Modified ion exchange separation for tungsten isotopic measurements from kimberlite samples using multi-collector inductively coupled plasma mass spectrometry

Abstract

Tungsten isotope composition of a sample of deep-seated rock can record the influence of core–mantle interaction of the parent magma (D. Brandon and R. J. Walker, Earth Planet. Sci. Lett., 2005, 232, 211–225). Samples of kimberlite, which is known as a carrier of diamond, from the deep mantle might exhibit effects of core–mantle interaction. Although tungsten isotope anomaly was reported for kimberlites from South Africa (K. D. Collerson, R. Schoenberg and B. S. Kamber, Geochim. Cosmochim. Acta, 2002, 66, A148), a subsequent investigation did not verify the anomaly (A. Scherstén, T. Elliot, C. Hawkesworth, and M. Norman, Nature, 2004, 427, 234–237). The magnesium-rich and calcium-rich chemical composition of kimberlite might engender difficulty during chemical separation of tungsten for isotope analyses. This paper presents a simple, one-step anion exchange technique for precise and accurate determination of tungsten isotopes in kimberlites using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). Large quantities of Ca and Mg in kimberlite samples were precipitated and removed with aqueous H₂SO₄. Highly pure fractions of tungsten for isotopic measurements were obtained following an anion exchange chromatographic procedure involving mixed acids. That procedure enabled efficient removal of high field strength elements (HFSE), such as Hf, Zr and Ti, which are small ions that carry strong charges and develop intense electrostatic fields (F. Albarède, An Introduction to Geochemistry, Cambridge University Press, Cambridge, UK, 2003). The tungsten yields were 85%–95%. Advantages of this system include less time and less use of reagents. Precise and accurate isotopic measurements are possible using fractions of tungsten that are obtained using this method. The accuracy and precision of these measurements were confirmed using various silicate standard rock samples, JB-2, JB-3 and AGV-1.