Lithium behaviour during partial melting

Lithium has been receiving increased attention due to its relevance to the green energy transition and increasing demand. One of the main sources of Li is peraluminous granitoids and pegmatites. Current models for the genesis of such enriched lithologies include the fractional crystallisation of melts derived from sedimentary protoliths (e.g., Linnen et al., 2012); the low-volume melting of Li-rich protoliths (e.g., Shaw et al., 2016); and the subsequent re-melting of S-type orthogneisses (Ballouard et al., 2024; Koopmans et al., 2024). Hence, understanding the behaviour of Li during partial melting is critical, regardless of the preferred model. However, modelling studies have shown contrasting trends in melt Li concentrations during partial melting reactions due to the use of different distribution coefficient sets (Ballouard et al., 2024; Koopmans et al., 2024).  

 

In this study, we coupled existing and newly acquired mineral Li concentration data from high-grade terranes, and phase equilibria and trace element modelling to improve our understanding of Li behaviour during partial melting. Our main results highlight that: (1) when present cordierite is the main mineral host of Li in metapelitic migmatites; and (2) the experimentally observed temperature-dependent variation of cordierite Li concentrations (Evensen and London, 2003) is also observed in natural samples, but – as expected – with different concentrations levels. Moreover, our results show that the mineral distribution coefficients of natural samples do not match experimentally derived Li distribution coefficients for cordierite and biotite. We discuss possible reasons for divergent distribution coefficients based on the comparison of physicochemical parameters associated with these distribution coefficients and typical high-grade metapelites. Our findings also indicate that low-pressure melting with peritectic cordierite will suppress melt Li concentrations, in agreement with Ballouard et al. (2024).

 

This study highlights how metamorphic minerals and melting conditions affect Li behaviour during partial melting. Moreover, our findings and discussion highlight that further experimental and natural constraints on Li distribution coefficients are necessary for a precise understanding of how Li is mobilised during metapelite melting.

 

References:

Ballouard et al. (2024) Contrib to Min and Petr 178(11), 75

Evensen and London (2003) Contrib to Min and Petr 144, 739-757

Koopmans et al. (2024) Geology 52(1), 7-11

Linnen et al (2012) Elements 8(4), 275-280

Shaw et al. (2016) Precamb Res 281, 338-362

 

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement no. 956125.