Experimental determination of Li diffusion rates and mechanisms in K-feldspar

The rapid diffusivity of Li in plagioclase has been used in several studies – employing diffusion chronometry – to quantify the timescales of short-lived magmatic processes, such as degassing, or decompression-induced crystal growth [e.g. 1,2]. However, experimentally determined diffusion rates of Li in K-feldspar (K-fsp) have not been published yet, hindering its use as a diffusion chronometer. Furthermore, previous studies indicate that Li in feldspars and other silicate minerals may show a complex diffusion behavior, with diffusion along interstitial sites as well as along metal sites, producing a characteristic isotope effect [e.g. 3,4].

Here, we performed a series of diffusion couple experiments using oriented K-fsp crystal cubes (Or₇₂, Or₈₀, Or₉₄) in contact with synthetic, Li-doped glass cubes of K-fsp composition (Or₆₀), in order to quantify the (chemical) diffusion rate of Li in K-fsp (D_Li) and its dependence on the feldspar composition and the crystallographic orientation. The experiments were conducted in rapid-heat / rapid-quench cold seal pressure vessels at temperatures between 540°C and 940°C and pressures between 50 MPa and 200 MPa. In the run products (crystal and glass cubes), Li concentration and Li isotopic profiles (δ⁷Li) were analyzed using femtosecond-laser ablation-sector field-ICP-MS and femtosecond-laser ablation-multicollector-ICP-MS, respectively.

Our results show that Li diffuses significantly faster in Or₇₂- and Or₈₀-crystals than in Or₉₄-crystals: values of D_Li for Or₇₂ are almost 1.5 orders of magnitude higher than D_Li for Or₉₄ at a given temperature. Diffusion rates parallel to the crystallographic b-axis vs. perpendicular to the b- and c-axes in Or₈₀- and Or₇₂-crystals are very similar, suggesting that the diffusion direction relative to the crystallographic orientation has little influence on D_Li. The experimentally-produced diffusion-driven δ⁷Li zoning in our K-fsp crystals implies that two diffusion mechanisms operate simultaneously, i.e. via interstitial sites and A sites. However, preliminary OH-concentration profiles measured by infrared microspectroscopy along the Li concentration profiles additionally indicate that Li-H inter-diffusion also influences the diffusivity of Li in K-fsp.        

 

References:

[1] Genareau & Clarke (2010): Am. Mineral., 95, 592–601.

[2] Neukampf et al. (2021): Geology, 49, 1–6.

[3] Dohmen et al. (2010): GCA, 74, 274-292.

[4] Pohl et al. (2024): Eur. J. Mineral., 36, 985–1003.