Rewriting Lithium’s Anatectic Narrative: A Hierarchical Framework for Mobility and Enrichment

Lithium-rich pegmatites and granites are commonly thought to form either by extreme fractionation of granitic magmas or low-degree crustal melting.  Yet, decades of debate leave striking questions unresolved. What are the mechanisms of Li release during crustal melting? Can crustal melting alone ever produce Li concentrations high enough to matter economically? These questions are particularly timely, as the idea that crustal anatexis alone can generate melts with sufficient Li to form economically viable ore deposits has gained renewed attention. Here, we present the first comprehensive database of Li concentrations in anatectic melt inclusions (i.e., melt inclusions hosted in perictectic minerals of migmatites and granulites), providing direct empirical constraints on the Li budget of primary crustal melts formed under common mid- to lower-crustal P–T–X_bulk conditions.

Lithium concentrations in these melts reach a maximum of ~600 μg/g during the earliest stages of fluid-absent biotite melting at 750–800 °C in cordierite-free metasedimentary rocks. Although these values are two to three times higher than those of typical S-type granites, they overlap the range of barren pegmatites and remain far below those of Li ore-forming systems. Integration of this dataset with thermodynamic and geochemical modelling shows that melting of Li-enriched sources or multi-stage melting can locally enhance melt Li contents, but are unlikely to directly generate high-grade Li deposits without subsequent melt differentiation. Without compelling evidence that strongly pre-enriched sources can preserve extreme Li anomalies (10 to 200 times crustal values) up to anatectic conditions, extreme post-anatectic differentiation emerges as a necessary condition for generating economically viable Li deposits. Nature demands more than a melting source: high-grade Li deposits of anatectic origin are earned in the details of differentiation.

Melt inclusions in anatectic rocks thus represent robust quantitative tracers of critical metal mobility, opening new avenues for future interrogation of fertile anatectic systems.