阿尔泰东南缘泥盆纪花岗质岩石的锆石U-Pb年龄、成因演化及构造意义：钙碱性-高钾钙碱性-碱性岩浆演化新证据

中国阿尔泰广泛发育的花岗质岩石已获得大量研究,但是其东南缘研究薄弱,制约了对整个阿尔泰造山带构造岩浆演化的认识。本文新获得阿尔泰东南缘四个花岗质岩体(昆格依特、库吉尔特、布铁乌及卡拉特玉别)锆石U-Pb年龄,分别为382±4Ma、381±4Ma、385±5Ma和363±6Ma。岩石学、地球化学特征等显示这些花岗质岩石具有高钾钙碱性、准铝质—弱过铝质的I型特点,全岩εNd(t)值为-2.42~-0.53,Nd模式年龄tDM为1.6~1.3Ga;锆石εHf(t)值为-3.44~+13.26,绝大多数为正值,锆石Hf二阶段模式年龄tDM-2为2.5~0.6Ga,表明源区物质组成复杂,有较多的新生幔源物质参与花岗质岩石的形成,并含有古老地壳成分。综合已有年龄分析显示,中国阿尔泰花岗质岩石的形成时代可分为480~440Ma(峰期460Ma)、420~390Ma(峰期400Ma)、390~370Ma(峰期380Ma)、370~360Ma(峰期365Ma)、360~350Ma。处于岩浆发育峰期的早泥盆世(420~390Ma)多为准铝-过铝质的钙碱性系列;中晚泥盆世(390~360Ma)多为准铝—弱过铝质的高钾钙碱性系列;370~360Ma为高钾钙碱性系列。该地区363Ma的高钾钙碱性花岗质岩石的确定,为进一步厘定整个阿尔泰泥盆纪花岗质岩浆由钙碱性(480~390Ma),到高钾钙碱性(390~360Ma),再到354Ma的布尔根碱性花岗岩的演变特点提供了新的证据,进一步揭示阿尔泰造山带该时期由俯冲增生演变到碰撞及后碰撞的演化过程。     

诸广山南部高坪地区细粒花岗岩LA-ICP-MS锆石U-Pb年龄及其地质意义

诸广山南部高坪地区钻探工程揭露到较多细粒花岗岩岩脉,部分岩脉与矿体空间关系密切。本文利用LA-ICP-MS锆石U-Pb定年法测得区内两组细粒花岗岩年龄分别为252.9±1.9 Ma(MSWD=1.8)和244±2 Ma(MSWD=1.4)。结合细粒花岗岩中晶质铀矿U-Pb定年结果,证实其形成于印支期。本文结合粤北地区铀成矿规律及区域铀成矿特征,对高坪地区铀成矿潜力及找矿方向进行初步探讨,认为:(1)印支期,诸广山南部先后广泛侵入粗粒斑状黑云母花岗岩与细粒花岗岩,其中细粒花岗岩多沿断裂发育;(2)至燕山晚期,沿断裂侵入基性脉岩;(3)高坪地区细粒花岗岩与基性脉岩叠合部位为铀成矿的有利空间。     

Quantitative Separation of As(III) and As(V) from a Synthetic Water Solution Using Ion Exchange Columns in the Presence of Fe and Mn Ions

The quantitative separation of As(III) from a water sample containing As(III) and As(V) in the presence of Fe and Mn in an ion exchange resin (AG1 X8) column for the speciation of arsenic is described. Individual and combined effects of Fe and Mn on the separation of As(III) from the solution have been studied separately. In absence of Fe and Mn, the ratio between the As(T) concentration in the eluent and the As(III) concentration in the original sample has been found to be 0.9717 under optimum process conditions. The presence of Fe(II) in the water sample increased the As(T) concentration in the eluent whereas Mn(II) decreased it. Combined effects of Fe and Mn on the percentage increment in the eluent arsenic concentration have been expressed by additive and interactive models. The interactive model has been developed by a statistical software with a 95 % confidence level. In most of the cases the error on the determination of the As(III) concentration had a minimum when using the interactive model.     

U‐Pb Detrital Zircon Analysis – Results of an Inter‐laboratory Comparison

Inter‐laboratory comparison of laser ablation ICP‐MS and SIMS U‐Pb dating of synthetic detrital zircon samples provides an insight into the state‐of‐the art of sedimentary provenance studies. Here, we report results obtained from ten laboratories that routinely perform this type of work. The achieved level of bias was mostly within ± 2% relative to the ID‐TIMS U‐Pb ages of zircons in the detrital sample, and the variation is likely to be attributed to variable Pb/U elemental fractionation due to zircon matrix differences between the samples and the reference materials used for standardisation. It has been determined that ~ 5% age difference between adjacent age peaks is currently at the limit of what can be routinely resolved by the in situ dating of detrital zircon samples. Precision of individual zircon age determination mostly reflects the data reduction and procedures of measurement uncertainty propagation, and it is largely independent of the instrumentation, analytical technique and reference samples used for standardisation. All laboratories showed a bias towards selection of larger zircon grains for analysis. The experiment confirms the previously published estimates of the minimum number of grains that have to be analysed in order to detect minor zircon age populations in detrital samples.     

