High precision titanium isotope measurements on geological samples by high resolution MC-ICPMS

A method has been developed for the precise and reproducible measurement of Ti isotopes in natural materials using high resolution MC-ICPMS. Instrumental mass fractionation is internally corrected using Ti-49/Ti-47. Replicate measurements of synthetic standard solutions, terrestrial rocks, and the carbonaceous chondrite Allende yield a long-term reproducibility (2 sigma) of 0.28 epsilon, 0.34 epsilon, and 0.28 epsilon for Ti-50/Ti-47, Ti-48/Ti-47, and Ti-46/Ti-47, respectively. Isobaric interferences from Ca-46.48(+), V-50(+), Cr-50(+), and doubly charged Zr can be corrected for reliably in separated Ti solutions with Ca/Ti < 5, V/Ti < 0.3, Cr/Ti < 0.2, and Zr/Ti < 1, respectively. Such elemental ratios are achieved easily in most samples using the anion-exchange procedure presented. Single and double charged polyatomic ions can either be resolved, e.g., (NAr+)-N-14-Ar-36, or do not compromise the measurements. The method has been successfully applied to terrestrial rocks, meteorites, and various mineral separates. Terrestrial samples and standards agree within analytical uncertainties but are consistently different from the recommended values of Niederer et al. [F.R. Niederer, D.A. Papanastassiou, G.J. Wasserburg, Geochim. Cosmochim. Acta 45 (1981) 1017] with the largest effect on Ti-50/Ti-47. The new results provide evidence that the recommended terrestrial Ti-50/Ti-47 is not well constrained; our data are higher by similar to 13 epsilon than the recommended value. Better agreement is found with the values recommended by Heydegger et al. [H.R. Heydegger, J.J. Foster, W. Compston, Earth Planet. Sci. Lett. 58 (1982) 406]. Our best estimate for the isotopic composition of terrestrial Ti (relative to Ti-49/Ti-47 = 0.749766) is: Ti-50/Ti-47 = 0.73010, Ti-48/Ti-47 = 10.06884, and Ti-46/Ti-47 = 1.09325. This corresponds to an atomic weight of 47.877, significantly different from the value of 47.867 recommended by IUPAC. A 50Ti/47Ti anomaly for bulk Allende of 3.37 +/- 0.51 epsilon is found, while for Ti-48/Ti-47 and Ti-46/Ti-47 the data are identical to the terrestrial value within the uncertainties. This is consistent with other recently reported differences between the bulk Earth and chondrites in neutron-rich nuclides. (c) 2006 Elsevier B.V. All rights reserved.