LA-ICP-MS imaging in the geosciences and its applications to geochronology

Abstract

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) mapping is a rapidly expanding field in the geosciences because of the wealth of spatial information it provides on the petrogenesis of igneous, metamorphic and sedimentary rocks and ore mineral formation. The technique has also opened many new applications in forensics, archaeology and the imaging of trace metals in biological tissues. Instrumentation advances in both LA and ICP-MS systems now permit precise isotopic analysis with laser spot sizes of <10 μm and sub-ppm detections limits. In particular LA-quadrupole (Q)-ICP-MS systems facilitate mapping of large numbers of elements across nearly the entire mass range of the periodic table. A range of key applications in petrogenesis studies and in the fields of environmental and biological sciences is reviewed. Furthermore, technical considerations including a practical guide for setting up LA-ICP-MS multi-element mapping experiments and the latest innovations in dedicated data reduction and image processing packages for the visualization, interrogation and extraction of quantitative data from LA-ICP-MS maps are discussed, with particular emphasis placed on quadrupole systems. A key issue is imaging artefacts (spectral skew) caused by interaction between the laser repetition rate and the total sweep cycle time, particularly when coupling a sequential Q-ICP-MS analyser to modern low dispersion (fast-washout) LA cells. Running at high repetition rates is recommended for low-dispersion cells as it enables faster scanning while minimizing temporal variations in signal intensity caused by pulsing of the laser. Advances in LA-ICP-MS instrumentation and methodologies (data processing, visualization and extraction) have enabled simultaneous acquisition of compositional and U-Pb age information at high-spatial resolution which has great potential in geochronological studies. Formation histories of complex polyphase accessory minerals such as zircon, titanite or monazite, where discrete age domains are often linked with specific rock-forming processes and their physical conditions (i.e. U-Pb petrochronology), are now directly accessible. Moreover, fine-scale processes affecting the U-Pb systematics of accessory minerals can be constrained when LA-ICP-MS mapping is combined with other textural (e.g. CL or Raman) imaging techniques. U-Pb analysis of materials with variable initial Pb concentrations and/or heterogeneous genetic origin such as carbonates also benefit from such a mapping approach as areas with sufficient spread in 238U/204Pb ratio (μ) can be targeted, while simultaneous imaging of diagnostic trace elements allows identification and exclusion of zones affected by alteration or detrital contamination. The quasi-simultaneous detection offered by LA-time-of-flight (TOF)-ICP-MS allows rapid multi-element analysis of very fast transient signals (i.e. laser ablation imaging in low dispersion laser ablation cells). The approach is ideally suited to 2D and 3D imaging of biological and geological materials and will likely replace LA-Q-ICP-MS for such applications once more routinely available.