High-precision rare earth element, nickel, and chromium chemistry of chert microbands pre-screened with in-situ analysis

The interbedded chert and iron minerals of banded iron formation (BIF) provide an important window into the chemistry of the Precambrian ocean. Chert microbands have better preservation potential than the iron-rich bands but pose analytical challenges. The generally much lower concentration of trace elements in chert require better detection limits and make chert more susceptible to the influence of foreign materials such as ash or detritus. Without screening for chemical complexity, bulk chemical analyses may have limited significance. Here we report a new analytical protocol consisting of two steps. First it uses exploratory laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) in situ analysis for identifying suitable chert areas to be sub-sampled for solution ICP-MS analysis. Second, the subsampled chert is digested and dried down, volatilizing the bulk of the matrix as SiF4. The solution can then be admitted to the ICP-MS at much lower dilution factors than common rock samples, resulting in detection limits for many of the rare earth elements at double or single digit ppt levels. This approach produces high quality trace element data even for cherts with exceedingly low trace element contents. Significantly, such data approach primary chemical composition of the BIF better than conventional bulk analysis. It is also demonstrated, through selective digestions, that the most common types of mineralogical heterogeneity in cherts – carbonate and iron-oxide minerals – have minimal effect on the REE + Y composition, at least in studied cherts from the Abitibi Greenstone Belt. The proposed combination of techniques will allow future studies of more refined trace element chert geochemistry, providing a better understanding of Precambrian oceans and BIF deposition.

Research highlights
► We use LA-ICP-MS analysis to show REE + Y heterogeneity in cherts.
► Some of this heterogeneity is not petrographically visible.
► Heterogeneous cherts may deviate from seawater Ni–Cr behavior.
► This method of screening cherts should be applied in deep time studies of BIF.