Zhu-Yin Chu

Precise determination of Sm, Nd concentrations and Nd isotopic compositions at the nanogram level in geological samples by thermal ionization mass spectrometry

In this paper, a high sensitivity method for measurements of Nd isotopes as NdO+, on a TIMS using a single W filament with TaF5 as an ion emitter is presented. Although analyzing Nd isotopes as oxides (NdO+) is a well known technique, this is the first report to analyze Nd isotopic compositions as oxides using W filaments and the TaF5 emitter. When 0.5–1 ng loads of a Neodymium isotopic reference reagent, JNdi-1, were measured using this method, the ion yields were found to be mostly in excess of 15% and could be as high as 32%. Internal precision on 143Nd/144Nd could be better than 10 ppm (2SE) for 1 ng JNdi-1 loads and better than 15 ppm (2SE) for 0.5 ng JNdi-1 loads; thirteen replicates of 0.5–1 ng JNdi-1 loads yielded a 143Nd/144Nd value of 0.512112 ± 0.000028 (2SD). Compared with the previously reported NdO+ measurement method using the Re (or W) filaments + Silica-gel + H3PO4 loading techniques, this method has advantages including higher sensitivity, a more stable ion beam, and no need for oxygen gas to be bled into the ion source chamber. Sm isotopes were analyzed as Sm+ using the W filaments and the TaF5 emitter, and high sensitivity and good ion beam stability were also obtained. Several international rock reference materials, including an ultramafic rock reference material USGS PCC-1 that contains very low amounts of Sm and Nd, were analyzed with full column chemistry and the TaF5 method, and the results of Sm, Nd concentrations and Nd isotopic data are in good agreement with the reported values. Combined with a highly efficient and low-blank column chemistry to separate Nd from Sm, Ce, and Pr, this method holds potential to analyze Sm, Nd concentrations and Nd isotopic compositions of highly depleted peridotites; very small aliquots of minerals such as garnets; extra-terrestrial materials of limited sample size; and environmental samples that contain very low quantities of Sm and Nd.

A practical method for determination of molybdenite Re-Os age by inductively coupled plasma-mass spectrometry combined with Carius tube-HNO3 digestion

A simplified method for the determination of molybdenite Re-Os ages using inductively coupled plasma-mass spectrometry (ICP-MS) is presented. By the means of Carius tube method, molybdenite and pyrite were digested using concentrated HNO3, and were then changed into MoO3 and Fe(NO3)3 precipitates, respectively. Rhenium was determined directly by ICP-MS after removal of Os by heating for the molybdenite supernatant or by cation-exchange purification for the pyrite supernatant. Osmium distilled as OsO4 from the supernatant was trapped using pure water and could be directly analyzed by ICP-MS. This method was validated using two molybdenite reference materials, GBW 04435 and GBW 04436, and their Re-Os ages obtained are 220.3 ± 1.1 Ma (1.0%, 2s), and 140.5 ± 0.9 Ma (1.2%, 2s), respectively, consistent with literature values. The proposed Re-Os dating method was applied to molybdenite and pyrite sampled from a porphyry Mo-deposit. The results show that this deposit is the oldest Mo-deposit so far found in China.

Precise and accurate determination of Sm, Nd concentrations and Nd isotopic compositions in geological samples by MC-ICP-MS

In this work, we established a highly reproducible analysis of Sm, Nd concentrations and Nd isotopic compositions in geological samples by isotope dilution analysis with MC-ICP-MS. This technique is superior in terms of the analytical reproducibility or rapidity of analysis compared with quadrupole ICP-MS or with thermal ionization mass spectrometry (TIMS) isotope dilution techniques. Samples were spiked with 149Sm–150Nd enriched tracer and then digested by a commonly used HF, HNO3 and HClO4 acid protocol. The bulk rare earth elements (REEs) were separated from the sample on a standard cation exchange resin, and further purified on Eichrom Technologies Ln Resin, to obtain Sm and Nd fractions prior to mass spectrometric measurements. Replicate analyses of international certified reference materials (CRMs) demonstrate that our obtained 147Sm/144Nd and 143Nd/144Nd isotopic ratios are in good agreement with previously published values from isotope dilution methods. In addition to determining the concentrations of Sm and Nd, the Nd isotopic composition can be measured simultaneously during Nd isotope dilution run. Additionally, a mineral Sm-Nd isochronal age that is identical to, within error, a U-Th-Pb zircon age for the same rock is further measured and validates the robustness of the present protocol. Therefore, the high actual sample throughput inherent to the MC-ICP-MS can be fully exploited for the determination of Sm and Nd concentrations and Nd isotopic compositions.

