The antiquity of lanthanide tetrad effect and super-chondritic Y/Ho ratio in seawater

Pure and pristine Archaean and Palaeoproterozoic banded iron formations (BIFs) are excellent marine geochemical archives, especially of original rare earths and yttrium (REY) characteristics. As recently demonstrated for the Mt.Ruker BIF in Antarctica[1], ultrapure BIFs preserve seawater-like REY distribution with heavy REYSN enrichment and light REYSN depletion (SN: shale-normalized). Our results indicate that a prominent feature of the REY signature of seawater throughout Earth’s history has been a super-chondritic Y/Ho ratio which falls between that of potential detritus and the maximum Y/Ho ratio of modern seawater. Another more subtle and fragile proxy of modern seawater is the W-type lanthanide tetrad effect (LTE) which results from the slightly differing bonding characteristics of individual REY in chemical complexes, due to the specific electron configuration of the REY3+ ions. This W-type LTE is found in pure marine carbonate rocks and in BIFs, and confirms the primary and marine origin of their REY distribution. The preservation of uniform, super-chondritic Y/Ho ratios in combination with the W-type LTE in chert and Fe-oxide BIF bands is an excellent tool to test the pristineness of geochemical signals in such samples as well as evidence for the primary origin of banding in BIFs.

[1] Ernst D. M. and Bau M. (2021) Banded iron formation from Antarctica: The 2.5 Ga old Mt. Ruker BIF and the antiquity of lanthanide tetrad effect and super-chondritic Y/Ho ratio in seawater. Gondwana Research 91, 97–111.