Apatite sulfur signatures reveal melt–fluid remiscibility at low temperature in felsic systems

The transition from magma-dominated to fluid-dominated systems marks a critical stage where metal elements combine with ligands, enabling their transport and, in some cases, the formation of economically significant mineral to ore deposits. Pegmatites, often viewed as frozen products of this magmatic–hydrothermal transition, have an origin that remains debated. Based on new sulfur and neodymium isotope data from apatite hosted in tonalites and pegmatites from the Western Adamello and Listino ring complex (Adamello batholith, Italy), we show that pegmatites are directly derived from their tonalite hosts but crystallized following two distinct processes. Sulfur-poor, intermediate- to low-δ 34 S (<12 ‰) pegmatites follow the typical tonalite liquid line of descent, consistent with crystallization from a fluid-saturated granitic melt that underwent undercooling. In contrast, sulfur-rich, high-δ 34 S (≥12 ‰) pegmatites show intermediate volatile and isotopic compositions between silicate melt and aqueous fluid endmembers, as reproduced by our crystal–melt–fluid fractionation model. These signatures indicate low-temperature (≤650°C) remiscibility between melt and fluid, leading to late-stage sulfur enrichment and increase in δ 34 S values of pegmatitic liquids. We tentatively propose that closing the melt–fluid miscibility gap to a supercritical liquid at low temperature may enhance interaction between volatile, metal, and lithophile elements, providing one mechanism for concentrating critical elements and generating magmatic–hydrothermal mineral and ore deposits.