Experimental constraints on Li isotope fractionation during clay formation

Abstract

Knowledge of the lithium (Li) isotope fractionation factor during clay mineral formation is a key parameter for Earth system models. This study refines our understanding of isotope fractionation during clay formation with essential implications for the interpretation of field data and the global geochemical cycle of Li. We synthesised Mg-rich layer silicates (stevensite and saponite) at temperatures relevant for Earth surface processes. The resultant solids were characterised by X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FT-IR) to confirm the mineralogy and crystallinity of the product. Bulk solid samples were treated with ammonium chloride to remove exchangeable Li in order to distinguish the Li isotopic fractionation between these sites and structural (octahedral) sites. Bulk solids, residual solids and exchangeable solutions were all enriched in 6 Li compared to the initial solution. On average, the exchangeable solutions had δ 7 Li values 7 lower than the initial solution. The average difference between the residue and initial solution δ 7 Li values (∆ 7 Li residue−solution) for the synthesised layer silicates was-16.6±1.7 at 20 • C, in agreement with modelling studies, extrapolations from high temperature experimental data and field observations. Three bonding environments were identified from 7 Li-NMR spectra which were present in both bulk and residual solid 7 Li-NMR spectra, implying that some exchangeable Li remains after treatment with ammonium chloride. The 7 Li-NMR peaks were assigned to octahedral, outer-sphere (interlayer and adsorbed) and pseudo-hexagonal (ditrigonal cavity) Li. By combining the 7 Li-NMR data with mass balance constraints we could calculate a fractionation factor, based on a Monte Carlo minimum misfit method, for each bonding environment. The calculated values are-21.5±1.1 ,-0.2±1.9 and 15.0±12.3 for the octahedral, outer-sphere and pseudo-hexagonal sites respectively (errors 1σ). The bulk fractionation factor (∆ 7 Li bulk−solution) is dependent on the chemistry of the initial solution. The higher the Na concentration in the initial solution the lower the bulk δ 7 Li value. We suggest this is due to Na outcompeting Li for interlayer sites and as interlayer Li has a high δ 7 Li value relative to octahedral Li, increased Na serves to lower the bulk δ 7 Li value. Three experiments conducted at higher pH exhibited lower δ 7 Li values in the residual solid. This could either be a kinetic effect, resulting from the higher reaction rate at high pH, or an equilibrium effect resulting from reduced Li incorporation in the residual solid and/or a change in Li speciation in solution. This study highlights the power of 7 Li-NMR in experimental studies of clay synthesis to target site specific Li isotope fractionation factors which can then be used to provide much needed constraints on field processes.

The Response of Magnesium, Silicon, and Calcium Isotopes to Rapidly Uplifting and Weathering Terrains: South Island, New Zealand

“…This co-relationship between the two systems can be modeled using a Rayleigh relationship (Vigier et al, 2009;Pogge von Strandmann et al, 2012, 2014b, 2017Murphy et al, 2019), and by assuming a continental crust starting composition (Savage et al, 2010;Sauzéat et al, 2015). Using a kaolinite fractionation factor of α = 0.979 for Li isotopes (Pistiner and Henderson, 2003;Hindshaw et al, 2019), and of 0.9989 for Si isotopes (close to values for allophane, which is compositionally similar to kaolinite; Ziegler et al, 2005;Delstanche et al, 2009), we can model the co-isotopic behavior for the western samples with between 20 and 60% of Li and Si removal (Figure 4E). This implies that both Li and Si in these samples are only being affected by primary silicate dissolution, and the formation of single type of clay mineral (or a combination of minerals with the same fractionation factors).…”

Experimental determination of Li isotope behaviour during basalt weathering

“…This range is due to the formation of secondary minerals (clays, zeolites and Fe-Mn-Al oxyhydroxides) during weathering, where these minerals preferentially take up the light isotope, 6 Li, driving residual river and soil waters isotopically heavy (Vigier et al, 2008;Wimpenny et al, 2010). These secondary minerals also adsorb Li onto their exchangeable sites, imposing the same isotopic fractionation direction (Hindshaw et al, 2019;Pistiner and Henderson, 2003). Hence, Li isotopes in natural waters are controlled by the ratio of primary mineral dissolution (low d 7 Li) relative to secondary mineral formation (driving solution d 7 Li high).…”

“…However, while the general mechanics of Li isotope fractionation during weathering are fairly well understood, there is less information available on the precise processes that d 7 Li traces, such as whether the primary fractionation mechanism is adsorption onto secondary mineral surfaces, uptake into interlayer sites, or via structural or lattice-bound incorporation into neoformed minerals (Hindshaw et al, 2019;Pistiner and Henderson, 2003;Pogge von Strandmann et al, 2010;Wimpenny et al, 2015). Further, fractionation factors and reaction kinetics are also poorly known.…”

Diffusive processes in aqueous glass dissolution

“…17 For Li, longer bond lengths, less stiff bonds and lower bond energies are associated with higher coordination number sites. 10,13 As such, when there is precipitation, 6 Li is preferentially partitioned into the higher CN octahedral and pseudo-hexagonal sites (CN of six) of clay secondary phases, 7 Li preferentially remains in solution (CN of four) 10,18,19 and the solution becomes isotopically heavier with time. [20][21][22][23][24] Further, Li can be adsorbed into the interlayer sites of smectites or onto the adsorption sites of gibbsite and ferrihydrite, although the former is not expected to be associated with any isotopic fractionation.…”

“…[20][21][22][23][24] Further, Li can be adsorbed into the interlayer sites of smectites or onto the adsorption sites of gibbsite and ferrihydrite, although the former is not expected to be associated with any isotopic fractionation. 10,14,18 Here, isotope fingerprinting techniques 25 have been applied to glass leachates to probe whether the dissolution of a simplified simulant nuclear waste glass was rate-limited by diffusion. A seven-oxide borosilicate simplified glass analogue, 6 Li-Mg-EM, was synthesised based on an existing Li-free analogue of a complex simulant Magnox (Mg-Al alloy fuel cladding) waste glass of 25 wt% waste loading.…”