Lithium isotopic fractionation in olivine crystal in presence of iron at variable temperatures: A first-principles study

Abstract

Using a combination of first-principles density functional theory (DFT) and density functional perturbation theory (DFPT), we investigate the influence of Fe on the inter-site (i.e, between interstitial and substitutional sites) equilibrium isotopic fractionation of Li in olivine (M2SiO4; M: Mg, Fe). Our calculations indicate that among the substitutional sites, Li as well as Fe (Fe2+ and Fe3+) prefer to get incorporated in the smaller and more regular M1-octahedral site as opposed to the larger and more distorted M2-octahedral site in the olivine crystal structure. Calculation of the isotopic fractionation of Li between the interstitial and substitutional sites of olivine indicates that there is a Fe-concentration dependent enrichment of the interstitial site by the heavier Li isotope relative to the substitutional site. It is found that with the incorporation of 3.125 % Fe in the octahedral site of forsterite (Mg2SiO4), there is an enhancement of the ‘isotopic fractionation factor’ (103ln(αinterstitial-substitutional)) of Li by 41.751 ‰ at 300 K and 2.812 ‰ at 1200 K, which increases with further rise in Fe concentration. The correlation between the average Li-O bond length and the ‘reduced partition function ratios’ (103ln (β7-6)) of 7Li/6Li is found to break down in the presence of local charge imbalance. Our calculations offer further insights into the intricate nature of Li isotopic fractionation in natural olivine, which has long been used as a geochemical tracer to infer geological processes in the Earth.