Chuanwei Zhu

Characteristics of Cd isotopic compositions and their genetic significance in the lead-zinc deposits of SW China

Up to now, the evaporation and condensation, as well as the biological absorption and inorganic absorptions, have been proved to be major factors in Cd isotope fractionation. And Cd isotopes have been widely applied in studies on the universal evolution and marine environment and so on. However, only a few researches have been conducted in applying Cd isotopes to trace the source of metallogenic material and the evolution of the ore-forming fluid in a complex mineralization environment, especially in a hydrothermal ore-formation system. We measured the Cd isotopic compositions of sphalerite, galena, and ores from five lead-zinc deposits in SW China, and found that the δ114/110Cd values varied from −1.53‰ to 0.34‰, with a total range of 1.87‰, which is greater than most of measured geological samples. Meanwhile, through contrasting the Cd content with Cd isotopic compositions of different deposits, it may be concluded that different genetic lead-zinc deposits have different Cd content and isotopic compositions, which could be a tool for the studies on the origin of ore deposits. Also, the biomineralization and crystal fractionation may also result in Cd isotope fractionation. In a word, although the research of Cd isotopes is presently at the preliminary stage (especially in hydrothermal ore-formation system), this study demonstrated that Cd isotopes can give a clue in tracing the evolution of ore-forming fluid and metallogenic environment.

Zn/Cd ratios and cadmium isotope evidence for the classification of lead-zinc deposits

Lead-zinc deposits are often difficult to classify because clear criteria are lacking. In recent years, new tools, such as Cd and Zn isotopes, have been used to better understand the ore-formation processes and to classify Pb-Zn deposits. Herein, we investigate Cd concentrations, Cd isotope systematics and Zn/Cd ratios in sphalerite from nine Pb-Zn deposits divided into high-temperature systems (e.g., porphyry), low-temperature systems (e.g., Mississippi Valley type [MVT]) and exhalative systems (e.g., sedimentary exhalative [SEDEX]). Our results showed little evidence of fractionation in the high-temperature systems. In the low-temperature systems, Cd concentrations were the highest, but were also highly variable, a result consistent with the higher fractionation of Cd at low temperatures. The δ114/110Cd values in low-temperature systems were enriched in heavier isotopes (mean of 0.32 ± 0.31‰). Exhalative systems had the lowest Cd concentrations, with a mean δ114/110Cd value of 0.12 ± 0.50‰. We thus conclude that different ore-formation systems result in different characteristic Cd concentrations and fraction levels and that low-temperature processes lead to the most significant fractionation of Cd. Therefore, Cd distribution and isotopic studies can support better understanding of the geochemistry of ore-formation processes and the classification of Pb-Zn deposits.

An analytical method for precise determination of the cadmium isotopic composition in plant samples using multiple collector inductively coupled plasma mass spectrometry

Isotope techniques can be applied to discover the migration and transformation of metal elements in plants. However, only a few studies on Cd isotopes in plants have been carried out so far. In this study, an optimized analytical method consisting of digestion, purification and determination of Cd isotopes in plants was developed. Three Cd standard solutions as well as four plant species (Solanum nigrum, Ricinus communis, Cyperus alternifolius and Pteris vittata), which were grown in soil or hydroponic cultures, were repeatedly analyzed for Cd isotopes using Multiple Collector Inductively Coupled Plasma Mass Spectrometry (MC-ICPMS). The factors that affected the accuracy of Cd isotopic determination, such as isobaric interference and instrumental mass fractionation, have been carefully evaluated and corrected. The purification procedure yielded a Cd recovery of not less than 95% and effectively eliminated the spectral interference of Pd, In and Sn as well. The analysis of pure Cd standard materials showed accurate isotope values, which matched with the results of previously published methods. This technique provided an average long-term external reproducibility of ±0.09‰ for δ114/110Cd (2SD). The overall δ114/110Cd values of four plant species ranged from −0.39‰ to −0.08‰ and provided direct evidence for Cd isotopic fractionation in herbaceous plants.

Fractionation of Stable Cadmium Isotopes in the Cadmium Tolerant Ricinus communis and Hyperaccumulator Solanum nigrum.

Cadmium (Cd) isotopes provide new insights into Cd uptake, transport and storage mechanisms in plants. Therefore, the present study adopted the Cd-tolerant Ricinus communis and Cd-hyperaccumulator Solanum nigrum, which were cultured under controlled conditions in a nutrient solution with variable Cd supply, to test the isotopic fractionation of Cd during plant uptake. The Cd isotope compositions of nutrient solutions and organs of the plants were measured by multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS). The mass balance of Cd isotope yields isotope fractionations between plant and Cd source (δ114/110Cdorgans-solution) of −0.70‰ to −0.22‰ in Ricinus communis and −0.51‰ to −0.33‰ in Solanum nigrum. Moreover, Cd isotope fractionation during Cd transport from stem to leaf differs between the Cd-tolerant and -hyperaccumulator species. Based on these results, the processes (diffusion, adsorption, uptake or complexation), which may induce Cd isotope fractionation in plants, have been discussed. Overall, the present study indicates potential applications of Cd isotopes for investigating plant physiology.

Precise Mo isotope ratio measurements of low-Mo (ng g−1) geological samples using MC-ICP-MS

Although molybdenum (Mo) isotopic compositions of carbonatites, phosphorites and siliceous rocks can be used as proxies to reconstruct conditions of marine chemistry throughout geological time, only a few studies have, so far, analysed these low-Mo (ng g−1) geological samples because of analytical limitations. In this study, a low blank, high yield two-column Mo purification procedure was developed for various low-Mo geological samples. The sample-standard bracketing (SSB) and double-spike (DS) methods for mass fractionation correction were used to compare the accuracy of Mo isotope ratio measurements. Six Mo reference materials, NIST SRM 3134 Mo, JMC Mo, SC+1 and SC−1 (eluted fractions of Sigma-Aldrich Mo), CRM GSR-6 limestone and USGS BCR-2 basalt, were used as quality controls. The results showed that the Mo delta values of reference materials and geological samples corrected by the SSB and DS methods were, within error, consistent with each other and the DS method was the method of choice for samples with <0.5 μg g−1 Mo. The average instrument long-term (over 1 year) external reproducibility of NIST SRM 3134 Mo was better than ±0.03‰ amu−1 (2SD, n = 288) and the analytical precision of low-Mo (101 to 103 ng g−1) geological samples was better than ±0.04‰ amu−1. This method can facilitate Mo isotope ratio measurements in geological samples with a low Mo content, offering a possibility to study a wider range of Mo reservoirs in geological processes.