Depth profile analyses by femtosecond laser ablation (multicollector) inductively coupled plasma mass spectrometry for resolving chemical and isotopic gradients in minerals

Abstract

Abstract. Femtosecond laser ablation (fs-LA) coupled to a multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) instrument 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 isotopic zoning of the elements Li, Mg, and Fe in magmatic crystals. Here, we present a novel sampling technique employing a fs-LA system that is equipped with a computer numerical control (CNC) laser stage, using the open-source software LinuxCNC. Combining this laser set up with ICP-MS or MC-ICP-MS allows us to perform depth profile analyses of major and trace elements, respectively, as well as metal stable isotope variations of Fe and Mg in olivine crystals and in experimental diffusion couples. Samples are ablated in circular patterns with profile diameters of 100–200 µm using a laser spot size of 25–30 µm. Depending on the scan speed and the repetition rate of the laser, each ablated sample layer is between 300 nm and 3.0 µm thick. The integrated signal of one ablated layer represents one data point of the depth profile. We have tested this technique by analyzing 5–50 µm deep depth profiles (consisting of 15–25 individual layers) of homogeneous and chemically zoned olivine crystal cuboids. The minor and trace element analyses of the zoned cuboids, conducted by fs-LA-ICP-MS, were compared with “horizontal” profiles analyzed in polished sections of the cuboids with electron probe microanalysis (EPMA). Furthermore, we analyzed Fe–Mg isotopic depth profiles of the same cuboids with fs-LA-MC-ICP-MS, of which the chemically zoned ones also showed isotopic zoning at identical scales. Isotopic depth profiles were also conducted on an unzoned olivine cuboid that was coated with a 26Mg- and 56Fe-enriched olivine thin film (of ∼ 800 nm) in order to investigate top-to-bottom contamination during depth profiling. Our results indicate that (i) concentration data acquired by fs-LA depth profiling match well with EPMA data, (ii) precise and accurate Fe and Mg isotopic data can be obtained (i.e., precision and accuracy are ≤ 0.12 ‰ and ≤ 0.15 ‰ for both δ26Mg and δ56Fe, respectively), and (iii) potential top-to-bottom contamination during depth profiling of isotope ratios can be avoided. The technique presented herein is particularly suitable for the investigation of minerals or glasses with chemical and/or isotopic gradients (e.g., diffusion zoning) vertical to planar surfaces. It can also be applied in materials sciences in order to analyze thin films, coatings, or surface contaminations on solids.