Slab devolatilization and fluid evolution in the Mariana forearc constrained by noble gases, halogens, and iodine isotopes

Subduction zone forearcs represent the first major devolatilization window of the downgoing slab and play a critical, yet poorly constrained, role in regulating volatile recycling between Earth’s surface and interior. Here we present the first coupled dataset of dissolved noble gases (He–Ne–Ar–Kr–Xe), halogens (Cl–Br–I), and iodine isotopes from Mariana forearc borehole fluids to better constrain the sources, transport, and modification of slab-derived volatiles. The fluids reflect mixing between seawater and deep mantle and slab-derived components. Halogen systematics identify altered oceanic crust containing carbonates and iron oxyhydroxides as the dominant slab fluid source, with volatile release linked to decarbonation and mineral breakdown. Iodine isotopes indicate that these fluids tap ancient reservoirs isolated within the slab for tens to hundreds of millions of years prior to subduction. Ultra-high-precision (sub-permil) xenon isotope measurements provide the first identification of resolvable Xe isotope anomalies in forearc fluids, with heavy Xe excesses uniquely fingerprinting contributions from both mantle and U-rich slab lithologies. In contrast, elemental noble gas systematics reveal strong fractionation during slab processing, with preferential loss of light noble gases relative to heavier species. This is expressed as systematic neon depletion and decoupling of helium from heavier noble gases, consistent with diffusive loss during progressive burial and heating of the slab. Together, these results demonstrate that the Mariana forearc system acts as an efficient filter of slab-derived volatiles, selectively removing light noble gases and limiting their recycling to the deeper mantle.