Earth’s K recycling: Perspective from K isotopes of intracontinental magmas

Subduction continuously transports crustal potassium (K) into the mantle, yet the fate of this recycled K remains poorly constrained. K isotope compositions of mantle-derived magmas offer insights into the mantle’s K budget. Magmas formed during initial subduction generally exhibit higher δ⁴¹K values than typical arc lavas, with both being isotopically heavier than mid-ocean-ridge basalts. This suggests a preferential release of isotopically heavy K from subducting materials, implying the presence of isotopically light slab residues. Consequently, a low-δ⁴¹K signature was expected for orogenic and intracontinental magmatism if these sources were incorporating recycled materials. However, analyses of volcanic samples from intracontinental settings and lavas from orogenic belts reveal that they do not exhibit lighter K isotope compositions. Instead, their δ⁴¹K values overlap those of arc lavas and mid-ocean-ridge basalts. To explain this discrepancy, we propose two scenarios. First, the light K isotope signature may not have been transferred into melts during partial melting of the mantle that was metasomatized by dehydrated slab melts, as indicated by low-δ⁴¹K signatures in mantle peridotites. Second, while initial melts may have been enriched in light K isotopes, fractional crystallization of K-rich minerals during K-rich magma ascent could have preferentially incorporated these light isotopes, resulting in an enrichment of heavy K isotopes in the residual melts. This mechanism is supported by the presence of phenocrysts with low δ⁴¹K values in basaltic lavas. These findings highlight the significant role of K-rich minerals in controlling recycled K and its isotopes during the recycling processes.