Improved in situ Sr isotopic analysis by a 257 nm femtosecond laser in combination with the addition of nitrogen for geological minerals

In situ Sr isotope analysis of geological materials by laser ablation multiple collector-inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) is a powerful tracer technique for tracking magmatic source components and geological processes. However, the accuracy and precision of the Sr-87/Sr-86 ratio are limited in the analysis of natural minerals because of the low-Sr concentration, the isobaric interference or small grains with complex textural contexts, especially for transparent minerals such as feldspars. In this study, analytical results demonstrated that ablation rates in fs laser ablation were consistent for various samples (0.08-0.11 mu m per pulse), but those in ns laser ablation were obviously material properties-dependent, such as the rates of 0.026 mu m per pulse and 0.144 mu m per pulse for feldspar and pyrite, respectively. In addition, at similar energy fluences, the sensitivities of Sr in feldspars analyzed by the fs laser were 3.4 times higher than those analyzed by the ns laser due to the higher ablation efficiency of the fs pulse. These advantages of the fs laser not only offer the benefit of eliminating or weakening the matrix effect during the laser ablation processes but also help to improve the analytical precision for transparent minerals. We also demonstrated that the isobaric interferences of calcium dimers and argides (CaAr+ + CaCa+) and Kr+ were dramatically reduced by factors of 6.5-11.7 and 5-12.5 in the presence of 6-12 ml min(-1) N-2, respectively. Furthermore, with the addition of N-2 (12 ml min(-1)), the sensitivity of Rb was inhibited, resulting in a decrease of 1.47 times in Rb/Sr signal ratios. Due to the effect of suppressing interferences by adding N-2, both the stability and accuracy of the Sr-87/Sr-86 and Sr-84/Sr-86 ratios show improvement, especially for the Rb-rich feldspars. Combining the advantages of the fs laser system with the addition of nitrogen, an improved in situ Sr isotope analytical method is then developed. The satisfactory accuracy and precision of the Sr-87/Sr-86 ratio from natural plagioclases, a K-feldspar with high Rb/Sr ratios (0.46) and a low-Sr clinopyroxene were obtained, demonstrating the reliability of the proposed method. Four feldspars, which have different contents of the major elements, Sr and Rb, showed homogeneous Sr isotope compositions and were recommended as potential suitable reference materials for in situ Sr isotope analysis. As an application, two plagioclases in mafic microgranular enclaves (MMEs) with small grain sizes (200-300 mu m) and wide ranges of Rb/Sr ratios were analyzed and showed obvious variations of the Sr-87/Sr-86 ratios from core to rim, which indicated that the proposed method in this study can provide high spatial resolution geochemical information for a single mineral.