Crystallisation-driven volatile exsolution (second boiling) is widely invoked to pressurise shallow magma reservoirs, yet its ability to generate eruption-triggering overpressure and detectable ground deformation is poorly quantified. We couple phase equilibria with thermal and viscoelastic models of sill-like intrusions to map pressure evolution and geodetic detectability of second boiling. Thermodynamic calculations for basaltic and rhyolitic magmas from Askja (Iceland) and Puyehue-Cordón Caulle (Chile) are embedded in conductive thermal models for thin (20 m) and thick (200 m) sills at 2–4 km depth, and the resulting volume changes and magma compressibility are propagated into pressurisation histories and volume-change rates. Hydrous magmatic sills can develop overpressures of 20–30 MPa on timescales of months to centuries, depending on initial sill size and crustal parameters, whereas relatively dry basalt undergoes cyclic behaviour with contraction followed by modest re-pressurisation. For a given composition and depth, thin and thick sills attain similar overpressures but differ by an order of magnitude in volume-change rate. Only small, volatile-rich sills exceed typical InSAR resolution thresholds. In felsic crust, viscoelastic relaxation further dampens overpressure and surface deformation, consistent with globally observed reduced deformation at arc volcanoes. Our results define a regime of “stealth” pressurisation in shallow hydrous reservoirs, in which magma emplaced centuries earlier may accumulate eruption-scale overpressure with little or no unrest detectable by short temporal baseline geodetic monitoring. This mechanism can help explain unexpected explosive eruptions with little precursory deformation.
Himalayan leucogranites, some of the best-known high-silica granites in the world, are associated with abundant rare-metal mineralization, particularly lithium pegmatites in the Higher Himalaya. Although structural deformation has been proposed to play a major role in magmatic fractionation, how it enhances this process remains poorly understood. Here, we examine the Gyirong leucogranite, a representative Higher Himalayan leucogranite adjacent to the South Tibetan Detachment System (STDS), through detailed analyses on minerals and whole-rock samples along the transect. Mineral microstructures reflect the granites experienced high-temperature melt-present deformation, with typical differential stresses of the STDS. Systematic geochemical variations are observed, with increasing Li, Rb, and Cs and decreasing La, Sr, and Zr toward the STDS, reflecting a clear magmatic fractionation trend. The muscovite compositional zonings indicate that fractionated melt is concentrated at the top of the pluton without magmatic recharge signatures. According to the systematic geochemical composition, we divide the Gyirong leucogranite into cumulate and residual melt parts. The calculated crystallinity of cumulate part is higher than the maximum packing fraction, reflecting deformation must play a role. Forward modeling shows that at least 200 kyr is required to achieve the observed geochemical profile, longer than the cooling duration estimated by thermal modeling. Thus, we propose that crustal shearing could increase the efficiency of crystal-melt segregation, leading to significant magmatic fractionation of high-silica granitic magmas over short periods of time. Such a process is intrinsically linked to the Himalayan uplift, providing a critical mechanism for rare-metal elements enrichment.
