinations made by laser ablation multi-collector inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS) are required due to high data productivity and consequently high reference material consumption rate. This study examines a new natural zircon for Zr isotope ratios by double spike thermal ionisation mass spectrometry (TIMS), and for Hf isotopes by bulk solution nebuliser (SN)-MC-ICP-MS with both Zr and Hf determined by LA-MC-ICP-MS. A total of five zirconium isotope measurements from drilled zircons, determined by TIMS, yield a mean δ94/90ZrIPGP-Zr value of -0.09 ± 0.06‰ (2s). Five and eight hafnium isotope measurements for powders from the drilled zircons and Ban-1-4 by SN-MC-ICP-MS, yield mean 176Hf/177Hf ratios of 0.282985 ± 0.000011 (2s) and 0.282982 ± 0.000007 (2s), respectively. The mean δ94/90ZrIPGP-Zr value and 176Hf/177Hf ratio determined by LA-MC-ICP-MS analyses are -0.06 ± 0.09‰ (2s, n = 504) and 0.282985 ± 0.000035 (2s, n = 327), respectively. The isotopic homogeneities suggest that the Ban-1 zircon is a suitable reference material for microbeam Zr and Hf isotopic measurements.  相似文献   

10.
This work presents an evaluation of various methods for in situ high‐precision Sr and Pb isotopic determination in archaeological glass (containing 100–500 μg g?1 target element) by nanosecond laser ablation multi‐collector‐inductively coupled plasma‐mass spectrometry (ns‐LA‐MC‐ICP‐MS). A set of four soda‐lime silicate glasses, Corning A–D, mimicking the composition of archaeological glass and produced by the Corning Museum of Glass (Corning, New York, USA), were investigated as candidates for matrix‐matched reference materials for use in the analysis of archaeological glass. Common geological reference materials with known isotopic compositions (USGS basalt glasses BHVO‐2G, GSE‐1G and NKT‐1G, soda‐lime silicate glass NIST SRM 610 and several archaeological glass samples with known Sr isotopic composition) were used to evaluate the ns‐LA‐MC‐ICP‐MS analytical procedures. When available, ns‐LA‐MC‐ICP‐MS results for the Corning glasses are reported. These were found to be in good agreement with results obtained via pneumatic nebulisation (pn) MC‐ICP‐MS after digestion of the glass matrix and target element isolation. The presence of potential spectral interference from doubly charged rare earth element (REE) ions affecting Sr isotopic determination was investigated by admixing Er and Yb aerosols by means of pneumatic nebulisation into the gas flow from the laser ablation system. It was shown that doubly charged REE ions affect the Sr isotope ratios, but that this could be circumvented by operating the instrument at higher mass resolution. Multiple strategies to correct for instrumental mass discrimination in ns‐LA‐MC‐ICP‐MS and the effects of relevant interferences were evaluated. Application of common glass reference materials with basaltic matrices for correction of ns‐LA‐MC‐ICP‐MS isotope data of archaeological glasses results in inaccurate Pb isotope ratios, rendering application of matrix‐matched reference materials indispensable. Correction for instrumental mass discrimination using the exponential law, with the application of Tl as an internal isotopic standard element introduced by pneumatic nebulisation and Corning D as bracketing isotopic calibrator, provided the most accurate results for Pb isotope ratio measurements in archaeological glass. Mass bias correction relying on the power law, combined with intra‐element internal correction, assuming a constant 88Sr/86Sr ratio, yielded the most accurate results for 87Sr/86Sr determination in archaeological glasses  相似文献   

11.
The in situ measurement of Sr isotopes in carbonates by MC‐ICP‐MS is limited by the availability of suitable microanalytical reference materials (RMs), which match the samples of interest. Whereas several well‐characterised carbonate reference materials for Sr mass fractions > 1000 µg g?1 are available, there is a lack of well‐characterised carbonate microanalytical RMs with lower Sr mass fractions. Here, we present a new synthetic carbonate nanopowder RM with a Sr mass fraction of ca. 500 µg g?1 suitable for microanalytical Sr isotope research (‘NanoSr’). NanoSr was analysed by both solution‐based and in situ techniques. Element mass fractions were determined using EPMA (Ca mass fraction), as well as laser ablation and solution ICP‐MS in different laboratories. The 87Sr/86Sr ratio was determined by well‐established bulk methods for Sr isotope measurements and is 0.70756 ± 0.00003 (2s). The Sr isotope microhomogeneity of the material was determined by LA‐MC‐ICP‐MS, which resulted in 87Sr/86Sr ratios of 0.70753 ± 0.00007 (2s) and 0.70757 ± 0.00006 (2s), respectively, in agreement with the solution data within uncertainties. Thus, this new reference material is well suited to monitor and correct microanalytical Sr isotope measurements of low‐Sr, low‐REE carbonate samples. NanoSr is available from the corresponding author.  相似文献   

