Rutile's pressure-temperature-time evolution dictates its role as a depth-dependent subduction-zone water regulator

Abstract

The deep Earth water cycling is critically regulated by hydrogen storage capabilities of nominally anhydrous minerals (NAMs). Rutile TiO₂, a prevalent NAM in subduction zones, incorporates measurable hydrogen via defect-mediated mechanisms. However, its molecular-level structural evolution and hydrogen content variations under subduction-relevant conditions remain poorly constrained. This study integrates spatially resolved Raman imaging and Fourier transform infrared spectroscopy to quantitatively elucidate the microstructural and hydrogen dynamics in rutile across various pressure-temperature-time (P-T-t) trajectories. The results reveal that during prolonged cold subduction (geothermal gradient of ∼5 °C/km), pressure enhances microstructural ordering and crystallinity in rutile, leading to hydrogen loss. Conversely, thermal activation generates microstructural defects, which serves as additional sites for hydrogen incorporation. Quantitative hydrogen tracking demonstrates that rutile undergoes ∼50 % dehydration in anhydrous environments, which is inhibited by the rutile-to-akaogiite transformation. This transformation initiates at ∼8 GPa due to molecular heterogeneity, yet the kinetic stability provided by polar covalent TiO bonds enables rutile to persist up to ∼20 GPa. Notably, rutile exhibits ∼12 % hydrogen uptake upon prolonged fluid infiltration along cold-to-hot subduction pathways (geothermal gradients of ∼5–20 °C/km). In the continuous aqueous environments typical of subduction zones, these findings establish rutile as a depth-dependent water regulator. It can transport water beyond its initial content to mantle depths exceeding ∼300 km, thus advancing our understanding of how NAMs modulate water cycling in subduction zones.