Fully quantitative trace element mapping of microbial carbonates by LA-ICP-TOF-MS: Background, 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 of sample powders retrieved from microdrilling 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 Image GEO193) coupled to inductively coupled plasma time-of-flight mass spectrometry (ICP-TOF-MS) and internal calibration via matrix-matching nanopowder carbonate reference materials to obtain rapid high-resolution quantitative trace element maps.

Our mapping results reveal that detrital elements such as, aluminium (Al), thorium (Th), bioactive elements such as cadmium (Cd) iron (Fe) barium (Ba) and nickel (Ni), and particle-reactive elements such as rare earth elements (REE) have concentrations distributions that correspond with the intrinsic bio-sedimentary layering in microbialite samples.

Here we present fully quantitative LA-ICP-TOF-MS trace element maps of a variety of microbialites from the Phanerozoic to the Archean. Our promising new method for rapidly obtaining spatial geochemical characteristics of microbialites that build the groundwork for follow-up research such as in situ U-Pb dating on truly authigenic carbonate phases or stable novel metal isotope analyses on individual layers to study bio-essential metal uptake in diverse microbial communities through deep time.