Lithium and boron diffusivity and isotopic fractionation in hydrated rhyolitic melts

Abstract

Lithium and boron are trace components of magmas, released during exsolution of a gas phase during volcanic activity. In this study, we determine the diffusivity and isotopic fractionation of Li and B in hydrous silicate melts. Two glasses were synthesized with the same rhyolitic composition (4.2 wt% water), having different Li and B contents; these were studied in diffusion-couple experiments that were performed using an internally heated pressure vessel, operated at 300 MPa in the temperature range 700–1250 °C for durations from 0 s to 24 h. From this we determined activation energies for Li and B diffusion of 57 ± 4 kJ/mol and 152 ± 15 kJ/mol with pre-exponential factors of 1.53 × 10–7 m2/s and 3.80 × 10–8 m2/s, respectively. Lithium isotopic fractionation during diffusion gave β values between 0.15 and 0.20, whereas B showed no clear isotopic fractionation. Our Li diffusivities and isotopic fractionation results differ somewhat from earlier published values, but overall confirm that Li diffusivity increases with water content. Our results on B diffusion show that similarly to Li, B mobility increases in the presence of water. By applying the Eyring relation, we confirm that B diffusivity is limited by viscous flow in silicate melts. Our results on Li and B diffusion present a new tool for understanding degassing-related processes, offering a potential geospeedometer to measure volcanic ascent rates.

Chemical interdiffusion between Na-tephritic and phonolitic melts at different T, H2O and fO2

“…Activation energies and ln D0 for both H2O concentrations plot to a single compensation law linear fit (Hart, 1981; Supplementary Figure 4). This is in line with previously published data, where H2O contents are known to reduce activation energy of diffusion (Watson, 1981(Watson, , 1994Behrens and Zhang, 2001;Spallanzani et al, 2022). Only Al shows an anomalous behaviour, where the fitting errors are larger for the 1.66 wt.% fit, and the data at 1150°C and 2.99 wt.% H2O does not allow to obtain a good Arrhenian fit.…”

“…Dissolved H2O in melt has already been demonstrated to strongly enhance diffusivities in silicate melts, both for major elements (e.g. Baker and Bossányi, 1994;González-García et al, 2017 and trace elements (Watson, 1981;Baker and Bossányi, 1994;Behrens and Zhang, 2001;Zhang et al, 2010;González-Garcia et al, 2018;Spallanzani et al, 2022). Our experimental dataset confirms this observation, and for the TP55 composition, major element diffusion is enhanced by 1.2-1.5 log units (corresponding to a factor of 20-30) when H2O contents increase from 0.3 to 3.3 wt.% at 1250 °C.…”

Li–Na interdiffusion and diffusion-driven lithium isotope fractionation in pegmatitic melts

“…Li-Na interdiffusion coefficients in the studied melt composition are in a similar range as Li and Na tracer diffusivities in other dry aluminosilicate melts, confirming little to no effect of aluminosilicate melt composition on Li diffusivity. Thus, added fluxes do not enhance the Li diffusivity in the same way as observed for H 2 O (Holycross et al, 2018;Spallanzani et al, 2022). Using melt viscosity as a proxy for the polymerization of the melt shows that water has a stronger potential to depolymerize a melt compared to other fluxing elements.…”

Lithium in felsic magmas: a volcanological perspective

“…Li diffusion coefficients (D Li ) are commonly determined experimentally by re-equilibrating Libearing glasses or crystals at a given temperature. In rhyolitic melts, D Li rhy increases from ~10 −9.5 to 10 −8.5 m 2 /s from 700 °C to 1,200 °C in hydrated rhyolitic melts (Holycross et al, 2018;Spallanzani et al, 2022), and increases from about 0.5 log unit from dry to hydrated rhyolites (Jambon and Semet, 1978;Holycross et al, 2018;Spallanzani et al, 2022) (Figure 3).…”