An Improved Fe—Ni Sulfide Fire Assay Method for Determination of Re, Platinum Group Elements, and Os Isotopic Ratios by Inductively Coupled Plasma- and Negative Thermal Ionization—Mass Spectrometry

An improved Fe–Ni sulfide fire assay method has been developed for determination of Re, the platinum group elements (PGE), and Os isotopic ratios using inductively coupled plasma–mass spectrometry (ICP-MS) and negative thermal ionization–mass spectrometry (NTI-MS). Recovery of Re using the neoclassical NiS fire assay technique is very low, but recoveries of up to 75% can be achieved by using Fe–Ni sulfide as a collector and Na2B4O7 as a flux. Using isotope dilution for determination of Re, a number of standard reference materials were analyzed for PGE and Re, and the results are consistent with their certified values. Multiple analyses of the reference standard GBW 07290 produced precisions ranging from 2.2% for Os to 5.9% for Ir. The detection limits are 2 pg g−1 for Ru, 1.5 pg g−1 for Rh, 25 pg g−1 for Pd, 23 pg g−1 for Re, 0.7 pg g−1 for Os, 1 pg g−1 for Ir, and 6 pg g−1 for Pt. The low procedural blank of Os (1.3 pg g−1) makes the Fe–Ni sulfide fire assay suitable for analysis of Os isotopic compositions. Using NTI-MS, the 187Os/188Os ratios were measured for WPR-1 and they are consistent with literature values.

Geochemical constraints on the petrogenesis of the Proterozoic granitoid gneisses from the eastern segment of the Central Tianshan Tectonic Zone, northwestern China

The Tianshan orogen is divided into the Northern, Central and Southern Tianshan tectonic zones by the northern and southern sutures on both sides of the Central Tianshan Tectonic Zone. The eastern segment of the Central Tianshan Tectonic Zone is characterized by the presence of numerous Precambrian metamorphic blocks and is unconformably overlain by Ordovician–Silurian and late Palaeozoic strata. The Precambrian Kumishi and Pargantag metamorphic blocks are the largest older blocks in the eastern segment of the Central Tianshan Tectonic Zone, consisting mainly of metamorphic granitoids and sedimentary rocks in greenschist to amphibolite facies. There are two major lithological assemblages of the metamorphic granitoids: (1) quartz dioritic gneisses, and (2) granodioritic–monzogranitic gneisses with a minor amount of tonalitic and syenogranitic gneisses in both the Kumishi and Pargantag blocks. The quartz dioritic gneisses are characterized by low Sr/Ce ( 2 O (2.65–4.04 wt %) contents and e Nd (t) values (−2.37–5.84), and negative Nb and Zr–Hf anomalies, as well as relatively flat chondrite-normalized REE patterns with slightly negative Eu anomalies, suggesting that the quartz dioritic gneisses were derived from partial melting of a depleted mantle source enriched by fluids and sedimentary melts from the subducted slab. However, most of granitic gneiss samples display high K 2 O contents, low Al 2 O 3 /(FeO * + MgO + TiO 2 ) values, and relatively flat chondrite-normalized REE patterns with intensively negative Eu anomalies. Integrated low e Nd (t) values and older T DM model ages suggest that crustal materials played a significant role in the petrogenesis of these granitoid gneisses and that they were mainly derived from the partial melting of calc-alkaline mafic to intermediate rocks in the crust. Also, variations in geochemical features between the Kumishi–Gangou and Pargantag regions, such as Zr and Hf, may reflect geographic variability in the development of coeval granitic magmas. Tectonic discrimination for granitoid, using trace elements, together with Nd isotopic data, demonstrates that these granitoid gneisses in the eastern segment of the Central Tianshan Tectonic Zone formed in a continental margin arc during late Mesoproterozoic times.

A rapid single column separation scheme for high-precision Sr–Nd–Pb isotopic analysis in geological samples using thermal ionization mass spectrometry

Thermal ionization mass spectrometry (TIMS) is considered the most accurate technique for determining Sr–Nd–Pb isotopic ratios in geological samples. However, time-consuming and complex sample separation procedures greatly hinder the instrumental measurement efficiency. In this study, a single-column separation chemistry procedure for Sr–Nd–Pb from single rock dissolution was developed. The chemistry procedure was designed to minimize the number of evaporation steps and considerably shorten the separation time, enabling high throughput for TIMS. In contrast to conventional three-column separation procedures (∼3 days), this technique was characterized by high efficiency superiority in terms of separation time (∼8 hours), a 3-fold enhancement in the separation efficiency. The stability of our procedure was demonstrated by replicated TIMS measurements of 87Sr/86Sr, 143Nd/144Nd, 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios for six international silicate rock reference materials, spanning a wide range of bulk compositions. The analytical results obtained for these standards agreed well with published data. The external reproducibility (2 RSD, n = 10) of a BCR-2 standard sample was ±0.0020% for 87Sr/86Sr, ±0.0023% for 143Nd/144Nd, and ±0.021–0.033% for 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios.