12.
This study presents a high‐precision Cd isotope measurement method for soil and rock reference materials using MC‐ICP‐MS with double spike correction. The effects of molecular interferences (e.g., 109Ag1H+, 94Zr16O+, 94Mo16O+ and 70Zn40Ar+) and isobaric interferences (e.g., Pd, In and Sn) to Cd isotope measurements were quantitatively evaluated. When the measured solution has Ag/Cd ≤ 5, Zn/Cd ≤ 0.02, Mo/Cd ≤ 0.4, Zr/Cd ≤ 0.001, Pd/Cd ≤ 5 × 10?5 and In/Cd ≤ 10?3, the measured Cd isotope data were not significantly affected. The intermediate measurement precision of pure Cd solutions (BAM I012 Cd, Münster Cd and AAS Cd) was better than ± 0.05‰ (2s) for δ114/110Cd. The δ114/110Cd values of soil reference materials (NIST SRM 2709, 2709a, 2710, 2710a, 2711, 2711a and GSS‐1) relative to NIST SRM 3108 were in the range of ?0.251 to 0.632‰, the δ114/110Cd values of rock reference materials (BCR‐2, BIR‐1, BHVO‐2, W‐2, AGV‐2, GSP‐2 and COQ‐1) varied from ?0.196‰ to 0.098‰, and that of the manganese nodule (NOD‐P‐1) was 0.163 ± 0.040‰ (2s, n = 8). The large variation in Cd isotopes in soils and igneous rocks indicates that they can be more widely used to study magmatic and supergene processes.  相似文献   

13.
The interest in variations of barium (Ba) stable isotope amount ratios in low and high temperature environments has increased over the past several years. Characterisation of Ba isotope ratios of widely available reference materials is now required to validate analytical procedures and to allow comparison of data obtained by different laboratories. We present new Ba isotope amount ratio data for twelve geological reference materials with silicate (AGV‐1, G‐2, BHVO‐1, QLO‐1, BIR‐1, JG‐1a, JB‐1a, JR‐1 and JA‐1), carbonate (IAEA‐CO‐9) and sulfate matrices (IAEA‐SO‐5 and IAEA‐SO‐6) relative to NIST SRM 3104a. In addition, two artificially fractionated in‐house reference materials BaBe12 and BaBe27 (δ137/134Ba = ?1.161 ± 0.049‰ and ?0.616 ± 0.050‰, respectively) are used as quality control solutions for the negative δ‐range. Accuracy of our data was assessed by interlaboratory comparison between the University of Bern and the United States Geological Survey (USGS). Data were measured by MC‐ICP‐MS (Bern) and TIMS (USGS) using two different double spikes for mass bias correction (130Ba–135Ba and 132Ba–136Ba, respectively). MC‐ICP‐MS measurements were further tested for isobaric and non‐spectral matrix effects by a number of common matrix elements. The results are in excellent agreement and suggest data accuracy.  相似文献   

14.
Progressive leaching of plagioclase for Sr isotopes and microdrilling for Sr and Pb isotopes from grains of plagioclase and orthopyroxene from the Critical Zone and the Lower Zone indicates that these minerals are not in isotopic equilibrium. Leaching suggests Critical Zone plagioclase either lost Rb or had a more radiogenic Sri rim relative to the core, whereas plagioclase from an Upper Zone sample is isotopically homogeneous for Sri. Microdrilling analyses of plagioclase from the Lower and Critical Zones consistently have a higher initial 87Sr/86Sr (Sri) and a less radiogenic modeled 238U/204Pb composition (μ2) than coexisting orthopyroxene. The range of calculated Sri for plagioclase and orthopyroxene is 0.70506–0.70662(34) and 0.70290–0.70654(36), respectively. The average difference in Sri between mineral pairs was 0.00095. The range of calculated μ2 for plagioclase and orthopyroxene is 9.42–10.30 (average 9.7) and 9.83–15.75 (average 10.1), respectively. The range of measured 208Pb/206Pb for plagioclase and orthopyroxene is 34.757–36.439(33) and 36.669–41.845(85), respectively. One orthopyroxenite without evidence for more than one population of crystal size distribution, nonetheless had Sri = 0.70654 (36) with calculated μ2 of 10.32 for larger grains as compared with Sri = 0.70290 (32) and calculated μ2 of 9.97 for smaller grain-size fractions. Isotopic results from this study demonstrate that whole-rock isotopic data may not provide the appropriate level of detail necessary to address some processes in the Bushveld Complex. However, systematic changes have the potential to elucidate the timing of contamination with regard to other processes (crystal aging, compaction-driven recrystallization, and mineral exsolution) occurring within a slowly cooled crystal–liquid–vapor mush system.  相似文献   

