High-precision zirconium stable isotope measurements of geological reference materials as measured by double-spike MC-ICPMS
Introduction
The age of plasma-source multiple collection mass spectrometry has seen a relative boom in the number of different stable isotope systems that have been developed and applied to solve various geo- and cosmo-chemical problems over the last two decades (see review by Teng et al., 2017). While relatively mature stable isotope systems continue to provide powerful constraints on various natural processes, the development and subsequent application of new systems is needed in order to constrain different earth and planetary processes.
High-field strength elements (HFSEs) have been extensively studied within the field of high-temperature geochemistry. These elements are classified on the basis of exhibiting a small ionic radii (Z) relative to a high cationic charge (r), and although all elements displaying a Z/r > 2 are considered high-field strength (Rollinson, 1993), this classification has traditionally been applied to Hf, Zr, Ti, Nb and Ta within a geological context (Salters, 1998). Owing to their high Z/r ratio, the HFSEs are not readily incorporated in the crystal lattice of most mantle minerals, and, as such, behave as incompatible elements during mantle melting events (e.g. Salters and Shimizu, 1988). Furthermore, they are generally considered poorly mobile in low pressure aqueous fluids and, as such, are comparatively resistant to late metamorphic and alteration effects (Winchester and Floyd, 1977). Consequently the HFSEs serve as good tools to trace historic mantle depletion events and the petrogenesis of igneous rocks (e.g. Kelemen, 1990; Woodhead et al., 1993; Niu, 1997, Niu, 2004; van Westrenen et al., 2001; Weyer et al., 2003).
Despite their unique geochemical behaviour, the HFSEs have received relatively little attention in terms of stable isotope geochemistry. Only Ti has been extensively investigated for mass-dependant stable isotope variations in nature (e.g. Millet et al., 2016). Zirconium is a key member of the HFSE group and occurs as a transition metal, d-block element within the periodic table. It behaves as a refractory incompatible element during partial melting of Earth's mantle, and is consequently enriched in the crust (>100 μg g−1; Ronov and Yaroshevsky, 1969) relative to bulk silicate earth (~10 μg g−1; McDonough and Sun, 1995). Zirconium has five stable isotopes: 90Zr (51.45%); 91Zr (11.22%); 92Zr (17.15%); 94Zr (17.38%); 96Zr (2.80%), thus making it an interesting target for examining mass-dependent stable isotope variations within geological processes. Additionally, as Zr has more than four stable isotopes, it is a suitable candidate for the application of the double spike technique to discriminate for processing or analytically induced mass bias effects. Zirconium isotopes have been studied before but largely from a mass independent perspective (e.g. Schönbächler et al., 2004; Iizuka et al., 2016), although Akram & Schönbächler (2016) did examine a selection of synthetic standards and geo-reference materials and documented a range of mass dependent isotopic variations.
Recently, non-traditional stable isotopes have served as a powerful tool to study igneous differentiation and the temporal and spatial evolution of Earth's mantle (e.g. Dauphas et al., 2009; Chen et al., 2013; Millet et al., 2016; Greber et al., 2017a, Greber et al., 2017b). Because Zr is among one of the most refractory trace elements found on Earth it is comparatively resistant to evaporation and condensation, metamorphism and late alteration, thus making it relatively easy to understand primary igneous processes. Furthermore, seeing as the highly refractory mineral zircon represent a major host of Zr, it is possible that Zr isotopes could serve as a powerful tracer for understanding the evolution of source and growth of continental crust through time, without suffering from the effects of metamorphism and alteration that other stable isotope systems are susceptible to.
Here we present data for one zircon mineral separate reference material and six terrestrial silicate rocks and explore the utility of Zr isotopes as tracers of igneous processes on Earth. A new analytical method for the determination of high precision (<±0.05‰, 2sd) Zr isotope ratios within terrestrial silicate rock and mineral separate samples is also presented in detail.
Section snippets
Double-spike preparation and calibration
The double-spike technique is a well-established method for the determination of high-precision isotope ratios in various earth and planetary materials. The advantages of the double-spike technique have been known since early measurements of radiogenic isotope ratios by thermal ionization instruments (TIMS; Compston and Oversby, 1969; Gale, 1970; Dodson, 1970; Russell, 1971; Cumming, 1973). More recently, the application of the double-spike technique has been extended to generate high-precision
Reagents, standards and reference materials
All laboratory work was carried out within class 100 laminar flow workstations as part of the clean geochemistry facility at the Institute de Physique du Globe de Paris (IPGP). Concentrated reagents (HCl, HNO3, HF) were purchased from BASF France as Selectipur® grade, and were further purified by sub-boiling distillation using Savillex™ Teflon stills. All further dilutions of reagents were performed using 18.2 MΩ cm ultra-pure H2O from a Merk™ Millipore Milli-Q® (MQ) purification system.
As no
Comparison of different sample dissolution techniques
Because a large fraction of the Zr within igneous rocks is likely to be hosted within highly refractory zircon grains it was necessary to examine the effectiveness of different dissolution techniques for Zr isotopes. As described in Section 3.2 multiple dissolutions using either Teflon or Parr bombs have been performed on the GA whole rock powder and on uncrushed grains of the Plešovice zircon.
The δ94/90ZrIPGP-Zr values for four individual measurements of the granitic whole rock reference
Conclusions
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A new method is presented for the determination of stable Zr isotopes within natural samples using a two step ion exchange chromatography with AG1-X8 and DGA resins, and analysis by MC-ICPMS using a 91Zr–96Zr double-spike to correct for mass discrimination.
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The chemical purification method effectively separates Zr from the sample matrix, including Mo, and we demonstrate that the purity of Zr in the processed samples is suitable for accurate determination of isotope ratios using the 91Zr–96Zr
Acknowledgements
We thank the two anonymous reviewers for their thoughtful comments that have greatly clarified the manuscript and Klaus Mezger for efficient editorial handling. Funding for this work comes from the European Research Council [ERC Starting grant PRISTINE: 637503]. Pascale Louvat and Thibaut Sontag are acknowledged for technical support of the multi-collector facility at IPGP. Marc-Alban Millet (Cardiff) is thanked for discussion regarding double-spike and also for providing some of the reference
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