Late Cretaceous arc igneous activity: the Eğrikar Monzogranite example

The geochemical and Sr–Nd–Pb isotope properties, as well as the Laser Ablation Inductively Coupled Plasma and Mass Spectrometry (LA-ICP-MS) U–Pb zircon age, of E?rikar Monzogranite in the eastern Pontides, are primarily investigated in this study with the aim of determining its magma source and geodynamic evolution. The U–Pb zircon age obtained from E?rikar Monzogranite is 78 ± 1.5 Ma, thereby re?ecting the age of monzogranite. The I-type E?rikar Monzogranite comprises quartz, plagioclase (An_35–45), orthoclase, muscovite, and biotite. The geochemical analyses of the E?rikar Monzogranite indicate being medium K calc-alkaline, peraluminous, and resembling magmatic arc granite. The E?rikar Monzogranite is enriched in large ion lithophile elements and light rare earth elements relative to high field strength elements. Chondrite-normalized rare earth element patterns have concave upward shapes (La_N/Yb_N 2.47–8.58) with pronounced negative Eu anomalies (Eu_N/Eu* = 0.29–0.65). Initial εNd_(i) values vary between 1.85 and 2.18 and initial ⁸⁷Sr/⁸⁶Sr values between 0.7048 and 0.7067. Fractionation of plagioclase, hornblende, and apatite played an important role in the evolution of E?rikar Monzogranite. The crystallization temperatures of the melts ranged from 770°C to 919°C based on zircon and apatite saturation temperatures. The geochemical and isotopic data suggest being generated by the partial melting of ma?c lower crustal sources.     

Reliable Determination of Ge in Solid Environmental Samples Using a Chemical Preparation Procedure Developed for Si Isotopes and ICP‐MS Analysis

Germanium (Ge) exists at trace levels in the Earth's crust and is a powerful geochemical tracer of the silicon (Si) cycle. This study proposes a simple and reliable method for determining Ge contents in environmental samples using ICP‐MS. As Si and Ge have very similar chemical properties, we investigated the applicability of the chemical preparation procedure developed for Si isotopes for the determination of Ge in environmental samples. Advantages of this procedure are as follows: (a) efficient removal of the matrix and main interferences affecting Ge determinations by ICP‐MS, (b) a low limit of detection (6 ng l^?1), (c) relative repeatability of approximately 3% obtained on ⁷⁴Ge and (d) robustness and accuracy based on agreement within errors with the published Ge values for rock reference materials (BHVO‐2, AGV‐2 and BCR‐2). This procedure allowed revision of the Ge values of three soil reference materials (1.67 ± 0.09 μg g^?1, 2.41 ± 0.18 μg g^?1, 1.89 ± 0.10 μg g^?1 for GBW 07401, GBW 07404 and GBW 07407, respectively) and proposal of a value for the plant reference material ERM‐CD281 (70 ± 3 μg g^?1). This method provides a convenient procedure for determining Ge mass fractions in environmental samples and opens the possibility of coupling two tracers of the Si biogeochemical cycle with a single measurement procedure.     

The provenance of Western Irish Namurian Basin sedimentary strata inferred using detrital zircon U–Pb LA‐ICP‐MS geochronology

The Western Irish Namurian Basin (WINB) preserves classic examples of basin floor sequences through to slope deposits and deltaic cyclothems. Despite over 50 years of research into the WINB, its sediment provenance remains highly contested. Sedimentological arguments, including palaeocurrent vectors and palaeoslope indicators have been invoked to propose a sediment source from the NW or the west (i.e. from within Laurentia). These same indicators have been subsequently reinterpreted to reflect a southern provenance. It is not clear from sedimentological arguments alone which interpretation more accurately reflects the infilling of the WINB. Regional‐scale constraints on WINB provenance may be obtained with detrital zircon U–Pb geochronology. U–Pb LA‐ICP‐MS detrital zircon analysis was undertaken on samples from three sandstone units at different stratigraphic levels within the WINB siliciclastic sedimentary fill (Ross Formation, Tullig Sandstone, Doonlicky Sandstone). The samples are dominated by 500–700 Ma zircons, which can be correlated with Cadomian–Avalonian orogenic activity within terranes to the south of the WINB (Avalonia/Ganderia, Armorica and Iberia). In contrast, Eastern Laurentia, to the north of the WINB, was devoid of orogenic activity at this time. WINB samples also yield age populations younger than 500 Ma, and older than 700 Ma. These are not diagnostic of a particular source terrane and thus could be derived from terranes north and/or south of the WINB. WINB detrital zircon age spectra can be reconciled by an Avalonian or combined Avalonian–Laurentian provenance for WINB sedimentary strata. Further research is required in order to distinguish between these two possibilities. Copyright © 2011 John Wiley & Sons, Ltd.     