15.
The isotopic composition of lithium (Li) in clinopyroxene (Cpx), determined via in situ micro-analysis, has been employed as a potential geochemical tool for studying various geological processes such as crust-mantle recycling, silicate weathering and fluid-rock interaction. To obtain precise and accurate Li isotopic compositions in Cpx by LA-MC-ICP-MS, synthetic Cpx matrix-matched reference materials (RMs) were prepared in this study. Six Cpx-matrix RMs were prepared by mixing metallic oxides with GSP-2 (granodiorite) or pure L-SVEC solution and melting them into glasses (GSP-2 + oxide; L-SVEC + oxide). Two representative synthetic glasses, CPXA01 and CPXB01, were subjected to a series of analyses to investigate the possible qualification of the RMs for in situ Li isotope measurement by LA-MC-ICP-MS, including elemental homogeneity analysis (elemental mapping analysis and spot analysis), Li isotopic homogeneity analysis and accurate Li isotopic determination. The applicability of the synthetic Cpx-matrix RMs was highlighted by comparing the δ7Li values of three natural Cpx calibrated against the synthetic Cpx-matrix RMs and other commonly used RMs with different matrices (NIST SRM 612, BCR-2G, GOR128-G, StHs6/80-G, KL2-G and T1-G), respectively. Additionally, CPXB01-05 RMs with the same matrix but different Li contents were prepared to explore the Li content mismatch effect, which is significant for accurate determination of in situ Li isotopic composition by LA-MC-ICP-MS. The results of the cross-calibration of Li isotopes in CPXA01 and CPXB01 suggested no obvious Li isotopic fractionation between the two types of glasses (GSP-2 + oxide; L-SVEC + oxide). Thus, the two methods of producing Cpx-matrix RMs are suitable for preparing the matrix-matched RMs for in situ microanalysis for Li isotopes.  相似文献   

16.
A critical study of 311 published WR chemical analyses, isotopic and mineral chemistry of anorthosites and associated rocks from eight Proterozoic massif anorthosite complexes of India, North America and Norway indicates marked similarities in mineralogy and chemistry among similar rock types. The anorthosite and mafic-leucomafic rocks (e.g., leuconorite, leucogabbro, leucotroctolite, anorthositic gabbro, gabbroic anorthosite, etc.) constituting the major part of the massifs are characterized by higher Na2O + K2O, Al2O3, SiO2, Mg# and Sr contents, low in plagioclase incompatible elements and REE with positive Eu anomalies. Their δ 18O‰ (5.7–7.5), initial 87Sr/86Sr (0.7034–0.7066) and ɛ Nd values (+1.14 to +5.5) suggest a depleted mantle origin. The Fe-rich dioritic rocks occurring at the margin of massifs have isotopic, chemical and mineral composition more close to anorthosite-mafic-leucomafic rocks. However, there is a gradual decrease in plagioclase content, An content of plagioclase and XMg of orthopyroxene, and an increase in mafic silicates, oxide minerals content, plagioclase incompatible elements and REE from anorthosite-mafic-leucomafic rocks to Fe-rich dioritic rocks. The Fe-rich dioritic rocks are interpreted as residual melt from mantle derived high-Al gabbro melt, which produced the anorthosite and mafic-leucomafic rocks. Mineralogically and chemically, the K-rich felsic rocks are distinct from anorthosite-mafic-leucomafic-Fe-rich dioritic suite. They have higher δ 18O values (6.8–10.8‰) and initial 87Sr/86Sr (0.7067–0.7104). By contrast, the K-rich felsic suites are products of melting of crustal precursors.  相似文献   

