Potassium isotope fractionation during silicate-carbonatite melt immiscibility and phlogopite fractional crystallization

Potassium (K) isotopes have been used as a tracer of K recycling in the Earth, but K isotope fractionation during magma evolution is poorly constrained. Here, we present K isotope data for a magmatic suite of alkaline silicate-carbonatite affinity. The suite was formed from liquid-liquid immiscibility and subsequent phlogopite fractionation. The K isotopic signatures of different rock types are in the following order: alkaline silicate lavas (delta K-41 = -0.424 to 0.090 parts per thousand) > carbonatitic silicate lavas (delta K-41 = -0.640 to -0.035 parts per thousand) > carbonatites (delta 41K = -0.858 and -0.258 parts per thousand). Phlogopite phenocrysts in the silicate lavas are isotopically lighter (delta K-41 = -0.628 to -0.534 parts per thousand) than the lavas in which they occur (Delta K-41(Phlogopite-whole rock) = -0.502 to -0.109 parts per thousand). Correlations between delta K-41 values and chemical proxies of melt immiscibility and phlogopite fractionation indicate that K isotopes are significantly fractionated by both processes at a similar to 0.6 parts per thousand magnitude. Such K isotope variation overlaps the range of delta K-41 in arc lavas. Compilations of literature data further confirm the critical roles of melt immiscibility and phlogopite fractionation in K isotope variations of high-K lavas (K2O >1 wt%) from post-collision orogenic and intra-continental settings. In comparison, basaltic arc lavas are depleted in K2O (mostly <1 wt%) and lack evidence of significant phlogopite fractionation. The K isotope variations of arc lavas are mainly controlled by their mantle sources, which were metasomatized by melt or fluid released from the subducting slab. Therefore, K recycling and K isotope variation are controlled by distinct mechanisms in different tectonic settings.