Weidong Sun

Fluids released from subducting slabs play an important role in arc magmatism and global volatile cycles, yet resolving their lithological origins and spatial distribution remains a major challenge. Here, we integrate molybdenum (Mo) isotope systematics with improved P-wave tomography across the Kamchatka arc to trace the origins and transport pathways of slab-derived fluids. Arc lavas exhibit δ ^98/95 Mo values systematically higher than the depleted mantle and correlate positively with fluid-mobile element enrichment. We identify a distinctive “dumbbell-shaped” isotopic pattern cospatial with prominent low-velocity zones in the mantle wedge. The anomalies further align with regions of marked velocity reductions in the incoming Pacific Plate, indicating that the fluid budget of the volcanic front is directly inherited from heterogeneous serpentinization of the subducting lithospheric mantle prior to subduction. Our results demonstrate that preexisting slab hydration exerts first-order control on subduction zone chemical architecture, providing a predictive framework for understanding global volatile cycling.

Hydration of the lithospheric mantle is a critical part of deep water cycling and is essential in shaping Earth's habitable environments. At low water/rock ratios, water is stored in nominally anhydrous minerals or hydrous minerals; however, at high water/rock ratios, the leaching effect of water on mantle minerals remains poorly understood. Here, we present hydrothermal experiments of the ‘phlogopite + H ₂ O’ system at 500°C–950°C and 0.9–1.8 GPa, showing that incongruent dissolution of phlogopite in water produces corundum (Al ₂ O ₃ ) as an insoluble residual phase at pressures above ∼1.5 GPa. Water leaches soluble components (K, Mg and Si) from phlogopite, leaving corundum as the residue under lithospheric mantle P‒T conditions. This provides a mechanistic explanation for sapphire formation in the lithospheric mantle, consistent with natural sapphire gemstones occurring as mantle xenoliths and xenocrysts within alkaline basalts. Sapphires form through hydrous leaching of phlogopite at depths greater than ∼50 km and are subsequently transported to the surface by alkaline basaltic magmas originating from at least ∼70 km depth. The widespread sapphire deposits associated with alkaline basalts in East China indicate extensive hydrous leaching of the lithospheric mantle, with water ultimately sourced from the subducted slab, mantle transition zone or mantle plumes.

Carbonatite-associated rare earth element (REE) deposits are currently the primary source of REE resources. Their formation requires REE enrichment during prolonged magma evolution, achieved by suppressing REE-rich mineral crystallization and promoting REE-enriched brine melt formation. Our experiments on the fractional crystallization of carbonatitic magmas indicate that pressure (emplacement depth) is the primary factor controlling REE enrichment. High-pressure ( >0.3 GPa) promotes early olivine crystallization, depleting silica and suppressing REE-rich apatite formation. Deep emplacement also delays aqueous fluid exsolution, thereby stabilizing brine melts that enhance phosphate dissolution and prevent REE dispersion into apatite. In contrast, low-pressure conditions ( <0.3 GPa) lead to exsolution of REE-poor hydrothermal fluids, dispersing REE into magmatic apatite and preventing the deposition of economically significant REE-carbonates in subsequent hydrothermal stages. Our pressure-dependent model highlights deep emplacement as crucial for passive REE enrichment in residual brine melts, driving large-scale mineralization through precipitation of burbankite and/or bastnäsite.

A large hydrothermal field at depths >4300 meters was found on the east Caroline plate in the western Pacific Ocean. Here, we show that large hydrothermal pipes with steep walls and breccia-dominated bottoms suggest explosions of billion metric tons of TNT (trinitrotoluene) equivalent. More than 800 short-duration seismic events were detected within 28 days along a 150-kilometer profile, indicating widespread ongoing explosive gas release. We suggest that billions of tons of hydrogen gas were produced through serpentinization associated with fluid infiltration via fractures formed by bending of the oceanic plate. Hydrogen was retained in fractures sealed by mud and carbonate. However, hydrogen alone cannot produce the explosive capacity responsible for the formation of pipes. Oxygen released from circulating seawater can be concentrated at crustal depths and then mixed with mantle-derived hydrogen. Explosion of the gas mixture releases large amounts of heat that triggers further explosions of compressed hydrogen gas and results in the formation of hydrothermal pipes.

The Kunlun hydrothermal system near the Mussau Trench in the western Pacific consists of 20 large round/oval craters of hundreds of meters in diameter. The total area is 11.1 square kilometers, i.e., over a hundred times larger than the Lost City. Four of the larger craters were prospected using the human occupied vehicle Fendouzhe and have steep walls similar to those of kimberlite pipes, with depths of up to 130 meters. Ecosystems have been observed in smaller pits on the bottom of these pipes. Hydrogen concentrations of ~5.9 to 6.8 millimoles per kilogram in hydrothermal fluids have been obtained using in situ Raman spectra. The estimated total hydrogen flux of Kunlun is 4.8 × 10 ¹¹ moles per year, which is >5% of the estimated global submarine abiotic hydrogen flux. Hydrogen-rich, alkaline fluids generated by serpentinization have formed large-scale carbonate rocks within the pipes below the carbonate compensation depth, and both dolomite and calcite have been identified.