Haifeng Fan

Molybdenum isotopic records across the Precambrian-Cambrian boundary

Molybdenum (Mo) isotopic data from early Cambrian formations in southern China demonstrate the importance of non-euxinic sediments as a promising archive for coeval seawater Mo records. For the first time, in analogue to noneuxinic, nonskeletal carbonates, we report that the pristine phosphorites also can preserve the ambient fluid, which make the pristine phosphorites a new potential fingerprinting tool to record the coeval paleoseawater. Pristine phosphorite and dolomite samples of two early Cambrian formations in southern China suggest that early Cambrian seawater may have had δ 97/95 Mo values of at least 1.4‰, similar to modern oceans. Low δ 97/95 Mo (as low as −0.5‰) values were obtained from reworked phosphorite, but evidence suggests that these are reflecting Mo related to Fe oxyhydroxide minerals rather than the coeval seawater. In contrast, late Precambrian dolomite just below the transition to the Cambrian would indicate a δ 97/95 Mo of coeval seawater slightly heavier than that of the Mesoproterozoic (0.9‰–1‰ versus 0.8‰), suggesting that oceanic circulation patterns may have been thoroughly reorganized by that time, and may have triggered biological diversification from the Ediacaran to the early Cambrian.

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.

Marine Redox Conditions in the Early Cambrian Ocean: Insights from the Lower Cambrian Phosphorite Deposits, South China

It is generally considered that a significant change in oceanic redox conditions occurred during the Ediacaran–Cambrian transition. However, there are currently two major conflicting views on the degree of oxygenation of deep water (oxic vs. ferruginous) during this interval. To date, the oxygenation conditions of the Early Cambrian ocean have not been well constrained. The oxygenation magnitude and mechanism of the Early Cambrian ocean could be critical to the significant biological evolution of the “Cambrian Explosion”. To constrain the Early Cambrian oceanic redox environment, we conducted an integrated study on iron and sulfur isotopes and redox-sensitive elements (Mo, U, and V) of Lower Cambrian phosphorite deposits from two shallow sections (Meishucun and Gezhongwu) and a deeper water section (Zunyi) from the Yangtze Platform, South China. The near zero δ56Fe values from the two shallow sections studied here reflect oxic conditions in the lower phosphorite deposition. An obvious positive shift in δ56Fe and redox-sensitive element content was observed in the middle parts of the two shallow water sections, which might reflect loss of light iron by dissimilatory iron reduction during early diagenesis under suboxic shallow water in the platform. However, the highly positive δ56Fe values in the deep section could reflect a lower oxidation degree of dissolved Fe(II) under anoxic deep water. The data suggest redox-stratified oceanic conditions during the Early Cambrian, in which completely oxygenated shallow water (platform) coexisted with anoxic deep water (slope). We propose that prolonged upwelling of dissolved organic carbon (DOC)-, Fe(II)- and phosphorus-rich anoxic deep water in a redox-stratified ocean could have increased exchange with the open ocean, resulting in major phosphorite deposition in oxic-suboxic conditions. The progressive oxygenation of the ocean may have facilitated the Early Cambrian biotic diversification.

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.

Calibrating NIST SRM 683 as a new international reference standard for Zn isotopes

Zinc isotopes have been increasingly applied in cosmochemistry, geochemistry, and environmental sciences. A direct comparison of data obtained in different laboratories is a prerequisite to these applications. The JMC 3-0749L Zn solution is the international primary standard for Zn isotopes, but it is nearly exhausted and no longer available. Thus, it is necessary to calibrate a new reference material for Zn isotope analysis. Herein, we measured the Zn isotope composition and examined the homogeneity of the Standard Reference Material 683 of the National Institute of Standard and Technology (NIST SRM 683, a pure Zn metal nugget of 140 grams) with the double-spike method using Multiple Collector-Inductively Coupled Plasma-Mass Spectrometry (MC-ICP-MS). Five metal nuggets of NIST SRM 683 were intensively sampled by micro-drilling, and they exhibited δ66ZnJMC-Lyon of 0.13 ± 0.06‰ (here and throughout the text, errors are given as two-standard deviation (2SD), N = 32), 0.13 ± 0.04‰ (N = 31), 0.13 ± 0.04‰ (N = 30), 0.13 ± 0.04‰ (N = 30), and 0.12 ± 0.06‰ (N = 30). These values were identical to those of two Zn metal nuggets (0.11 ± 0.02‰ and 0.12 ± 0.02‰) previously reported, which confirmed the accuracy of our data and the isotopic homogeneity of NIST SRM 683. We also digested one whole nugget with δ66ZnJMC-Lyon of 0.12‰ ± 0.04‰ (N = 44), which was identical to that of the drilling samples. Our results indicated that within our analytical precision of 0.04, NIST SRM 683 is homogeneous in Zn isotopic compositions with δ66ZnJMC-Lyon of 0.12 ± 0.04‰ (N = 295) at the 95% confidence level, which is close to the Zn isotope value of bulk silicate Earth. Therefore, we believe that NIST SRM 683 is a good candidate for the next primary standard for Zn isotopes.

Determination of total selenium in geological samples by HG-AFS？ After enrichment with thiol cotton fiber

Standard reference material and different geological samples were dissolved by system A(the mixture of nitric,perchloric acids) and system B(the mixture of nitric,perchloric and hydrofluoric acids),and total Se in all the samples was measured by hydride generation atomic fluorescence spectrometry(HG-AFS) after enrichment with thiol cotton fiber(TCF).The agreement is very good(within error) for most of the samples,particularly for those having recommended values with two different digestion methods.The Se concentrations found in our study by the two methods are identical,the correlation coefficient is 0.9986;the relative standard deviation(RSD) for the determination of 0.04 μg/g Se is 10.2%;and the recovery of systems A and B are 96%—106% and 99%—104%,respectively.