Titanium-rich phase stability in subducting oceanic crust and implications for molybdenum and water cycles

Titanium-rich phases in subducting oceanic crust are key hosts for high field strength elements (HFSEs) and molybdenum, and potential carriers of water to the deep mantle. We constrained their stability through high-pressure experiments (4−12 GPa, 1000−1200 °C) on anhydrous and hydrous basaltic compositions. Rutile transforms to an α-PbO2-type Fe−Ti oxyhydroxide (α-phase) above ∼6 GPa, and Ti-bearing ε-FeOOH (ε-phase) appears at 12 GPa and 1000 °C in hydrous systems. At 1200 °C, the α-phase appears at 6 GPa in hydrous mid-ocean ridge basalt (MORB), but only at 8 GPa in anhydrous MORB. Mo compatibility decreases in the sequence rutile > α-phase > ε-phase (DMoα/rutile = 0.2−1, DMoε/α = 0.2; DMo—Mo partition coefficient), consistent with mineral−melt partitioning (DMorutile/melt = 1.3−27, DMoα/melt = 0.8−3.5), indicating progressive Mo release during subduction. Both α- and ε-phases incorporate significant H2O, enabling deep water transport beyond lawsonite stability. These phase relations and partitioning data provide essential constraints for modeling coupled HFSEs, Mo, and H2O cycling through subduction zones.