Depth profile sampling by femtosecond-LA-(MC-)ICP-MS for high spatial resolution analyses of chemical and isotopic diffusion profiles

Femtosecond laser ablation coupled to MC-ICP-MS has been proven to be a powerful means to analyze isotope ratios of “non-traditional” stable isotope systems with high spatial resolution, precision and accuracy. The technique has been successfully applied e.g. to investigate diffusion-generated Li- and Fe-Mg isotopic zoning in magmatic crystals. Here, we present a novel sampling technique employing a fs-LA system that is equipped with a CNC-controlled laser stage. This set up allows to perform depth profile analyses of major and trace elements as well as metal stable isotope variations (Li-Mg-Fe) and is particularly suitable for experimental samples with a well-defined crystal interface. Samples are ablated in circular patterns with diameters of 100-200 µm and the integrated signal of one layer represents one data point of the depth profile. Depth resolution is between 300 nm and 2.5 µm and depends on the scan speed and the repetition rate of the laser. We have tested this technique by analyzing chemical diffusion profiles in olivine crystal cubes which are compared to “horizontal” profile data acquired by electron microprobe analyses. Furthermore, Fe-Mg isotopic depth profiles were analyzed in a diffusion couple consisting of a 25Mg- and 57Fe-doped olivine thin film in contact with a natural olivine crystal cube. Our results indicate (i) that concentration data acquired by LA depth profiling match well with electron microprobe data, and (ii) that precise and accurate δ25Mg, δ26Mg, δ56Fe and δ57Fe data can be obtained.