Methodology and Application of Hafnium Isotopes in Ilmenite and Rutile by MC‐ICP‐MS

Abstract

The high abundances of the high field‐strength elements in ilmenite and rutile make these minerals particularly suitable for hafnium isotopic investigations. We present a technique for separating Hf by ion exchange chemistry from high‐TiO2 (> 40% m/m) minerals to achieve precise Hf isotopic composition analyses by MC (multiple collector)‐ICP‐MS. Following digestion and conversion to chlorides, the first elution column is used to separate iron and the rare earth elements, the second column is designed to separate most of the titanium from Hf, an evaporation step using HClO4 is then performed to remove any trace of HF in preparation for the third column, which is needed to eliminate any remaining trace of titanium. The modified chemistry helped to improve the yields from < 10 to > 78% as well as the analytical precision of the processed samples (e.g., sample 2033‐A1, 176Hf/177Hf = 0.282251 ± 25 before vs. 0.282225 ± 6 after). The technique was tested on a case study in which the Hf isotopic ratios of ilmenite and rutile (analysed prior to the chemistry improvement) were determined and permitted to evaluate that the origin of rutile‐bearing ilmenite deposits is from the same or similar magma than their, respectively, associated Proterozoic anorthosite massifs (Saint‐Urbain and Lac Allard) of the Grenville Province in Québec, Canada.

Evaluation of the digestion capability of ammonium bifluoride for the determination of major and trace elements in Ti-rich minerals by ICP-MS

“…Ti-rich minerals, especially rutile, are known to be refractory samples and oen take 72-168 hours to be completely dissolved using acid digestion. 5,7,8,11,15 A digestion time from 0.5 to 10 h was tested for 50 mg each of rutile RGPMR, ilmenite ore GBW07841, ilmenite PAK122, titanite AFG200 and perovskite RUS100, while 300 mg of NH 4 HF 2 and 230 °C were used. The recoveries of Ti were evaluated as a function of digestion time at 230 °C as shown in Fig.…”

“…1,5,6 The Hf isotopic compositions of ilmenite and rutile may provide provenance information on complex geological histories. 7,8 Titanite and perovskite contain considerable amount of rare earth elements (REEs), and Sm-Nd isotopic compositions are useful for understanding the age and source of Ti-rich rocks. 9,10 Recently, the mass-dependent Ti isotope ratio as a power tool for studying magmatic processes has attracted much attention for some Ti-rich minerals, such as rutile, ilmenite and titanite.…”

“…[11][12][13] Complete dissolution is a prerequisite for the accurate determination of elements and isotopic compositions in geological materials. 14 Many digestion methods, such as openvessel acid digestion, 9,11 closed-vessel acid digestion 7,8,15 and alkali fusion, 16,17 have been developed for the decomposition of Ti-rich minerals. Alkali fusion is efficient due to a high digestion temperature of >700 °C, but this increases the total dissolved solid content and could lead to a high blank.…”

“…11 Closed-vessel acid digestion achieves a higher digestion temperature (190 °C) compared to open-vessel acid digestion, but it still takes several days to dissolve rutile. 7,8 For example, closed-vessel acid digestion with a mixture of acids (5 mL HF + 0.7 mL HNO 3 + 0.7 mL HClO 4 ) took 5 days to digest 100 mg of rutile at 190 °C. 7 A rapid and simple digestion technique using solid ammonium biuoride (NH 4 HF 2 ) has been developed to digest refractory geological materials, such as zircon, for 3 h at 230 °C.…”

“…7,8 For example, closed-vessel acid digestion with a mixture of acids (5 mL HF + 0.7 mL HNO 3 + 0.7 mL HClO 4 ) took 5 days to digest 100 mg of rutile at 190 °C. 7 A rapid and simple digestion technique using solid ammonium biuoride (NH 4 HF 2 ) has been developed to digest refractory geological materials, such as zircon, for 3 h at 230 °C. 18 Lately, NH 4 HF 2 (or NH 4 F) has become popular for complete digestion of various geological rocks and minerals for the determination of the total elements and isotopic compositions, as it exhibits many advantages including efficiency, safety, simple operation and low blank.…”