Evaluation of sample dissolution method for Sm-Nd isotopic analysis of scheelite

Abstract

Sm-Nd isotope systematics of scheelite is becoming a powerful geochronometer for dating the Au and W ore deposits. However, Sm-Nd isotopic analysis of scheelite is relatively difficult due to the formation of large amounts of tungstic acid (H₂WO₄) precipitates during the conventional acid decomposition of scheelite. In this study, both the low temperature attack and Teflon bomb acid dissolution methods for Sm-Nd isotope analysis of scheelite have been thoroughly investigated. It has been shown that, for both low temperature attack and Teflon bomb methods, although a large amount of tungstic acid precipitate remains during the 2.5 M HCl acidification process prior to cation exchange separation, only less than 10% of the Sm and Nd remains in the tungstic acid precipitate. On the other hand, it has been found that, for isotope dilution analysis, the spike-sample isotopic equilibration was only completely achieved for the Teflon bomb method and not completely achieved for the low temperature attack method, indicating that the low temperature attack method has possibly not liberated all the rare earth elements (REEs) from the scheelite crystal structure. Consequently, for high precision scheelite Sm-Nd dating purposes, the complex Teflon bomb rather than the simple low temperature attack method should be used in order to completely digest scheelite samples; alternatively, since it has been found that the tungstic acid precipitate formed during the acid dissolution process can be completely dissolved by self-purified low-blank 30% NaOH, a simple low temperature attack method combined with low-blank 30% NaOH dissolution of the remaining tungstic acid precipitate can also possibly be used to completely dissolve the scheelite sample for Sm-Nd isotopic analysis.