17.
The Parashi granitoid of northeasternmost Colombia intrudes the Upper Cretaceous to Lower Paleocene accretionary complex formed by the collision of the Caribbean arc and the continental margin of South America. This granitoid presently separated of the continental margin includes a major quartzdiorite body with andesite to dacite dikes and mafic enclaves. Zircon U–Pb LA-MC-ICP-MS and K–Ar geochronology on the quartzdiorite and the dikes suggest that crystallization extended from ca. 47 to 51 Ma. Major and trace elements are characterized by a medium-K, immature continental arc signature and high Al2O3, Na2O and Ba–Sr contents. Initial 87Sr/86Sr isotopic values range between 0.7050 and 0.7054, with 143Nd/144Nd = 0.51235–0.51253, εNd and εHf values from −0.81 to −4.40 and −4.4 and −5.2. Major and trace element ratios and isotopic modeling suggest that sedimentary and/or quartzofeldspathic crustal sources were mixed with a mafic melt input. The petrotectonic and geological constraints derived from this granitoid suggest that Parashi plutonism records an immature, oblique subduction-zone setting in which the presence of a high-temperature mantle realm and strong plate coupling associated to upper crust subduction caused the partial fusion of a previously tectonically underplated mafic crust and associated metasediments exposed in the continental margin. The limited temporal expression of this magmatism and the transition to a regional magmatic hiatus are related to a subsequent change to strongly and slow oblique tectonics in the Caribbean–South America plate interactions and the underflow of a relatively thick slab of Caribbean oceanic crust.  相似文献   

18.
《Applied Geochemistry》2006,21(3):419-436
Geochemical maps expressing areal distributions of chemical elements in the earth’s land surface have been published in several countries in relation to various global environment issues. The authors have applied a radiogenic isotope ratio, 87Sr/86Sr, to geochemical mapping in order to understand the geological origin, transportation and dispersion system of chemical elements in the earth’s land surface. The Sr isotope ratio is a useful tracer for distinguishing the geological origin of surficial deposits, especially in areas where surface exposure of bedrocks is low, because it is not significantly altered by the processes of weathering and transportation. Most bedrocks in the Japanese islands are covered by plants, soils and urban areas. In this study, 142 of 1219 stream sediments (<180 μm) collected from the northeastern part of Aichi Prefecture, in the central part of Japan (75 km × 30 km), were analyzed. Their Sr isotope ratios range from 0.7086 to 0.7315 with an average of 0.7129, except for one sample. This average is higher than the mean of the upper crust of the Japan Arc (the Japanese Island Crustal Composite, JICC), 0.7077. This difference can be attributed to the below-average presence of young volcanic rocks, generally having lower 87Sr/86Sr values, and the above-average presence of granitic rocks, in the study area compared with the surface exposure of the Japan Arc. The first factor controlling the distribution of Sr isotope ratios is the bedrock distributed around the sampling points. Regional variation in the 87Sr/86Sr value shows that it is higher in the western and southeastern parts, where sedimentary rocks and metamorphic rocks are distributed, and that it is lower mainly in the central part, where granitic rocks are distributed. The 87Rb/86Sr–87Sr/86Sr plot for stream sediments more clearly reveals the differences and similarities of bedrocks. In some locations, the distribution of Sr isotope ratios does not correspond to that of bedrocks on the geological map. One reason is the existence of unmapped bedrock, for example, small intrusive masses of granite. The other is fluvial transportation and dispersion. The distribution of the isotope ratios suggests that some stream sediments include surficial deposits from a few km upstream. Application of the Sr isotope ratio to geochemical mapping is useful for revealing both the distribution of unexposed bedrocks and the transportation of surficial deposits. Information on unexposed bedrocks will be expected to contribute to the improvement of geological mapping.  相似文献   

