Rapid quantitative trace element mapping of microbial carbonates by LA-ICP-TOF-MS: Context, results and perspectives

Trace element compositions of carbonate microbialites are valuable proxies to reconstruct shallow water microbial environments through Earth's history. Most of the published trace element data of microbial carbonates are obtained from bulk digestion or carbonate leaching from sample powders or via in situ laser ablation analysis calibrated to external reference materials such as NIST glasses. However, due to the complex formation mechanism of microbial carbonates, huge compositional differences are associated with spatial and lithological sample heterogeneities that cannot be fully resolved with the current analytical methodologies.

Here, we describe a new method using a high-frequency laser ablation (LA) system (NWR ImageGEO193) coupled to inductively coupled plasma time-of-flight mass spectrometry (ICP-TOF-MS) and external calibration via matrix-matched nanopowder carbonate reference materials to obtain rapid high-resolution quantitative trace element maps.

Our mapping results reveal that detrital elements such as thorium (Th), aluminium (Al), bioactive elements such as cadmium (Cd) iron (Fe) and nickel (Ni), and rare earth elements (REE) have concentration distributions that correspond with the intrinsic bio-sedimentary layering of the samples. Fully quantitative trace element maps are the foundation for follow-up research such as in situ U-Pb dating on truly authigenic carbonate phases or stable novel metal isotope analyses on individual lithified microbial layers to study bio-essential metal uptake in diverse microbial communities through deep times.

Therefore, quantitative trace element mapping via LA-ICP-TOF-MS is a promising method that can rapidly obtain the spatial geochemical characteristics of microbial carbonates.