Li and K isotope fractionation controlled by minerals during magmatic differentiation of Changbaishan, NE China

This study aims to elucidate the behavior of Li and K isotopes during the differentiation of continental basaltic magmas. We conducted comprehensive petrographic, whole-rock and mineral geochemical, and Li-K isotopic analyses on basalts and trachytes and their phenocrysts from the Changbaishan volcano in NE China. The geochemical characteristics of these rocks indicate that magma evolution was predominantly driven by fractional crystallization, involving the removal of an assemblage of olivine, plagioclase, and/or sanidine. The δ⁷Li values of the basalts range from +1.7 to +3.7 ‰, which are significantly lower than those of the trachytes (+3.2 to +9.4 ‰). The δ⁷Li values of olivine, plagioclase, and sanidine show notable variations, ranging from −3.7 to +0.8 ‰, −2.3 to +5.6 ‰, and + 0.4 to +7.1 ‰, respectively. Correlations of δ⁷Li with Eu/Eu^⁎, along with Rayleigh distillation modeling, suggest that fractional crystallization of plagioclase has led to significant Li isotope fractionation. The δ⁴¹K values across the samples range from −0.62 to −0.36 ‰, with basalts (δ⁴¹K = −0.58 to −0.36 ‰) exhibiting significantly heavier K isotope compositions than trachytes (δ⁴¹K = −0.62 to −0.46 ‰). Furthermore, correlations of δ⁴¹K with Na₂O + K₂O, K₂O, and Th, combined with Rayleigh fractionation modeling results, indicate that fractional crystallization of K-rich and isotopically heavy K sanidine (δ⁴¹K = −0.46 to +0.35 ‰) resulted in significant K isotope fractionation within the magma. These findings demonstrate the potential of Li and K isotopes as effective tracers for magma differentiation.