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Zirconium (Zr) stable isotopes recently emerged as potential tracers of magmatic processes and, as a result, their behavior in high-temperature environments have been the focus of extensive characterization. In contrast, few studies have focused on Zr behavior and isotopic fractionation in low temperature or aqueous environments. Here, we describe a new analytical routine for highly precise and accurate analysis of Zr isotopes of water samples, using a combination of double-spike and iron co-precipitation methods. To assess the impact of potential systematic biases a series of experiments were conducted on natural and synthetic water samples. Our results show that the spike-to-sample ratio, matrix composition, and high field-strength element (HFSE) concentration have negligible effects on measured seawater Zr isotopic compositions, and that the Fe co-precipitation method used yields accurate and precise Zr isotope data. We thus apply this method to natural seawater samples collected from a water column profile in the Pacific Ocean off the coast of California, with depths ranging from 5 to 711 m. We find that the natural seawater samples are highly fractionated relative to solid-Earth values and display marked variability in δ94/90Zr as a function of depth, ranging from ∼ +0.650 ‰ near the surface, to + 1.530 ‰ near the profile bottom, with an analytical uncertainty of ± ∼0.045 ‰ (2 SE, external reproducibility). The δ94/90Zr value of seawater is much higher than that of Earth’s mantle and continental crust, which has a δ94/90Zr value near zero, indicating the presence of processes in the hydrosphere capable of inducing large mass-dependent fractionation. Furthermore, the seawater δ94/90Zr value exhibits systematic variations with respect to water depth and salinity, suggesting that Zr isotopic compositions may be sensitive to seawater chemical properties and source highlighting its potential utility as a tracer of biogeochemical processes within the ocean.
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Rapid boron isotope and concentration measurements of silicate geological reference materials dissolved through sodium peroxide sintering
Understanding the movement of fluids in the solid Earth system is crucial for answering a wide range of important questions in Earth science. Boron (B) is a perfect tracer for geofluids because of its high solubility and large isotopic fractionation that depends on both temperature and alkalinity. However, the high volatility of boron in acidic solutions at moderate temperatures presents a significant challenge for accurate measurements of the boron concentration and boron isotopic ratios for silicate rock samples. To circumvent this problem, most laboratories use low-temperature dissolution methods that involve concentrated hydrofluoric acid with or without mannitol. However, hydrofluoric acid is highly hazardous and the controlled temperature condition may be difficult to monitor. As a result, relatively few silicate samples have been analyzed for high precision B concentration and isotopic composition measurements, which hinders our understanding of the behavior of B in the solid earth system and the utility of this powerful tracer. Here we report B concentrations and isotopic compositions of the most commonly used geological reference standards dissolved through sodium peroxide sintering and purified using a rapid single-column exchange chromatographic procedure. This streamlined method effectively removes Na and Si from the sample matrix and generates accurate B concentration and isotopic data in as little as a day without the need for expensive lab equipment and reagents. Sintering is already routinely used to dissolve zircon-bearing silicate samples as it ensures complete dissolution. Besides the analysis of boron, other elemental and isotopic analyses can be performed using aliquots of the same dissolution, which greatly speeds up the chemical processing time and reduces uncertainties associated with sample heterogeneity. Using this method, large amounts of material can be processed for ion-exchange chromatography without the need of splitting each sample into separate beakers for dissolution as is often required for the HF + mannitol dissolution method. This new method can rapidly expand the available dataset of the boron concentration and boron isotopes of silicate materials which will certainly advance our understanding of many geologic problems involving fluids.
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- Award ID(s):
- 1714892
- PAR ID:
- 10354646
- Date Published:
- Journal Name:
- Journal of Analytical Atomic Spectrometry
- Volume:
- 36
- Issue:
- 10
- ISSN:
- 0267-9477
- Page Range / eLocation ID:
- 2153 to 2163
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D1JA00195G
