In situ Sr isotopic analysis of low Sr silicates using LA-ICP-MS

We have developed a LA-ICP-MS technique for the in situ determination of Sr-87/Sr-86 in silicates having low Sr concentrations (30-400 mu g g(-1)). Because of its high sensitivity we used a single collector sector-field ICP-MS, which was combined with a 193 nm Nd:YAG laser ablation system. Experiments were performed using the fast electrical scan mode (0.1 s per pass) of the mass spectrometer and a spot size of 50 mm. Various corrections were made for the determination of Sr-87/Sr-86, such as dead time of the ion counting system, blank, isobaric interferences of Kr and Rb, mass discrimination for Sr and Rb. An external precision (1 SD) of the Sr isotope measurements of about 0.0002-0.0004 were achieved for samples having Sr of 100-400 mu g g(-1) and Rb/Sr < 0.1. Lower Sr concentrations (30-100 mu g g(-1)) yielded higher SD values of up to about 0.0015. The LA-ICP-MS data for MPI-DING, USGS and NIST reference glasses agree within uncertainty limits with high-precision TIMS values. We have applied this technique for in situ isotopic analysis of 100-300 mm glassy melt inclusions from Hawaii and of small glass fragments of oceanic basalts.