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Lithium represents a fast-diffusing element in a variety of minerals and melts. Its high diffusivity in plagioclase, for example, 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].

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 diffusion rate of Li in K-fsp and its dependence on the feldspar composition and the crystallographic orientation. The experiments were conducted in rapid-heat / rapid-quench CSPVs at temperatures between 540°C and 940°C and pressures between 50 and 200 MPa. In the run products, Li concentration and Li isotopic profiles (δ7Li) were analyzed using fs-LA-sector field-ICP-MS and fs-LA-MC-ICP-MS, respectively. Our results show that Li diffuses significantly faster in Or72- and Or80-crystals than in Or94-crystals: values of DLi for Or72 are almost 1.5 orders of magnitude higher than DLi for Or94 at a given temperature. The experimentally-produced diffusion-driven δ7Li zoning in our K-fsp crystals indicates that two diffusion mechanisms operate simultaneously, i.e. via interstitial and A sites. Hence, we are applying a multispecies diffusion model to determine “site-specific” diffusion coefficients (DLi) and isotope fractionation factors (βLi).

References:

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

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

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