Evaluation of Sr chemical purification technique for natural geological samples using common cation-exchange and Sr-specific extraction chromatographic resin prior to MC-ICP-MS or TIMS measurement

In this paper, sample preparation protocols based on common cation-exchange and Sr-specific extraction chromatographic resin were evaluated and investigated for natural geological samples prior to 87Sr/86Sr ratio measurement using MC-ICP-MS or TIMS. Several CRMs and real geological samples were digested using HF, HNO3 and HClO4 in closed vessels prior to sequential chemical purifications and then Sr isotopic ratios were determined by MC-ICP-MS or TIMS. Because HREEs reside in the Sr fraction when common cation-exhange resin is used, this purification technique is unsuitable prior to MC-ICP-MS analysis, as doubly loaded HREE interfere on Sr masses. We observe an obviously positive relationship between the radiogenic 87Sr/86Sr ratio and the content of HREEs in the Sr fractions, as well as a negative relationship between un-radiogenic 84Sr/86Sr or 84Sr/88Sr ratios and the content of HREEs in the Sr fractions during MC-ICP-MS Sr isotopic measurement. Such effects are insignificant for TIMS measurement, because ionization temperatures are generally lower and can be well controlled during the analyses. In contrast to the traditional Sr purification method (cation exchange resin), Sr-specific extraction chromatographic resin produces high purity Sr fractions, making it feasible for both MC-ICP-MS and TIMS.

Simultaneous Determination of 143Nd/144Nd and 147Sm/144Nd Ratios and Sm–Nd Contents from the Same Filament Loaded with Purified Sm–Nd Aliquot from Geological Samples by Isotope Dilution Thermal Ionization Mass Spectrometry

Isotope dilution thermal ionization mass spectrometry (ID-TIMS) is the standard technique used to achieve precise (143)Nd/(144)Nd and (147)Sm/(144)Nd isotope ratios and accurate elemental concentrations of Sm-Nd. However, in previous studies, purified Sm and Nd fractions must be individually loaded onto different filaments for their accurate determination using TIMS because of severe isobaric interferences. Thus, the classical ID-TIMS technique is time consuming and laborious. In this study, a new method is proposed, which is able to acquire both ratios of (143)Nd/(144)Nd and (147)Sm/(144)Nd and concentrations of Sm-Nd simultaneously on the same filament arrangement. The measurement time and filament consumption are reduced by 50% with the current method, and therefore, the operation cost of TIMS is significantly reduced. A mixed (152)Sm-(148)Nd spike was employed to achieve accurate results after spike subtraction and isobaric interference corrections. Results obtained from a series of standard rock samples are in good agreement with recommended values, within ±0.003% for the (143)Nd/(144)Nd ratio and ±1% for the (147)Sm/(144)Nd ratio.

Rapid separation scheme of Sr, Nd, Pb, and Hf from a single rock digest using a tandem chromatography column prior to isotope ratio measurements by mass spectrometry

A straightforward tandem column separation procedure is presented for the separation of Sr, Nd, Pb, and Hf from silicate materials. It allows rapid purification, without any intervening evaporation, of these four elements of great interest in Earth science and cosmochemistry. After sample loading, the upper Sr Spec column adsorbs Sr and Pb, while the lower TODGA Spec column adsorbs Hf and Nd. Strontium-lead and hafnium–neodymium elements are then back-extracted from the Sr Spec and TODGA Spec columns, respectively. The whole separation procedure, including column setup, cleaning, and pre-conditioning, takes approximately eight hours for separating a batch of 25 samples. The proposed procedure offers significant improvement in separation efficiency of these often-used four elements, compared with conventional four column methods. Fractions of Sr, Nd and Pb are then measured by TIMS and the Hf fraction is determined by MC-ICP-MS. The stability of this procedure was demonstrated by replicate measurements of 87Sr/86Sr, 143Nd/144Nd, 176Hf/177Hf, 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb isotope ratios of eight international silicate rock reference materials, spanning a wide range of bulk compositions. The analytical results obtained in this study agree well with published data. The external reproducibility (2RSD, n = 8) of standard BCR-2 was ±0.0026% for 87Sr/86Sr, ±0.0020% for 143Nd/144Nd, ±0.0049% for 176Hf/177Hf, and ±0.026–0.034% for 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb isotope ratios.

Single-step separation scheme and high-precision isotopic ratios analysis of Sr–Nd–Hf in silicate materials

Thermal ionization mass spectrometry and multiple-collector inductively coupled plasma mass spectrometry are considered to be “gold standards” for the determination of the isotope ratios of Sr–Nd and Hf in geological samples because of the extremely high precision and accuracy of these methods. However, the sample throughputs are hindered by time-consuming and tedious chemical procedures. Three-step ion exchange resin separation is traditionally employed to purify Sr–Nd–Hf from matrix elements. In this study, a one-step Sr–Nd–Hf separation scheme was developed to process geological samples. The separation scheme is based on the combined use of conventional AG50W-X12 cation-exchange resin and LN Spec extraction chromatographic material without any intervening evaporation step. The protocol not only prevents cross-contamination during operation using multiple-stage ion exchange resins but also significantly improves the efficiency of sample preparation. The stability of our chemical procedure was demonstrated by replicate measurements of 87Sr/86Sr, 143Nd/144Nd, and 176Hf/177Hf ratios in six international reference materials of silicate rocks. The analytical results obtained for these standard rocks compare well with the published data. The external reproducibility (2 SD, n = 10) of a BCR-2 standard sample was ±0.000018 for 87Sr/86Sr, ±0.000010 for 143Nd/144Nd, and ±0.000014 for 176Hf/177Hf.