Determination of platinum group elements using preconcentration by nickel sulfide fire assay , followed by tellurium coprecipitation

Platinum group elements (PGEs) and gold may provide direct information on deep mantle processes, because of their extremely high concentrations in the mantle relative to the crust. However, reports of PGE abundances in basaltic samples have been limited because of difficulties in PGE analysis and the complicated behavior of PGEs during basalt differentiation processes. The following analytical methods have been used for determining PGEs in geological rock samples. The NiS fire assay is widely used for collecting PGEs from rock samples. Neutron activation analysis (NAA) (e.g., Schmidt and Pernicka, 1994; Hoffman et al., 1978) or inductively coupled plasma mass spectrometry (ICP-MS) (e.g., Gros et al., 2002; Plessen and Erzinger, 1998; Yli et al., 1998) are used for analysis, because of their high sensitivity. Rock samples are fused with nickel, sulfur and alkaline fluxes. In the resulting melts, the metal-loving elements such as PGEs are collected into a NiS bead and can be readily separated from many cations which partition into a glass slag. Most nickel sulfide compounds are soluble in hydrochloric acid, while PGE sulfides are not. Therefore, PGEs can be simply separated from the nickel matrix by dissolving the NiS beads in HCl, and filtered off the matrix. To improve chemical recovery of the PGEs and Au, tellurium coprecipitation has often been carried out before the filteration of PGE sulfides (Shazali et al., 1987; Jackson et al., 1990). The fire assay-Te coprecipitation technique has the advantage that it can treat relatively large amounts of sample for analysis, leading to a reduction in heterogeneous sampling. In addition, the method is simple and achieves complete sample decomposition. However, contamination from the reagents used is significant, large amount of fusion charge are needed for this method. Since fire assay often suffers from blank problems, an acid digestion is sometimes preferred. To attain complete digestion of PGE-bearing refractory minerals and isotopic equilibrium between isotopes in the spike solution and those in the samples, acid digestion in a sealed, thick-walled Pyrex tube (Carius tube) at high temperatures (240oC) is used (Shirey and Walker, 1995). Rehkämper et al. (1998) used quartz lined Carius tubes and obtained procedural blanks of less than 20pg/g for all PGEs. Because isotope dilution analysis needs at least two isotopes free from isobaric interference, anion exchange separation is typically employed for separating the PGEs (Rehkämper et al., 1999; Pearson and Woodland, 2000). This technique combined with multicollector (MC)or quadropole (Q)-ICP-MS provides very precise analytical results, because chemical yields do not significantly affect the obtained results, and the isotopic ratio measurement is usually achieved with a high precision, even by Q-ICP-MS (RSD; ~0.5%). However, the ion-exchange separation and measurement are complicated and time-consuming because the sample solutions are obtained as separate fractions for each PGE. While the major trends for PGE analysis are described above, other methods, such as standard acid digestion followed by cation exchange separation (Jarvis et al., 1997; Ely et al., 1999) or isotope dilution analysis with Te coprecipitation (Enzweiler et al., 1995) have also been reported. In this study, we first tried the method of nickel sulfide fire assay followed by Te coprecipitation reported by Oguri et al. (1999) and examined whether this method is routinely applicable for the determination of trace PGEs in basalt samples, using two geological standard rocks (JP-1, peridotite, GSJ, split 10, position 72 and BHVO-2, Hawaiian basalt, USGS).