Hierarchy of processes for lithium mobility and enrichment in anatectic crust

Rare-metal granites and pegmatites are commonly interpreted to form either by extreme fractionation of granitic magmas or by low-degree crustal melting. Despite renewed interest in the anatectic model, the mechanisms governing Li release into melts and whether crustal melting alone can concentrate Li to ore-grade levels remain debated. Here, we present the first comprehensive database of Li concentrations in anatectic melt inclusions, providing direct constraints on the Li budget of primary melts formed under typical mid- to lower-crustal pressure-temperature (P-T) conditions and compositions. The dataset shows that Li release into anatectic melts is optimized during early, fluid-absent equilibrium melting of biotite at 750−800 °C in cordierite-free rocks, reaching up to ∼600 μg/g Li. Although these concentrations exceed those of S-type granites, they overlap the range of barren pegmatites, indicating that extreme fractionation is required to produce hard-rock Li ores. Integration of natural constraints with petrological modeling suggests that direct generation of ore-forming Li enrichments (3000−20,000 μg/g Li) requires strongly pre-enriched sources (600−2000 μg/g Li, i.e., >20 times ordinary crustal abundances). Given the lack of compelling evidence that such sedimentary sources retain Li anomalies up to anatectic conditions, the formation of hard-rock Li ores of anatectic origin likely reflects the convergence in space and time of favorable melting mechanisms, P-T conditions, and tectonic controls enabling rapid melt extraction and extensive fractionation—a convergence that, although rare, is preferentially achieved during the waning stages of orogenic cycles.