Apatite as a tool to quantify the volatile budget of felsic magma reservoirs

The presence of dissolved and exsolved volatiles in silicate melt plays a key role in magma differentiation by controlling their chemical and physical properties. Volatile concentrations in the melt are commonly determined from glass inclusions; however, these may have undergone post-entrapment modification or may be absent in the studied rocks. Alternatively, volatile-bearing minerals such as biotite, amphibole, and apatite offer a valuable means to reconstruct the volatile history of their parental melt. Here we investigate the volatile evolution of differentiating magmas from emplacement to interstitial melt extraction within the Western Adamello tonalite (WAT) and Re di Castello (RdC) units of the Adamello batholith (Italy), both displaying local crystal accumulation and melt segregation features. Apatite is a ubiquitous phase and exhibits a compositional trend of decreasing F, Cl and S with increasing Eu negative anomaly, the latter used here as a differentiation proxy. In both localities, apatite dominantly has low CO 2 contents (<300 μg/g), whereas S abundances are one order of magnitude higher in the RdC (600–2,000 μg/g) compared to the WAT (40–300 μg/g). In addition, some apatite cores are enriched in CO 2 (>1,000 μg/g). Variable F–Cl partitioning between apatite and its host biotite, along with the preservation of volatile zoning in apatite related to the pluton’s cooling history, suggests that volatiles in apatite did not fully re-equilibrate. Equilibrium melt volatile contents were calculated and show a consistent Cl decrease (∼500–50 μg/g) during differentiation, indicative of fluid saturation. Melt H 2 O and CO 2 contents, coupled with saturation curves, were used to retrieve crystallisation pressures of ∼200–300 MPa for low-CO 2 apatite, and ∼400–700 MPa for CO 2 -rich apatite, in good agreement with pressure estimates based on phase petrology and chemistry. Degassing fluid fluxes were computed by combining the calculated equilibrium fluid composition with available high-precision U–Pb zircon ages, revealing values 2 to 5 orders of magnitude lower than those of active volcanic systems, probably reflecting contrasting magma fluxes. Nevertheless, such low-density fluids, together with episodic magma recharge, likely promoted melt segregation and extraction. These results highlight apatite as a powerful proxy for evaluating the fluid saturation level of magmas, tracking the volatile budget of crystallising magma reservoirs, and reconstructing the architecture of transcrustal magmatic systems.