19.
Mafic granulite and spinel lherzolite xenoliths from Cenozoic alkaline basalts near Al-Ashkhara, eastern Oman, have been selected for a systematic mineralogical, geochemical and Sr–Nd–Pb isotopic study. This is the only place in E Arabia where samples of both lower crust and upper mantle can be examined. Lower crustal xenoliths consist of two mineralogically and chemically distinct groups: gabbronorite (subequal abundances of ortho- and clino-pyroxene and plagioclase) and plagioclase pyroxenite (dominant pyroxene and subordinate plagioclase). Temperature estimates for lower crustal xenoliths using the two pyroxene geothermometer (T-Wells) yield 810–865 °C. The mineral assemblage (spinel–pyroxene–plagioclase) and Al content in pyroxene indicate that plagioclase-bearing xenoliths equilibrated at 5–8 kbar (13 and 30 km depth) in the lower crust. εNd and 87Sr/86Sr calculated at 700 Ma for Al-Ashkhara lower crustal xenoliths (+ 6.4 to + 6.6; 87Sr/86Sr = 0.7028 to 0.7039) are consistent with the interpretation that juvenile, mafic melts were added to the lower crust during Neoproterozoic time and that there was no discernible contribution from pre-Neoproterozoic crust. Upper mantle xenoliths consist of both dry and hydrous (phlogopite-bearing) lherzolites. These peridotites are more Fe-rich than expected for primitive mantle or melt residues and probably formed by pervasive circulation of melts that have refertilized pre-existing mantle peridotites. Mineral equilibration temperatures range from 990 to 1070 °C. Isotopic compositions calculated at 700 Ma are εNd = + 6.8 to + 7.8 and 87Sr/86Sr = 0.7016 to 0.7025, indicating depleted upper mantle. Pb isotopic compositions indicate that the metasomatism was relatively recent, perhaps related to Paleogene tectonics and basanite igneous activity. Nd model ages for the spinel peridotite xenoliths range between 0.59 and 0.65 Ga. The xenolith data suggest that eastern Arabian lower crust is of hotspot origin, in contrast to western Arabian lower crust, which mostly formed at a convergent plate margin. Geochemical and isotopic differences between lower crust and upper mantle indicate that these are unrelated, possibly because delamination replaced the E Arabian mantle root in Neoproterozoic time.  相似文献   

20.
Calcite content, Sr concentrations, and isotopes of calcites in the Chinese deserts are systematically studied in this paper. Calcite contents, which are calculated according to acid-soluble Ca contents in the deserts, are generally higher in the sandy deserts than in the sandy lands and decrease roughly from northwest to northeast of China. Acid-soluble Sr is well correlated with calcite in the Chinese deserts, implying acid-soluble Sr comes mainly from the calcite dissolution. Sr concentrations in calcites, calculated on the basis of calcite contents and acid-soluble Sr concentrations in the deserts, have an inverse relation to calcite contents, essentially mirroring the degree of Sr substitution for Ca in the calcite development. Desert calcites have regional variations in Sr isotopic ratios. Calcite Sr isotopic ratios depend on geological settings and chemical weathering. The Badain Jaran, and Tengger deserts are probably affected by additional factors such as the remote groundwater cycle or overturning of underlying sand deposits.Only four deserts (Taklimakan, Qaidam, Badain Jaran, and Tengger) appear to be potential sources of eolian deposits in the Chinese Loess Plateau (CLP). Isotopic signatures of calcite Sr and silicate Nd further indicate that the Tengger desert was not an important source for eolian deposits in the CLP. Eolian calcite was probably enriched due to wind sorting from the potential sources to the CLP and suffered weathering–leaching after it accumulated in the CLP. Sr isotopic compositions and Ca/Sr molar ratios of calcites are different between the deserts and the Lingtai profile, due to the integrated effect of wind sorting and weathering–leaching.It is essential to calculate accurately the 87Sr/86Sr ratio and Sr concentration of eolian calcite entering the oceans according to geochemical data of the Chinese deserts, because of the importance of the Chinese deserts in the global dust cycle. The calculated Sr concentration and 87Sr/86Sr ratio of eolian calcite entering the North Pacific Ocean, are 11.75 μmol/g and 0.71032, respectively. The calculated values in this study are close to the recommended values by Jacobson [Jacobson A. D. (2004) Has the atmospheric supply of dissolved calcite dust to seawater influenced the evolution of marine 87Sr/86Sr ratios over the past 2.5 million years? Geochem. Geophys. Geosyst. 5(12), 1–9, Q12002. doi:10.1029/2004GC000750]. Using the same model as that of Jacobson (2004), the effect of Asia dust on the evolution of seawater Sr isotopes is evaluated. (87Sr/86Sr)seawater increases by 0.3 × 10−5 if the lower dust flux of 2.34 × 108 mol Sr/yr is used in the model, suggesting the little effect of Asian dust on the seawater Sr record in the Quaternary. The increase in (87Sr/86Sr)seawater is 1.5 × 10−5 if the higher value of 1.17 × 109 mol Sr/yr is used, as observed in the Quaternary Sr record. These results further support the suggestions of Jacobson (2004).  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号