Laser-ablation ICP-MS analysis of siliceous rock glasses fused on an iridium strip heater using MgO dilution

Trace element determination in rocks by fusion on an iridium strip heater followed by LA-ICP-MS analysis of the glass beads is extended here to SiO ₂-rich rocks; rapid fusion of samples with >55wt% SiO₂ is facilitated by dilution by high purity MgO. The method developed here can rapidly and accurately determine numerous trace elements in a large range of rock compositions in a short time (about 50 samples/day). Systematic evaluation for a large range of rock compositions (natural rocks and reference materials AGV-2, GSP-2, JG-1a) with SiO₂ contents between 45 and 80wt% shows that reproducibility and accuracy within 10% can be routinely achieved for most of the 28 trace elements investigated (Rb, Sr, Cs, Ba, Ti, Zr, Hf, Nb, Ta, Sc, V, Cr, Ni, Pb, Th, U, REE). The 40mg sample size is smaller than for XRF, INAA or solution-ICP-MS, detection limits are lower, and trace element palettes more complete than XRF and INAA. This microchemical method is thus attractive for the analysis of all natural geological materials as well as for experimental applications with small samples. Samples with SiO₂-contents >55wt% require hot and long melting to achieve homogeneous glasses and eliminate all residual minerals, particularly refractory accessory phases. Melting conditions of 1600°C and 20s for samples are recommended for SiO₂ contents between 55 and 70wt%, whereas 1800°C and 20-30s are often required for samples with >70wt% SiO₂. Problems are encountered for Pb and Cs due to volatilization on the Ir strip, for Sc due to interferences, and Zr and Hf due to their sequestration in refractory accessory minerals.