Banded Iron Formations (BIF) are chemical sediments that can be used to reconstruct ancient seawater chemistry over a huge timespan of Earth’s history (3.8-1.5 Ga). Fluctuations in the abundances and isotopic compositions of certain metals in BIF have been widely used to assess changes in the ancient environment through time [1,2]. Specifically, the lanthanides are commonly used for geochronology or to assess redox conditions and mixing of isotope reservoirs. To improve our understanding of the mechanisms that fractionate isotopes in oceans during BIF precipitation we used ab initio methods to calculate reduced partition functions (as 1000 lnβ) for aqueous species of La(III), Nd(III), Ce(III) and Ce(IV). The results are combined with aqueous speciation modelling of a simplified Precambrian seawater that contains the most relevant anions for complexation. Lanthanides show a high affinity for the formation of carbonato complexes in aqueous solution which is highly pH dependent leaving a smaller (< 1%) and lighter (-0.2 ‰) proportion of free lanthanides for adsorption onto sediment particles with evolution of the ocean to circumneutral conditions (pH~8). Isotope fractionation between aqueous species shows a typical proportionality to 1/T2 (ref. 3) which implies that incorporation of lanthanides into BIFs caused a smaller fractionation in the hotter ancient ocean (up to 60ºC; ref. 4). Therefore, the isotopic composition of lanthanides in BIFs represents a value closer to the seawater than in recent marine sediments. Isotope fractionation between Ce(III) and Ce(IV) species shows Δ1000 lnβ (~Δ142Ce) values of up to 0.42 indicating a significant fractionation of Ce due to oxidation that is potentially preserved in Precambrian marine sediments. In addition, nuclear field shift effects were calculated for La(III) and Ce(IV). Our results show that mass-independent isotope fractionation effects are insignificant between aqueous species.

Reference(s)

[1] Pons (2013) Geobiology, 11(3), 201-214; 

[2] Frei (2009) Nature, 461(7261), 250-253; 

[3] O’Neil (1986) Rev. in Min. 16, 1-40; 

[4] Feulner (2012) Rev. of Geophy., 50(2).