2D geochemical imaging of biogenic marine carbonates using LA-TOF-ICP-MS at 1 and 2 μm pixel resolution

Abstract

Many applications at the forefront of the study of the chemical composition of marine carbonates require in-situ micro-scale geochemical imaging. Such analyses are, however, challenging, requiring analytical techniques that are either expensive with limited accessibility (e.g. synchrotron X-Ray spectroscopy and secondary ion mass spectrometry), or time-consuming and able to only analyse a limited range of elements (e.g. electron microprobe). Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) as a tool for generating 2D images has grown in popularity, yet many analytical issues remain when generating high-spatial resolution geochemical images using this approach. Here we employ the imageGEO193 (ESL) fast wash-out laser ablation system coupled to the Nu Instruments Vitesse Time-of-Flight (TOF) ICP mass spectrometer, with its near-full mass spectra capabilities, to generate 2D geochemical images of a range of biogenic carbonates at ≤2 μm pixel resolution (pixel widths of either 1 or 2 μm) and at an unprecedented speed (200 pixels/s). We demonstrate sensitivity of ∼100 cps/μg g ⁻¹ at low mass rising to ∼1000 cps/μg g ⁻¹ at high mass based on analyses of reference materials JCp-1 (carbonate) and NIST SRM612 (silicate) with 1 μm wide square laser beams, and accuracy of ±7 % for elements present at concentrations >0.5 μg g ⁻¹ based on analyses of carbonate reference material JCt-1. By applying our quantitative method to a range of biogenic carbonates (coral skeletons, coralline algae, foraminifera), we demonstrate that considerable but coherent micron-scale compositional variability is the norm for nearly all quantified elements, including: Mg, Sr, Ba and U. This approach therefore has great potential to provide valuable insights into biomineralisation mechanisms and “vital effects”, ultimately facilitating more robust reconstructions of past environments.

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