A Method for Rapid Determination of Re and Os Isotope Compositions Using ID‐MC‐ICP‐MS Combined with the Sparging Method

A simple, rapid method for the determination of Re and Os concentrations and isotope compositions using isotope dilution multi‐collector inductively coupled plasma‐mass spectrometry (ID‐MC‐ICP‐MS) combined with Carius tube digestion and sparging introduction of Os was developed. For Os measurement, four channeltron ion counters to detect different Os isotopes were used simultaneously, which led to a drastic reduction in the measurement time. Rhenium isotopes were measured by means of eight Faraday cups with solution nebulisation and an ultrasonic membrane desolvator. The representative ¹⁸⁸Os count rate of an Os standard solution containing 50 pg of total Os was approximately 110000–120000 cps at the onset of measurement; the Re intensity of our in‐house 10 pg g^?1 standard solution reached 1820 V/μg g^?1 with a sample uptake rate of 95–99 μl min^?1. These values indicate that the sensitivity of the method was sufficient even for samples with low Re and Os concentrations, such as chert. As the temporal variations of the amplification efficiency of the ion counters differed from one another, we adopted a sample‐calibrator bracketing method to correct the measured Re and Os isotope ratios. The Re and Os concentrations via the isotope dilution method and the ¹⁸⁷Os/¹⁸⁸Os ratios of two sedimentary rock reference materials (JMS‐2 and JCh‐1) on the basis of the isotope ratios determined by the MC‐ICP‐MS and by negative thermal ionisation mass spectrometry (N‐TIMS) were comparable within their ranges. Based on Os isotope measurement of the IAG reference material [Durham Romil Os (DROsS)], the average difference from the recommended value and precision of Os isotope measurements by the sparging method in combination with multi‐ion‐counters were 0.72% and 0.76% [1RSD (%), n = 29], respectively. The precisions in the ¹⁸⁷Os/¹⁸⁸Os ratios [1RSD (%)] of JMS‐2, JCh‐1 and DROsS were 0.35–0.71, 1.56–3.31 and 0.99–1.28%, respectively, which depended on their Os ion intensities. No systematic difference was observed between the Re and Os geochemical compositions of JCh‐1 and JMS‐2 obtained by means of digestion with inverse aqua regia and CrO₃‐H₂SO₄ solutions, suggesting that either acid solution can be used for the sparging method of sedimentary rock samples. As CrO₃‐H₂SO₄ solution is believed to liberate predominantly the hydrogenous Re and Os fraction from organic‐rich sediment, the sparging method combined with CrO₃‐H₂SO₄ digestion and multi‐ion‐counters in the mass spectrometry is expected to be a powerful tool for reconstructing the secular change in marine Os isotope compositions with high sample throughput.     

Accurate Measurement of Rare Earth Elements by ICP‐MS after Ion‐Exchange Separation: Application to Ultra‐Depleted Samples

This study reports precise and accurate data for rare earth elements (REE) measured on eight geological reference materials, five enriched in REE (BE‐N, BHVO‐2, BR, BR‐24 and RGM‐1) and three very depleted in REE (BIR‐1, UB‐N and DTS‐2). Data were acquired by quadrupole ICP‐MS after isolation of the REE using an ion‐exchange chromatography procedure. All the measured REE abundances were similar within ≈ 5% (10% for the most REE‐depleted sample DTS‐2) to the high‐quality measurements previously published in the literature. We also show that by using an internal Tm spike, the reproducibility of the data was improved to ～ 1%. Applying this technique to the analysis of ultra‐depleted rock samples (sub ng g^?1), we show that significant improvements were obtained relative to the routine trace element measurement method. The chondrite‐normalised patterns were smooth instead of displaying irregularities. Although the classical method gives excellent results on REE‐rich samples, we believe that our technique improves the precision and accuracy of measurements for highly REE‐depleted rocks.     

VizualAge: A Novel Approach to Laser Ablation ICP‐MS U‐Pb Geochronology Data Reduction

VizualAge, a new computer software tool for analysing U‐Pb data obtained by laser ablation‐inductively coupled plasma‐mass spectrometry, was developed. It consists of a data reduction scheme (DRS) for Iolite (a general mass spectrometry data analysis tool) as well as visualisation routines. In addition to the U/Pb and Th/Pb ages calculated by Iolite’s U‐Pb geochronology DRS, VizualAge also calculates ²⁰⁷Pb/²⁰⁶Pb ages and common Pb corrections for each time‐slice of raw data. Importantly, VizualAge allows one to display a live concordia diagram for visualising data on such a diagram as an integration interval is being adjusted. This provides instantaneous feedback regarding discordance, uncertainty, error correlation and common Pb. Several zircon data sets were used to illustrate how the live concordia could be used as a powerful inspection tool, revealing a single analysis to consist of zones of concordance, metamict areas, as well as inherited cores or younger overgrowths. VizualAge also constructs histograms, conventional and Tera‐Wasserburg type concordia diagrams, as well as 3D U‐Th‐Pb and total U‐Pb concordia diagrams. The precision and accuracy of data reduced with VizualAge are demonstrated with examples of the Ple?ovice, Temora‐2 and Penglai zircon reference materials. Data for zircon from the Long Lake Batholith (Wyoming craton) were used to illustrate how VizualAge calculated common Pb corrections and helped to expose as yet unexplained difficulties with accurately determining ²⁰⁴Pb.